From 04fed990ee006cbb54458100526ba12b27117f85 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Mon, 10 Aug 2026 14:09:43 +0900
Subject: [PATCH 01/32] docs: add agent coding guidelines

---
 AGENTS.md | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 AGENTS.md

diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 00000000..bc0803f9
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,65 @@
+# AGENTS.md
+
+Behavioral guidelines to reduce common LLM coding mistakes. Merge with project-specific instructions as needed.
+
+**Tradeoff:** These guidelines bias toward caution over speed. For trivial tasks, use judgment.
+
+## 1. Think Before Coding
+
+**Don't assume. Don't hide confusion. Surface tradeoffs.**
+
+Before implementing:
+- State your assumptions explicitly. If uncertain, ask.
+- If multiple interpretations exist, present them - don't pick silently.
+- If a simpler approach exists, say so. Push back when warranted.
+- If something is unclear, stop. Name what's confusing. Ask.
+
+## 2. Simplicity First
+
+**Minimum code that solves the problem. Nothing speculative.**
+
+- No features beyond what was asked.
+- No abstractions for single-use code.
+- No "flexibility" or "configurability" that wasn't requested.
+- No error handling for impossible scenarios.
+- If you write 200 lines and it could be 50, rewrite it.
+
+Ask yourself: "Would a senior engineer say this is overcomplicated?" If yes, simplify.
+
+## 3. Surgical Changes
+
+**Touch only what you must. Clean up only your own mess.**
+
+When editing existing code:
+- Don't "improve" adjacent code, comments, or formatting.
+- Don't refactor things that aren't broken.
+- Match existing style, even if you'd do it differently.
+- If you notice unrelated dead code, mention it - don't delete it.
+
+When your changes create orphans:
+- Remove imports/variables/functions that YOUR changes made unused.
+- Don't remove pre-existing dead code unless asked.
+
+The test: Every changed line should trace directly to the user's request.
+
+## 4. Goal-Driven Execution
+
+**Define success criteria. Loop until verified.**
+
+Transform tasks into verifiable goals:
+- "Add validation" → "Write tests for invalid inputs, then make them pass"
+- "Fix the bug" → "Write a test that reproduces it, then make it pass"
+- "Refactor X" → "Ensure tests pass before and after"
+
+For multi-step tasks, state a brief plan:
+```
+1. [Step] → verify: [check]
+2. [Step] → verify: [check]
+3. [Step] → verify: [check]
+```
+
+Strong success criteria let you loop independently. Weak criteria ("make it work") require constant clarification.
+
+---
+
+**These guidelines are working if:** fewer unnecessary changes in diffs, fewer rewrites due to overcomplication, and clarifying questions come before implementation rather than after mistakes.

From c5f14562f719893788dc335cb143739423d6bf5c Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 09:03:56 +0900
Subject: [PATCH 02/32] =?UTF-8?q?refactor:=20Redis=20=EA=B8=B0=EB=B0=98=20?=
 =?UTF-8?q?=EC=84=B8=EC=85=98=20=EC=9D=B8=EC=A6=9D=20=EB=B0=A9=EC=8B=9D?=
 =?UTF-8?q?=EC=9C=BC=EB=A1=9C=20=EA=B0=9C=EC=84=A0?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .gitignore                                    |   2 +
 .../com/ktb/chatapp/config/OpenApiConfig.java |  16 +-
 .../ktb/chatapp/config/SecurityConfig.java    |   6 +-
 .../chatapp/controller/AuthController.java    | 101 +++++-------
 .../com/ktb/chatapp/dto/LoginResponse.java    |   1 -
 .../ktb/chatapp/dto/TokenRefreshResponse.java |   3 +-
 .../java/com/ktb/chatapp/model/Session.java   |  13 --
 .../chatapp/repository/SessionRepository.java |  13 --
 .../security/AuthenticatedUserPrincipal.java  |  31 ++++
 .../security/CustomBearerTokenResolver.java   |   1 -
 .../ktb/chatapp/service/SessionService.java   |  45 ++----
 .../service/UserDetailsServiceImpl.java       |  10 +-
 .../service/session/SessionMongoStore.java    |  41 -----
 .../service/session/SessionRedisStore.java    | 148 ++++++++++++++++++
 .../chatapp/service/session/SessionStore.java |   7 +
 .../socketio/AuthTokenListenerImpl.java       |  10 +-
 .../socketio/SocketIOEventListener.java       |  11 +-
 .../socketio/handler/ChatMessageHandler.java  |  17 --
 .../controller/OpenApiContractTest.java       |   2 +-
 .../chatapp/service/SessionServiceTest.java   |  51 ++----
 .../service/SessionServiceUnitTest.java       |  50 +++++-
 .../socketio/SocketIOEventListenerTest.java   |   5 +
 .../handler/ChatMessageHandlerTest.java       |  10 --
 apps/frontend/components/FileMessage.js       |  16 +-
 apps/frontend/contexts/AuthContext.js         |   8 +-
 .../files/__tests__/useChatFileUpload.test.js |   4 +-
 .../features/chat/files/useChatFileUpload.js  |   3 +-
 .../room/__tests__/useMessageHandling.test.js |   1 -
 .../room/__tests__/useRoomHandling.test.js    |   1 -
 .../features/chat/room/useFileHandling.js     |   3 +-
 .../features/chat/room/useRoomHandling.js     |   5 +-
 .../rooms/__tests__/ChatRoomsView.test.js     |   1 -
 .../rooms/__tests__/useRoomsSocket.test.js    |   1 -
 .../features/chat/rooms/useRoomsSocket.js     |   1 -
 .../frontend/lib/api/__tests__/client.test.js |  12 +-
 apps/frontend/lib/api/client.js               |   3 +-
 apps/frontend/lib/socket/SocketProvider.js    |   5 +-
 .../socket/__tests__/SocketProvider.test.js   |   2 -
 apps/frontend/pages/profile.js                |   3 +-
 apps/frontend/services/authService.js         |  15 +-
 apps/frontend/services/fileService.js         |   9 +-
 loadtest/load-test.js                         |   6 +-
 loadtest/ramp-up-test.js                      |  35 ++---
 43 files changed, 369 insertions(+), 359 deletions(-)
 create mode 100644 .gitignore
 delete mode 100644 apps/backend/src/main/java/com/ktb/chatapp/repository/SessionRepository.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/security/AuthenticatedUserPrincipal.java
 delete mode 100644 apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionMongoStore.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionRedisStore.java

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 00000000..60889873
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.DS_Store
+apps/backend/target/
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/OpenApiConfig.java b/apps/backend/src/main/java/com/ktb/chatapp/config/OpenApiConfig.java
index 60e87e67..46e242ff 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/config/OpenApiConfig.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/OpenApiConfig.java
@@ -42,7 +42,6 @@ public OpenAPI customOpenAPI() {
 
         // 보안 스키마 이름
         String jwtSchemeName = "Bearer Authentication";
-        String sessionSchemeName = "Session ID";
 
         return new OpenAPI()
                 .info(new Info()
@@ -52,10 +51,8 @@ public OpenAPI customOpenAPI() {
 
                                 ## 인증 방법
                                 1. `/api/auth/register` 또는 `/api/auth/login`으로 회원가입/로그인
-                                2. 응답으로 받은 `token`과 `sessionId`를 사용
-                                3. 이후 모든 요청에 다음 헤더 포함:
-                                   - `Authorization: Bearer {token}`
-                                   - `x-session-id: {sessionId}`
+                                2. 응답으로 받은 `token`을 사용
+                                3. 이후 모든 요청에 `Authorization: Bearer {token}` 헤더 포함
 
                                 ## 주요 기능
                                 - 사용자 인증 및 관리
@@ -88,12 +85,6 @@ public OpenAPI customOpenAPI() {
                                 .scheme("bearer")
                                 .bearerFormat("JWT")
                                 .description("JWT 토큰을 입력하세요 (Bearer 접두사 제외)"))
-                        // Session ID 헤더 보안 스키마
-                        .addSecuritySchemes(sessionSchemeName, new SecurityScheme()
-                                .type(SecurityScheme.Type.APIKEY)
-                                .in(SecurityScheme.In.HEADER)
-                                .name("x-session-id")
-                                .description("세션 ID를 입력하세요"))
                         // 공통 에러 응답 스키마
                         .addSchemas("ApiErrorResponse", new Schema<>()
                                 .type("object")
@@ -153,7 +144,6 @@ public OpenAPI customOpenAPI() {
                                                 .example("{ \"success\": false, \"code\": \"RATE_LIMIT_EXCEEDED\", \"message\": \"요청 한도를 초과했습니다.\" }")))))
                 // 글로벌 보안 요구사항 (일부 엔드포인트는 개별적으로 재정의)
                 .addSecurityItem(new SecurityRequirement()
-                        .addList(jwtSchemeName)
-                        .addList(sessionSchemeName));
+                        .addList(jwtSchemeName));
     }
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/SecurityConfig.java b/apps/backend/src/main/java/com/ktb/chatapp/config/SecurityConfig.java
index 4eb76010..a0609638 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/config/SecurityConfig.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/SecurityConfig.java
@@ -39,15 +39,11 @@ public class SecurityConfig {
             "Content-Type",
             "Authorization",
             "x-auth-token",
-            "x-session-id",
             "Cache-Control",
             "Pragma"
     );
 
-    private static final List<String> CORS_EXPOSED_HEADERS = List.of(
-            "x-auth-token",
-            "x-session-id"
-    );
+    private static final List<String> CORS_EXPOSED_HEADERS = List.of("Authorization");
 
     private static final List<String> CORS_ALLOWED_METHODS = List.of("GET", "POST", "PUT", "DELETE", "OPTIONS");
 
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/AuthController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/AuthController.java
index e95a11ed..840aa246 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/AuthController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/AuthController.java
@@ -4,6 +4,7 @@
 import com.ktb.chatapp.event.SessionEndedEvent;
 import com.ktb.chatapp.model.User;
 import com.ktb.chatapp.repository.UserRepository;
+import com.ktb.chatapp.security.AuthenticatedUserPrincipal;
 import com.ktb.chatapp.service.JwtService;
 import com.ktb.chatapp.service.SessionCreationResult;
 import com.ktb.chatapp.service.SessionMetadata;
@@ -68,7 +69,7 @@ public ResponseEntity<?> getAuthStatus() {
         return ResponseEntity.ok(Map.of("status", "active", "routes", routes));
     }
 
-    @Operation(summary = "회원가입", description = "새로운 사용자를 등록합니다. 등록 성공 시 JWT 토큰과 세션 ID가 반환됩니다.")
+    @Operation(summary = "회원가입", description = "새로운 사용자를 등록합니다. 세션과 JWT는 로그인 시 생성됩니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "201", description = "회원가입 성공",
             content = @Content(schema = @Schema(implementation = LoginResponse.class))),
@@ -85,8 +86,7 @@ public ResponseEntity<?> getAuthStatus() {
     @PostMapping("/register")
     public ResponseEntity<?> registerUser(
             @Valid @RequestBody RegisterRequest registerRequest,
-            BindingResult bindingResult,
-            HttpServletRequest request) {
+            BindingResult bindingResult) {
 
         // Handle validation errors
         ResponseEntity<?> errors = getBindingError(bindingResult);
@@ -108,13 +108,6 @@ public ResponseEntity<?> registerUser(
 
             user = userRepository.save(user);
 
-            // Create session with metadata
-            SessionMetadata metadata = new SessionMetadata(
-                    request.getHeader("User-Agent"),
-                    getClientIpAddress(request),
-                    request.getHeader("User-Agent")
-            );
-
             LoginResponse response = LoginResponse.builder()
                     .success(true)
                     .message("회원가입이 완료되었습니다.")
@@ -139,7 +132,7 @@ public ResponseEntity<?> registerUser(
         }
     }
     
-    @Operation(summary = "로그인", description = "이메일과 비밀번호로 로그인합니다. 성공 시 JWT 토큰과 세션 ID가 반환됩니다. 기존 세션은 자동으로 종료됩니다.")
+    @Operation(summary = "로그인", description = "이메일과 비밀번호로 로그인합니다. 성공 시 세션 ID가 포함된 JWT가 반환되며 기존 세션은 자동으로 종료됩니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "로그인 성공",
             content = @Content(schema = @Schema(implementation = LoginResponse.class))),
@@ -163,20 +156,18 @@ public ResponseEntity<?> login(
         if (errors != null) return errors;
         
         try {
-            // Authenticate user
-            User user = userRepository.findByEmail(loginRequest.getEmail().toLowerCase())
-                    .orElseThrow(() -> new UsernameNotFoundException("User not found"));
             Authentication authentication = authenticationManager.authenticate(
                     new UsernamePasswordAuthenticationToken(
-                            user.getEmail(),
+                            loginRequest.getEmail().toLowerCase(),
                             loginRequest.getPassword()
                     )
             );
 
             SecurityContextHolder.getContext().setAuthentication(authentication);
-            
-            // 단일 세션 정책을 위해 기존 세션 제거
-            sessionService.removeAllUserSessions(user.getId());
+            AuthenticatedUserPrincipal principal =
+                    (AuthenticatedUserPrincipal) authentication.getPrincipal();
+            User user = principal.getUser();
+            boolean replacingExistingSession = sessionService.getActiveSession(user.getId()) != null;
 
             // Create new session
             SessionMetadata metadata = new SessionMetadata(
@@ -188,6 +179,15 @@ public ResponseEntity<?> login(
             SessionCreationResult sessionInfo =
                     sessionService.createSession(user.getId(), metadata);
 
+            if (replacingExistingSession) {
+                eventPublisher.publishEvent(new SessionEndedEvent(
+                        this,
+                        user.getId(),
+                        "duplicate_login",
+                        "다른 기기에서 로그인하여 현재 세션이 종료되었습니다."
+                ));
+            }
+
             // Generate JWT token
             String token = jwtService.generateToken(
                 sessionInfo.getSessionId(),
@@ -198,13 +198,11 @@ public ResponseEntity<?> login(
             LoginResponse response = LoginResponse.builder()
                     .success(true)
                     .token(token)
-                    .sessionId(sessionInfo.getSessionId())
                     .user(AuthUserDto.from(user))
                     .build();
 
             return ResponseEntity.ok()
                     .header("Authorization", "Bearer " + token)
-                    .header("x-session-id", sessionInfo.getSessionId())
                     .body(response);
 
         } catch (UsernameNotFoundException | BadCredentialsException e) {
@@ -217,14 +215,11 @@ public ResponseEntity<?> login(
         }
     }
     
-    @Operation(summary = "로그아웃", description = "현재 세션을 종료합니다. x-session-id 헤더가 필요합니다.")
+    @Operation(summary = "로그아웃", description = "JWT가 가리키는 현재 세션을 종료합니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "로그아웃 성공",
             content = @Content(schema = @Schema(implementation = StandardResponse.class),
                 examples = @ExampleObject(value = "{\"success\":true,\"message\":\"로그아웃이 완료되었습니다.\"}"))),
-        @ApiResponse(responseCode = "400", description = "x-session-id 헤더 누락",
-            content = @Content(schema = @Schema(implementation = StandardResponse.class),
-                examples = @ExampleObject(value = "{\"success\":false,\"message\":\"x-session-id 헤더가 필요합니다.\"}"))),
         @ApiResponse(responseCode = "401", description = "인증 실패",
             content = @Content(schema = @Schema(implementation = StandardResponse.class))),
         @ApiResponse(responseCode = "500", description = "서버 내부 오류",
@@ -232,22 +227,15 @@ public ResponseEntity<?> login(
     })
     @PostMapping("/logout")
     public ResponseEntity<StandardResponse<Void>> logout(
-            HttpServletRequest request,
             Authentication authentication) {
 
         try {
-            // x-session-id 헤더 필수
-            String sessionId = extractSessionId(request);
-            if (sessionId == null || sessionId.isEmpty()) {
-                return ResponseEntity.status(HttpStatus.BAD_REQUEST)
-                        .body(StandardResponse.error("x-session-id 헤더가 필요합니다."));
-            }
-            
             if (authentication != null) {
                 // Spring Security 6 패턴: Authentication의 Details에서 userId 추출
                 @SuppressWarnings("unchecked")
                 Map<String, Object> details = (Map<String, Object>) authentication.getDetails();
                 String userId = (String) details.get("userId");
+                String sessionId = (String) details.get("sessionId");
                 
                 if (userId != null) {
                     sessionService.removeSession(userId, sessionId);
@@ -271,13 +259,13 @@ public ResponseEntity<StandardResponse<Void>> logout(
     }
     
 
-    @Operation(summary = "토큰 검증", description = "JWT 토큰과 세션의 유효성을 검증합니다. x-auth-token 또는 Authorization 헤더와 x-session-id 헤더가 필요합니다.")
+    @Operation(summary = "토큰 검증", description = "JWT 토큰과 해당 토큰이 가리키는 세션의 유효성을 검증합니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "토큰 검증 성공",
             content = @Content(schema = @Schema(implementation = TokenVerifyResponse.class))),
-        @ApiResponse(responseCode = "400", description = "토큰 또는 세션 ID 누락",
+        @ApiResponse(responseCode = "400", description = "토큰 또는 JWT 세션 ID 누락",
             content = @Content(schema = @Schema(implementation = TokenVerifyResponse.class),
-                examples = @ExampleObject(value = "{\"valid\":false,\"message\":\"토큰 또는 세션 ID가 필요합니다.\"}"))),
+                examples = @ExampleObject(value = "{\"valid\":false,\"message\":\"유효한 JWT가 필요합니다.\"}"))),
         @ApiResponse(responseCode = "401", description = "유효하지 않은 토큰 또는 만료된 세션",
             content = @Content(schema = @Schema(implementation = TokenVerifyResponse.class),
                 examples = @ExampleObject(value = "{\"valid\":false,\"message\":\"유효하지 않은 토큰입니다.\"}"))),
@@ -289,11 +277,11 @@ public ResponseEntity<StandardResponse<Void>> logout(
     public ResponseEntity<?> verifyToken(HttpServletRequest request) {
         try {
             String token = extractToken(request);
-            String sessionId = extractSessionId(request);
+            String sessionId = token != null ? jwtService.extractSessionId(token) : null;
             
             if (token == null || sessionId == null) {
                 return ResponseEntity.status(HttpStatus.BAD_REQUEST)
-                        .body(new TokenVerifyResponse(false, "토큰 또는 세션 ID가 필요합니다.", null));
+                        .body(new TokenVerifyResponse(false, "유효한 JWT가 필요합니다.", null));
             }
 
             // 토큰 유효성 검증
@@ -329,13 +317,13 @@ public ResponseEntity<?> verifyToken(HttpServletRequest request) {
         }
     }
     
-    @Operation(summary = "토큰 갱신", description = "만료된 토큰을 갱신합니다. 새로운 토큰과 세션 ID가 발급됩니다. 기존 세션은 종료됩니다.")
+    @Operation(summary = "토큰 갱신", description = "유효한 Redis 세션을 기준으로 JWT를 갱신합니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "토큰 갱신 성공",
             content = @Content(schema = @Schema(implementation = TokenRefreshResponse.class))),
-        @ApiResponse(responseCode = "400", description = "토큰 또는 세션 ID 누락",
+        @ApiResponse(responseCode = "400", description = "토큰 또는 JWT 세션 ID 누락",
             content = @Content(schema = @Schema(implementation = TokenRefreshResponse.class),
-                examples = @ExampleObject(value = "{\"success\":false,\"message\":\"토큰 또는 세션 ID가 필요합니다.\"}"))),
+                examples = @ExampleObject(value = "{\"success\":false,\"message\":\"유효한 JWT가 필요합니다.\"}"))),
         @ApiResponse(responseCode = "401", description = "유효하지 않은 사용자 또는 만료된 세션",
             content = @Content(schema = @Schema(implementation = TokenRefreshResponse.class),
                 examples = @ExampleObject(value = "{\"success\":false,\"message\":\"만료된 세션입니다.\"}"))),
@@ -347,11 +335,11 @@ public ResponseEntity<?> verifyToken(HttpServletRequest request) {
     public ResponseEntity<?> refreshToken(HttpServletRequest request) {
         try {
             String token = extractToken(request);
-            String sessionId = extractSessionId(request);
+            String sessionId = token != null ? jwtService.extractSessionIdFromExpiredToken(token) : null;
             
             if (token == null || sessionId == null) {
                 return ResponseEntity.status(HttpStatus.BAD_REQUEST)
-                        .body(new TokenRefreshResponse(false, "토큰 또는 세션 ID가 필요합니다.", null, null));
+                        .body(new TokenRefreshResponse(false, "유효한 JWT가 필요합니다.", null));
             }
 
             // 만료된 토큰이라도 사용자 정보는 추출 가능
@@ -361,7 +349,7 @@ public ResponseEntity<?> refreshToken(HttpServletRequest request) {
 
             if (userOpt.isEmpty()) {
                 return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
-                        .body(new TokenRefreshResponse(false, "사용자를 찾을 수 없습니다.", null, null));
+                        .body(new TokenRefreshResponse(false, "사용자를 찾을 수 없습니다.", null));
             }
 
 
@@ -369,31 +357,20 @@ public ResponseEntity<?> refreshToken(HttpServletRequest request) {
             var user = userOpt.get();
             if (!sessionService.validateSession(user.getId(), sessionId).isValid()) {
                 return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
-                        .body(new TokenRefreshResponse(false, "만료된 세션입니다.", null, null));
+                        .body(new TokenRefreshResponse(false, "만료된 세션입니다.", null));
             }
 
-            // 세션 갱신 - 새로운 세션 ID 생성
-            sessionService.removeSession(user.getId(), sessionId);
-            SessionMetadata metadata = new SessionMetadata(
-                    request.getHeader("User-Agent"),
-                    getClientIpAddress(request),
-                    request.getHeader("User-Agent")
-            );
-
-            SessionCreationResult newSessionInfo = sessionService.createSession(user.getId(), metadata);
-
-            // 새로운 토큰과 세션 ID 생성
             String newToken = jwtService.generateToken(
-                newSessionInfo.getSessionId(),
+                sessionId,
                 user.getEmail(),
                 user.getId()
             );
-            return ResponseEntity.ok(new TokenRefreshResponse(true, "토큰이 갱신되었습니다.", newToken, newSessionInfo.getSessionId()));
+            return ResponseEntity.ok(new TokenRefreshResponse(true, "토큰이 갱신되었습니다.", newToken));
 
         } catch (Exception e) {
             log.error("Token refresh error: ", e);
             return ResponseEntity.status(HttpStatus.UNAUTHORIZED)
-                    .body(new TokenRefreshResponse(false, "토큰 갱신 중 오류가 발생했습니다.", null, null));
+                    .body(new TokenRefreshResponse(false, "토큰 갱신 중 오류가 발생했습니다.", null));
         }
     }
     
@@ -411,14 +388,6 @@ private String getClientIpAddress(HttpServletRequest request) {
         return request.getRemoteAddr();
     }
     
-    private String extractSessionId(HttpServletRequest request) {
-        String sessionId = request.getHeader("x-session-id");
-        if (sessionId != null && !sessionId.isEmpty()) {
-            return sessionId;
-        }
-        return request.getParameter("sessionId");
-    }
-    
     private String extractToken(HttpServletRequest request) {
         String token = request.getHeader("x-auth-token");
         if (token != null && !token.isEmpty()) {
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/dto/LoginResponse.java b/apps/backend/src/main/java/com/ktb/chatapp/dto/LoginResponse.java
index 403fe804..863c2448 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/dto/LoginResponse.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/dto/LoginResponse.java
@@ -12,7 +12,6 @@
 public class LoginResponse {
     private boolean success;
     private String token;
-    private String sessionId;
     private AuthUserDto user;
     private String message;
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/dto/TokenRefreshResponse.java b/apps/backend/src/main/java/com/ktb/chatapp/dto/TokenRefreshResponse.java
index 6d0711d3..76fd7a81 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/dto/TokenRefreshResponse.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/dto/TokenRefreshResponse.java
@@ -3,7 +3,6 @@
 public record TokenRefreshResponse(
     boolean success,
     String message,
-    String token,
-    String sessionId
+    String token
 ) {
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/Session.java b/apps/backend/src/main/java/com/ktb/chatapp/model/Session.java
index 47071863..4222cf1d 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/Session.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/Session.java
@@ -6,26 +6,14 @@
 import lombok.Builder;
 import lombok.Data;
 import lombok.NoArgsConstructor;
-import org.springframework.data.annotation.Id;
-import org.springframework.data.mongodb.core.index.CompoundIndex;
-import org.springframework.data.mongodb.core.index.CompoundIndexes;
-import org.springframework.data.mongodb.core.index.Indexed;
-import org.springframework.data.mongodb.core.mapping.Document;
 
 @Data
 @Builder
 @NoArgsConstructor
 @AllArgsConstructor
-@Document(collection = "sessions")
-@CompoundIndexes({
-    @CompoundIndex(name = "userId_sessionId_idx", def = "{'userId': 1, 'sessionId': 1}", unique = true)
-})
 public class Session {
     public static final String SESSION_TTL = "30m";
 
-    @Id
-    private String id;
-
     private String userId;
 
     private String sessionId;
@@ -36,6 +24,5 @@ public class Session {
 
     private SessionMetadata metadata;
 
-    @Indexed(expireAfter = SESSION_TTL)
     private Instant expiresAt;
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/SessionRepository.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/SessionRepository.java
deleted file mode 100644
index db3ed042..00000000
--- a/apps/backend/src/main/java/com/ktb/chatapp/repository/SessionRepository.java
+++ /dev/null
@@ -1,13 +0,0 @@
-package com.ktb.chatapp.repository;
-
-import com.ktb.chatapp.model.Session;
-import org.springframework.data.mongodb.repository.MongoRepository;
-import org.springframework.stereotype.Repository;
-
-import java.util.Optional;
-
-@Repository
-public interface SessionRepository extends MongoRepository<Session, String> {
-    Optional<Session> findByUserId(String userId);
-    void deleteByUserId(String userId);
-}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/security/AuthenticatedUserPrincipal.java b/apps/backend/src/main/java/com/ktb/chatapp/security/AuthenticatedUserPrincipal.java
new file mode 100644
index 00000000..d3e1fb3f
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/security/AuthenticatedUserPrincipal.java
@@ -0,0 +1,31 @@
+package com.ktb.chatapp.security;
+
+import com.ktb.chatapp.model.User;
+import java.util.Collection;
+import java.util.List;
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+import org.springframework.security.core.GrantedAuthority;
+import org.springframework.security.core.userdetails.UserDetails;
+
+@Getter
+@RequiredArgsConstructor
+public class AuthenticatedUserPrincipal implements UserDetails {
+
+    private final User user;
+
+    @Override
+    public Collection<? extends GrantedAuthority> getAuthorities() {
+        return List.of();
+    }
+
+    @Override
+    public String getPassword() {
+        return user.getPassword();
+    }
+
+    @Override
+    public String getUsername() {
+        return user.getEmail();
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/security/CustomBearerTokenResolver.java b/apps/backend/src/main/java/com/ktb/chatapp/security/CustomBearerTokenResolver.java
index 4e4c34e4..a1cf3c5c 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/security/CustomBearerTokenResolver.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/security/CustomBearerTokenResolver.java
@@ -10,7 +10,6 @@
 public class CustomBearerTokenResolver implements BearerTokenResolver {
     
     private static final String CUSTOM_HEADER = "x-auth-token";
-    private static final String SESSION_HEADER = "x-session-id";
     
     @Override
     public String resolve(HttpServletRequest request) {
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/SessionService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/SessionService.java
index 4618267b..35b3a369 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/SessionService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/SessionService.java
@@ -19,6 +19,7 @@ public class SessionService {
     private final SessionStore sessionStore;
     public static final long SESSION_TTL_SEC = DurationStyle.detectAndParse(SESSION_TTL).getSeconds();
     private static final long SESSION_TIMEOUT = SESSION_TTL_SEC * 1000;
+    private static final long ACTIVITY_UPDATE_INTERVAL = 5 * 60 * 1000;
 
     private String generateSessionId() {
         return UUID.randomUUID().toString().replace("-", "");
@@ -36,9 +37,6 @@ private SessionData toSessionData(Session session) {
 
     public SessionCreationResult createSession(String userId, SessionMetadata metadata) {
         try {
-            // Remove all existing user sessions
-            removeAllUserSessions(userId);
-
             String sessionId = generateSessionId();
             long now = Instant.now().toEpochMilli();
             
@@ -82,54 +80,35 @@ public SessionValidationResult validateSession(String userId, String sessionId)
             }
 
             if (!sessionId.equals(session.getSessionId())) {
-                log.warn("Session ID mismatch for userId: {}. Provided: {}, Expected: {}", userId, sessionId, session.getSessionId());
+                log.warn("Session ID mismatch for userId: {}", userId);
                 return SessionValidationResult.invalid("INVALID_SESSION", "잘못된 세션 ID입니다.");
             }
 
             // Check if session has timed out
             long now = Instant.now().toEpochMilli();
             if (now - session.getLastActivity() > SESSION_TIMEOUT) {
-                log.warn("Session timed out for userId: {}, sessionId: {}", userId, sessionId);
+                log.warn("Session timed out for userId: {}", userId);
                 removeSession(userId, sessionId);
                 return SessionValidationResult.invalid("SESSION_EXPIRED", "세션이 만료되었습니다.");
             }
 
-            // Update last activity
-            session.setLastActivity(now);
-            session.setExpiresAt(Instant.now().plusSeconds(SESSION_TTL_SEC));
-            session = sessionStore.save(session);
+            if (now - session.getLastActivity() >= ACTIVITY_UPDATE_INTERVAL) {
+                if (!sessionStore.touch(userId, sessionId, now)) {
+                    return SessionValidationResult.invalid("INVALID_SESSION", "교체된 세션입니다.");
+                }
+                session.setLastActivity(now);
+                session.setExpiresAt(Instant.now().plusSeconds(SESSION_TTL_SEC));
+            }
 
             SessionData sessionData = toSessionData(session);
             return SessionValidationResult.valid(sessionData);
 
         } catch (Exception e) {
-            log.error("Session validation error for userId: {}, sessionId: {}", userId, sessionId, e);
+            log.error("Session validation error for userId: {}", userId, e);
             return SessionValidationResult.invalid("VALIDATION_ERROR", "세션 검증 중 오류가 발생했습니다.");
         }
     }
 
-    public void updateLastActivity(String userId) {
-        try {
-            if (userId == null) {
-                log.warn("updateLastActivity called with null userId");
-                return;
-            }
-
-            Session session = sessionStore.findByUserId(userId).orElse(null);
-            if (session == null) {
-                log.debug("No session found to update last activity for user: {}", userId);
-                return;
-            }
-
-            session.setLastActivity(Instant.now().toEpochMilli());
-            session.setExpiresAt(Instant.now().plusSeconds(SESSION_TTL_SEC));
-            sessionStore.save(session);
-            
-        } catch (Exception e) {
-            log.error("Failed to update session activity for user: {}", userId, e);
-        }
-    }
-
     public void removeSession(String userId, String sessionId) {
         try {
             if (sessionId != null) {
@@ -138,7 +117,7 @@ public void removeSession(String userId, String sessionId) {
                 sessionStore.deleteAll(userId);
             }
         } catch (Exception e) {
-            log.error("Session removal error for userId: {}, sessionId: {}", userId, sessionId, e);
+            log.error("Session removal error for userId: {}", userId, e);
             throw new RuntimeException("세션 삭제 중 오류가 발생했습니다.", e);
         }
     }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/UserDetailsServiceImpl.java b/apps/backend/src/main/java/com/ktb/chatapp/service/UserDetailsServiceImpl.java
index e6dbc587..ca9236da 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/UserDetailsServiceImpl.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/UserDetailsServiceImpl.java
@@ -2,14 +2,13 @@
 
 import com.ktb.chatapp.model.User;
 import com.ktb.chatapp.repository.UserRepository;
+import com.ktb.chatapp.security.AuthenticatedUserPrincipal;
 import lombok.RequiredArgsConstructor;
 import org.springframework.security.core.userdetails.UserDetails;
 import org.springframework.security.core.userdetails.UserDetailsService;
 import org.springframework.security.core.userdetails.UsernameNotFoundException;
 import org.springframework.stereotype.Service;
 
-import java.util.ArrayList;
-
 @RequiredArgsConstructor
 @Service
 public class UserDetailsServiceImpl implements UserDetailsService {
@@ -21,11 +20,6 @@ public UserDetails loadUserByUsername(String email) throws UsernameNotFoundExcep
         User user = userRepository.findByEmail(email.toLowerCase())
                 .orElseThrow(() -> new UsernameNotFoundException("User not found with email: " + email));
 
-        // username은 email로 설정
-        return new org.springframework.security.core.userdetails.User(
-                user.getEmail(),
-                user.getPassword(),
-                new ArrayList<>()
-        );
+        return new AuthenticatedUserPrincipal(user);
     }
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionMongoStore.java b/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionMongoStore.java
deleted file mode 100644
index e2725daa..00000000
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionMongoStore.java
+++ /dev/null
@@ -1,41 +0,0 @@
-package com.ktb.chatapp.service.session;
-
-import com.ktb.chatapp.model.Session;
-import com.ktb.chatapp.repository.SessionRepository;
-import java.util.Optional;
-import lombok.RequiredArgsConstructor;
-import org.springframework.stereotype.Component;
-
-/**
- * MongoDB implementation of SessionStore.
- * Uses SessionRepository for persistence.
- */
-@Component
-@RequiredArgsConstructor
-public class SessionMongoStore implements SessionStore {
-    
-    private final SessionRepository sessionRepository;
-    
-    @Override
-    public Optional<Session> findByUserId(String userId) {
-        return sessionRepository.findByUserId(userId);
-    }
-    
-    @Override
-    public Session save(Session session) {
-        return sessionRepository.save(session);
-    }
-    
-    @Override
-    public void delete(String userId, String sessionId) {
-        Session session = sessionRepository.findByUserId(userId).orElse(null);
-        if (session != null && sessionId.equals(session.getSessionId())) {
-            sessionRepository.delete(session);
-        }
-    }
-    
-    @Override
-    public void deleteAll(String userId) {
-        sessionRepository.deleteByUserId(userId);
-    }
-}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionRedisStore.java b/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionRedisStore.java
new file mode 100644
index 00000000..9dcfcbc3
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionRedisStore.java
@@ -0,0 +1,148 @@
+package com.ktb.chatapp.service.session;
+
+import com.ktb.chatapp.model.Session;
+import com.ktb.chatapp.service.SessionMetadata;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import lombok.RequiredArgsConstructor;
+import org.springframework.data.redis.core.StringRedisTemplate;
+import org.springframework.data.redis.core.script.DefaultRedisScript;
+import org.springframework.stereotype.Component;
+
+import static com.ktb.chatapp.service.SessionService.SESSION_TTL_SEC;
+
+@Component
+@RequiredArgsConstructor
+public class SessionRedisStore implements SessionStore {
+
+    private static final String KEY_PREFIX = "auth:session:";
+    private static final String SESSION_ID = "sessionId";
+    private static final String USER_ID = "userId";
+    private static final String CREATED_AT = "createdAt";
+    private static final String LAST_ACTIVITY = "lastActivity";
+    private static final String USER_AGENT = "userAgent";
+    private static final String IP_ADDRESS = "ipAddress";
+    private static final String DEVICE_INFO = "deviceInfo";
+    private static final DefaultRedisScript<Long> DELETE_IF_SESSION_MATCHES =
+            new DefaultRedisScript<>(
+                    "if redis.call('HGET', KEYS[1], 'sessionId') == ARGV[1] "
+                            + "then return redis.call('DEL', KEYS[1]) else return 0 end",
+                    Long.class
+            );
+    private static final DefaultRedisScript<Long> TOUCH_IF_SESSION_MATCHES =
+            new DefaultRedisScript<>(
+                    "if redis.call('HGET', KEYS[1], 'sessionId') == ARGV[1] then "
+                            + "redis.call('HSET', KEYS[1], 'lastActivity', ARGV[2]); "
+                            + "redis.call('EXPIRE', KEYS[1], ARGV[3]); return 1 else return 0 end",
+                    Long.class
+            );
+
+    private final StringRedisTemplate redisTemplate;
+
+    @Override
+    public Optional<Session> findByUserId(String userId) {
+        String key = buildKey(userId);
+        Map<Object, Object> values = redisTemplate.opsForHash().entries(key);
+        if (values.isEmpty()) {
+            return Optional.empty();
+        }
+
+        Long ttlSeconds = redisTemplate.getExpire(key);
+        Instant expiresAt = ttlSeconds != null && ttlSeconds > 0
+                ? Instant.now().plusSeconds(ttlSeconds)
+                : Instant.now();
+
+        return Optional.of(Session.builder()
+                .userId(value(values, USER_ID))
+                .sessionId(value(values, SESSION_ID))
+                .createdAt(longValue(values, CREATED_AT))
+                .lastActivity(longValue(values, LAST_ACTIVITY))
+                .metadata(readMetadata(values))
+                .expiresAt(expiresAt)
+                .build());
+    }
+
+    @Override
+    public Session save(Session session) {
+        String key = buildKey(session.getUserId());
+        Map<String, String> values = new HashMap<>();
+        values.put(USER_ID, session.getUserId());
+        values.put(SESSION_ID, session.getSessionId());
+        values.put(CREATED_AT, Long.toString(session.getCreatedAt()));
+        values.put(LAST_ACTIVITY, Long.toString(session.getLastActivity()));
+        writeMetadata(values, session.getMetadata());
+
+        redisTemplate.opsForHash().putAll(key, values);
+        redisTemplate.expire(key, Duration.ofSeconds(SESSION_TTL_SEC));
+        return session;
+    }
+
+    @Override
+    public boolean touch(String userId, String sessionId, long lastActivity) {
+        Long result = redisTemplate.execute(
+                TOUCH_IF_SESSION_MATCHES,
+                List.of(buildKey(userId)),
+                sessionId,
+                Long.toString(lastActivity),
+                Long.toString(SESSION_TTL_SEC)
+        );
+        return result != null && result == 1L;
+    }
+
+    @Override
+    public void delete(String userId, String sessionId) {
+        redisTemplate.execute(
+                DELETE_IF_SESSION_MATCHES,
+                List.of(buildKey(userId)),
+                sessionId
+        );
+    }
+
+    @Override
+    public void deleteAll(String userId) {
+        redisTemplate.delete(buildKey(userId));
+    }
+
+    private String buildKey(String userId) {
+        return KEY_PREFIX + userId;
+    }
+
+    private String value(Map<Object, Object> values, String key) {
+        Object value = values.get(key);
+        return value != null ? value.toString() : null;
+    }
+
+    private long longValue(Map<Object, Object> values, String key) {
+        String value = value(values, key);
+        return value != null ? Long.parseLong(value) : 0L;
+    }
+
+    private SessionMetadata readMetadata(Map<Object, Object> values) {
+        String userAgent = nullableValue(values, USER_AGENT);
+        String ipAddress = nullableValue(values, IP_ADDRESS);
+        String deviceInfo = nullableValue(values, DEVICE_INFO);
+        if (userAgent == null && ipAddress == null && deviceInfo == null) {
+            return null;
+        }
+        return new SessionMetadata(userAgent, ipAddress, deviceInfo);
+    }
+
+    private void writeMetadata(Map<String, String> values, SessionMetadata metadata) {
+        values.put(USER_AGENT, metadata != null ? emptyIfNull(metadata.userAgent()) : "");
+        values.put(IP_ADDRESS, metadata != null ? emptyIfNull(metadata.ipAddress()) : "");
+        values.put(DEVICE_INFO, metadata != null ? emptyIfNull(metadata.deviceInfo()) : "");
+    }
+
+    private String nullableValue(Map<Object, Object> values, String key) {
+        String value = value(values, key);
+        return value == null || value.isEmpty() ? null : value;
+    }
+
+    private String emptyIfNull(String value) {
+        return value != null ? value : "";
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionStore.java b/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionStore.java
index 49870236..138f5742 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionStore.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/session/SessionStore.java
@@ -24,6 +24,13 @@ public interface SessionStore {
      * @return the saved session
      */
     Session save(Session session);
+
+    /**
+     * Refresh session activity only if the stored session ID still matches.
+     *
+     * @return true when the current session was refreshed
+     */
+    boolean touch(String userId, String sessionId, long lastActivity);
     
     /**
      * Delete all sessions for a user
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/AuthTokenListenerImpl.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/AuthTokenListenerImpl.java
index 6357e1c6..36248851 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/AuthTokenListenerImpl.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/AuthTokenListenerImpl.java
@@ -19,7 +19,7 @@
 
 /**
  * Socket.IO Authorization Handler
- * socket.handshake.auth.token과 sessionId를 처리한다.
+ * socket.handshake.auth.token을 처리한다.
  */
 @Slf4j
 @Component
@@ -37,17 +37,17 @@ public AuthTokenResult getAuthTokenResult(Object _authToken, SocketIOClient clie
         try {
             var authToken = (Map<?, ?>) _authToken;
             String token = authToken.get("token") != null ? authToken.get("token").toString() : null;
-            String sessionId = authToken.get("sessionId") != null ? authToken.get("sessionId").toString() : null;
 
-            if (token == null || sessionId == null) {
-                log.warn("Missing authentication credentials in Socket.IO handshake - token: {}, sessionId: {}",
-                        token != null, sessionId != null);
+            if (token == null) {
+                log.warn("Missing token in Socket.IO handshake");
                 return new AuthTokenResult(false, "Authentication error");
             }
 
             String userId;
+            String sessionId;
             try {
                 userId = jwtService.extractUserId(token);
+                sessionId = jwtService.extractSessionId(token);
             } catch (JwtException e) {
                 return new AuthTokenResult(false, Map.of("message", "Invalid token"));
             }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListener.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListener.java
index ca61df4e..9a3c5556 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListener.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListener.java
@@ -24,11 +24,12 @@ public class SocketIOEventListener {
     @EventListener
     public void handleSessionEndedEvent(SessionEndedEvent event) {
         try {
-            socketIOServer.getRoomOperations("user:" + event.getUserId())
-                    .sendEvent(SESSION_ENDED, Map.of(
-                            "reason", event.getReason(),
-                            "message", event.getMessage()
-                    ));
+            var userOperations = socketIOServer.getRoomOperations("user:" + event.getUserId());
+            userOperations.sendEvent(SESSION_ENDED, Map.of(
+                    "reason", event.getReason(),
+                    "message", event.getMessage()
+            ));
+            userOperations.getClients().forEach(client -> client.disconnect());
             log.info("session_ended 이벤트 발송: userId={}, reason={}", event.getUserId(), event.getReason());
         } catch (Exception e) {
             log.error("session_ended 이벤트 발송 실패: userId={}", event.getUserId(), e);
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandler.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandler.java
index 08ae04c2..4894162c 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandler.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandler.java
@@ -16,8 +16,6 @@
 import com.ktb.chatapp.util.BannedWordChecker;
 import com.ktb.chatapp.websocket.socketio.ai.AiService;
 import com.ktb.chatapp.service.RoomActivityNotifier;
-import com.ktb.chatapp.service.SessionService;
-import com.ktb.chatapp.service.SessionValidationResult;
 import com.ktb.chatapp.service.RateLimitService;
 import com.ktb.chatapp.service.RateLimitCheckResult;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
@@ -45,7 +43,6 @@ public class ChatMessageHandler {
     private final UserRepository userRepository;
     private final FileRepository fileRepository;
     private final AiService aiService;
-    private final SessionService sessionService;
     private final RoomActivityNotifier roomActivityNotifier;
     private final BannedWordChecker bannedWordChecker;
     private final RateLimitService rateLimitService;
@@ -77,18 +74,6 @@ public void handleChatMessage(SocketIOClient client, ChatMessageRequest data) {
             return;
         }
 
-        SessionValidationResult validation =
-                sessionService.validateSession(socketUser.id(), socketUser.authSessionId());
-        if (!validation.isValid()) {
-            recordError("session_expired");
-            client.sendEvent(ERROR, Map.of(
-                    "code", "SESSION_EXPIRED",
-                    "message", "세션이 만료되었습니다. 다시 로그인해주세요."
-            ));
-            timerSample.stop(createTimer("error", "session_expired"));
-            return;
-        }
-
         // Rate limit check
         RateLimitCheckResult rateLimitResult =
                 rateLimitService.checkRateLimit(socketUser.id(), 10000, Duration.ofMinutes(1));
@@ -173,8 +158,6 @@ public void handleChatMessage(SocketIOClient client, ChatMessageRequest data) {
             // AI 멘션 처리
             aiService.handleAIMentions(roomId, socketUser.id(), messageContent);
 
-            sessionService.updateLastActivity(socketUser.id());
-
             // Record success metrics
             recordMessageSuccess(messageType);
             timerSample.stop(createTimer("success", messageType));
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/controller/OpenApiContractTest.java b/apps/backend/src/test/java/com/ktb/chatapp/controller/OpenApiContractTest.java
index e8b83e46..80106f49 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/controller/OpenApiContractTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/controller/OpenApiContractTest.java
@@ -50,6 +50,6 @@ void apiDocs_shouldExposeApiContracts() throws Exception {
                 .andExpect(jsonPath("$.servers[*].url", hasItem("http://localhost:5001")))
                 .andExpect(jsonPath("$.paths['/api/health']").exists())
                 .andExpect(jsonPath("$.components.securitySchemes['Bearer Authentication']").exists())
-                .andExpect(jsonPath("$.components.securitySchemes['Session ID'].name").value("x-session-id"));
+                .andExpect(jsonPath("$.components.securitySchemes['Session ID']").doesNotExist());
     }
 }
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceTest.java
index 1dea3427..b8318696 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceTest.java
@@ -1,12 +1,14 @@
 package com.ktb.chatapp.service;
 
 import com.ktb.chatapp.config.MongoTestContainer;
+import com.ktb.chatapp.config.RedisTestContainer;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.context.annotation.Import;
+import org.springframework.data.redis.core.StringRedisTemplate;
 import org.springframework.test.context.TestPropertySource;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -18,7 +20,7 @@
  * 백엔드 저장소(Redis/MongoDB)가 변경되어도 테스트 코드 수정이 불필요
  */
 @SpringBootTest
-@Import(MongoTestContainer.class)
+@Import({MongoTestContainer.class, RedisTestContainer.class})
 @TestPropertySource(properties = {
     "socketio.enabled=false"
 })
@@ -28,6 +30,9 @@ class SessionServiceTest {
     @Autowired
     private SessionService sessionService;
 
+    @Autowired
+    private StringRedisTemplate redisTemplate;
+
     private static final String TEST_USER_ID = "test-user-123";
     private static final String TEST_USER_ID_2 = "test-user-456";
     private static final String TEST_IP = "127.0.0.1";
@@ -67,6 +72,9 @@ void createSession_Success() {
         assertNotNull(result.getSessionData());
         assertEquals(TEST_USER_ID, result.getSessionData().getUserId());
         assertEquals(result.getSessionId(), result.getSessionData().getSessionId());
+        String redisKey = "auth:session:" + TEST_USER_ID;
+        assertTrue(Boolean.TRUE.equals(redisTemplate.hasKey(redisKey)));
+        assertThat(redisTemplate.getExpire(redisKey)).isPositive();
     }
 
     @Test
@@ -167,8 +175,8 @@ void validateSession_NonExistentUser_Failure() {
     }
 
     @Test
-    @DisplayName("세션 검증 - lastActivity 업데이트")
-    void validateSession_UpdatesLastActivity() throws InterruptedException {
+    @DisplayName("짧은 간격의 세션 검증은 lastActivity를 다시 저장하지 않는다")
+    void validateSession_DoesNotUpdateLastActivityWithinThrottleWindow() throws InterruptedException {
         // Given
         SessionMetadata metadata = createTestMetadata();
         SessionCreationResult created = sessionService.createSession(TEST_USER_ID, metadata);
@@ -181,42 +189,7 @@ void validateSession_UpdatesLastActivity() throws InterruptedException {
 
         // Then
         assertTrue(result.isValid());
-        assertThat(result.getSession().getLastActivity()).isGreaterThan(initialLastActivity);
-    }
-
-    // ============ 세션 활동 업데이트 테스트 ============
-
-    @Test
-    @DisplayName("lastActivity 업데이트 성공")
-    void updateLastActivity_Success() throws InterruptedException {
-        // Given
-        SessionMetadata metadata = createTestMetadata();
-        SessionCreationResult created = sessionService.createSession(TEST_USER_ID, metadata);
-        long initialLastActivity = created.getSessionData().getLastActivity();
-
-        Thread.sleep(100);
-
-        // When
-        sessionService.updateLastActivity(TEST_USER_ID);
-
-        // Then - 세션 데이터를 다시 가져와서 확인
-        SessionData activeSession = sessionService.getActiveSession(TEST_USER_ID);
-        assertNotNull(activeSession);
-        assertThat(activeSession.getLastActivity()).isGreaterThan(initialLastActivity);
-    }
-
-    @Test
-    @DisplayName("lastActivity 업데이트 - null userId 처리")
-    void updateLastActivity_NullUserId_NoException() {
-        // When & Then - 예외 발생하지 않아야 함
-        assertDoesNotThrow(() -> sessionService.updateLastActivity(null));
-    }
-
-    @Test
-    @DisplayName("lastActivity 업데이트 - 존재하지 않는 세션")
-    void updateLastActivity_NonExistentSession_NoException() {
-        // When & Then
-        assertDoesNotThrow(() -> sessionService.updateLastActivity("non-existent-user"));
+        assertThat(result.getSession().getLastActivity()).isEqualTo(initialLastActivity);
     }
 
     // ============ 세션 제거 테스트 ============
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceUnitTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceUnitTest.java
index 55901111..f6f6c666 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceUnitTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/SessionServiceUnitTest.java
@@ -15,7 +15,9 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
@@ -35,8 +37,8 @@ class SessionServiceUnitTest {
     private SessionService sessionService;
 
     @Test
-    @DisplayName("세션 생성은 기존 사용자 세션을 제거한 뒤 새 세션을 저장한다")
-    void createSession_RemovesExistingSessionsBeforeSave() {
+    @DisplayName("세션 생성은 단일 사용자 키에 새 세션을 저장한다")
+    void createSession_SavesSessionUsingSingleUserKey() {
         ArgumentCaptor<Session> sessionCaptor = ArgumentCaptor.forClass(Session.class);
         when(sessionStore.save(any(Session.class))).thenAnswer(invocation -> invocation.getArgument(0));
 
@@ -44,7 +46,7 @@ void createSession_RemovesExistingSessionsBeforeSave() {
                 USER_ID,
                 new SessionMetadata("agent", "127.0.0.1", "device"));
 
-        verify(sessionStore).deleteAll(USER_ID);
+        verify(sessionStore, never()).deleteAll(USER_ID);
         verify(sessionStore).save(sessionCaptor.capture());
         Session savedSession = sessionCaptor.getValue();
         assertThat(result.getSessionId()).isEqualTo(savedSession.getSessionId());
@@ -56,7 +58,7 @@ void createSession_RemovesExistingSessionsBeforeSave() {
     @Test
     @DisplayName("세션 생성 중 저장소 실패는 RuntimeException으로 래핑된다")
     void createSession_StoreFailure_ThrowsRuntimeException() {
-        doThrow(new IllegalStateException("store down")).when(sessionStore).deleteAll(USER_ID);
+        doThrow(new IllegalStateException("store down")).when(sessionStore).save(any(Session.class));
 
         RuntimeException exception = assertThrows(
                 RuntimeException.class,
@@ -64,7 +66,7 @@ void createSession_StoreFailure_ThrowsRuntimeException() {
 
         assertThat(exception).hasMessage("세션 생성 중 오류가 발생했습니다.");
         assertThat(exception).hasRootCauseInstanceOf(IllegalStateException.class);
-        verify(sessionStore, never()).save(any(Session.class));
+        verify(sessionStore).save(any(Session.class));
     }
 
     @Test
@@ -123,6 +125,44 @@ void validateSession_StoreFailure_ReturnsValidationError() {
         assertThat(result.getError()).isEqualTo("VALIDATION_ERROR");
     }
 
+    @Test
+    @DisplayName("오래된 활성 세션은 ID가 일치할 때만 원자적으로 갱신한다")
+    void validateSession_OldActiveSession_TouchesMatchingSession() {
+        Session oldSession = Session.builder()
+                .userId(USER_ID)
+                .sessionId(SESSION_ID)
+                .createdAt(Instant.now().minusSeconds(600).toEpochMilli())
+                .lastActivity(Instant.now().minusSeconds(600).toEpochMilli())
+                .expiresAt(Instant.now().plusSeconds(1200))
+                .build();
+        when(sessionStore.findByUserId(USER_ID)).thenReturn(Optional.of(oldSession));
+        when(sessionStore.touch(eq(USER_ID), eq(SESSION_ID), anyLong())).thenReturn(true);
+
+        SessionValidationResult result = sessionService.validateSession(USER_ID, SESSION_ID);
+
+        assertThat(result.isValid()).isTrue();
+        verify(sessionStore).touch(eq(USER_ID), eq(SESSION_ID), anyLong());
+    }
+
+    @Test
+    @DisplayName("활동 갱신 중 교체된 세션은 무효 처리한다")
+    void validateSession_ReplacedDuringTouch_ReturnsInvalidSession() {
+        Session oldSession = Session.builder()
+                .userId(USER_ID)
+                .sessionId(SESSION_ID)
+                .createdAt(Instant.now().minusSeconds(600).toEpochMilli())
+                .lastActivity(Instant.now().minusSeconds(600).toEpochMilli())
+                .expiresAt(Instant.now().plusSeconds(1200))
+                .build();
+        when(sessionStore.findByUserId(USER_ID)).thenReturn(Optional.of(oldSession));
+        when(sessionStore.touch(eq(USER_ID), eq(SESSION_ID), anyLong())).thenReturn(false);
+
+        SessionValidationResult result = sessionService.validateSession(USER_ID, SESSION_ID);
+
+        assertThat(result.isValid()).isFalse();
+        assertThat(result.getError()).isEqualTo("INVALID_SESSION");
+    }
+
     @Test
     @DisplayName("활성 세션 조회 중 저장소 실패는 null로 반환된다")
     void getActiveSession_StoreFailure_ReturnsNull() {
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListenerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListenerTest.java
index 42453f6a..bb09942d 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListenerTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/SocketIOEventListenerTest.java
@@ -1,12 +1,14 @@
 package com.ktb.chatapp.websocket.socketio;
 
 import com.corundumstudio.socketio.BroadcastOperations;
+import com.corundumstudio.socketio.SocketIOClient;
 import com.corundumstudio.socketio.SocketIOServer;
 import com.ktb.chatapp.dto.RoomResponse;
 import com.ktb.chatapp.event.RoomActivityEvent;
 import com.ktb.chatapp.event.RoomUpdatedEvent;
 import com.ktb.chatapp.event.SessionEndedEvent;
 import java.util.Map;
+import java.util.List;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -30,6 +32,7 @@ class SocketIOEventListenerTest {
     @Mock private SocketIOServer socketIOServer;
     @Mock private BroadcastOperations userOperations;
     @Mock private BroadcastOperations roomListOperations;
+    @Mock private SocketIOClient userClient;
 
     private SocketIOEventListener listener;
 
@@ -43,6 +46,7 @@ void handleSessionEndedEvent_sendsSessionEndedToUserRoom() {
         SessionEndedEvent event =
                 new SessionEndedEvent(this, "user-1", "duplicate_login", "ended");
         when(socketIOServer.getRoomOperations("user:user-1")).thenReturn(userOperations);
+        when(userOperations.getClients()).thenReturn(List.of(userClient));
 
         listener.handleSessionEndedEvent(event);
 
@@ -52,6 +56,7 @@ void handleSessionEndedEvent_sendsSessionEndedToUserRoom() {
         Map<String, String> payload = (Map<String, String>) payloadCaptor.getValue();
         assertEquals("duplicate_login", payload.get("reason"));
         assertEquals("ended", payload.get("message"));
+        verify(userClient).disconnect();
     }
 
     @Test
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandlerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandlerTest.java
index be9e2a56..eef128b9 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandlerTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ChatMessageHandlerTest.java
@@ -16,8 +16,6 @@
 import com.ktb.chatapp.service.RateLimitCheckResult;
 import com.ktb.chatapp.service.RateLimitService;
 import com.ktb.chatapp.service.RoomActivityNotifier;
-import com.ktb.chatapp.service.SessionService;
-import com.ktb.chatapp.service.SessionValidationResult;
 import com.ktb.chatapp.util.BannedWordChecker;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
 import com.ktb.chatapp.websocket.socketio.ai.AiService;
@@ -49,7 +47,6 @@ class ChatMessageHandlerTest {
     @Mock private UserRepository userRepository;
     @Mock private FileRepository fileRepository;
     @Mock private AiService aiService;
-    @Mock private SessionService sessionService;
     @Mock private RoomActivityNotifier roomActivityNotifier;
     @Mock private BannedWordChecker bannedWordChecker;
     @Mock private RateLimitService rateLimitService;
@@ -67,7 +64,6 @@ void setUp() {
                         userRepository,
                         fileRepository,
                         aiService,
-                        sessionService,
                         roomActivityNotifier,
                         bannedWordChecker,
                         rateLimitService,
@@ -80,10 +76,6 @@ void handleChatMessage_blocksMessagesContainingBannedWords() {
         SocketUser socketUser = new SocketUser("user-1", "tester", "session-1", "socket-1");
         when(client.get("user")).thenReturn(socketUser);
 
-        SessionValidationResult validResult = SessionValidationResult.valid(null);
-        when(sessionService.validateSession(socketUser.id(), socketUser.authSessionId()))
-                .thenReturn(validResult);
-
         RateLimitCheckResult allowedResult = RateLimitCheckResult.allowed(10000, 9999, 60, System.currentTimeMillis() / 1000 + 60, 60);
         when(rateLimitService.checkRateLimit(eq(socketUser.id()), anyInt(), any()))
                 .thenReturn(allowedResult);
@@ -123,8 +115,6 @@ void handleChatMessage_echoesSavedMessageToSenderSocket() {
         SocketUser socketUser = new SocketUser("user-1", "tester", "session-1", "socket-1");
         when(client.get("user")).thenReturn(socketUser);
 
-        when(sessionService.validateSession(socketUser.id(), socketUser.authSessionId()))
-                .thenReturn(SessionValidationResult.valid(null));
         when(rateLimitService.checkRateLimit(eq(socketUser.id()), anyInt(), any()))
                 .thenReturn(RateLimitCheckResult.allowed(10000, 9999, 60, System.currentTimeMillis() / 1000 + 60, 60));
 
diff --git a/apps/frontend/components/FileMessage.js b/apps/frontend/components/FileMessage.js
index 3af1edc9..9b75b5a2 100644
--- a/apps/frontend/components/FileMessage.js
+++ b/apps/frontend/components/FileMessage.js
@@ -29,14 +29,14 @@ const FileMessage = ({
   const messageDomRef = useRef(null);
   useEffect(() => {
     if (msg?.file) {
-      const url = fileService.getPreviewUrl(msg.file, user?.token, user?.sessionId, true);
+      const url = fileService.getPreviewUrl(msg.file, user?.token, true);
       setPreviewUrl(url);
       console.debug('Preview URL generated:', {
         filename: msg.file.filename,
         url
       });
     }
-  }, [msg?.file, user?.token, user?.sessionId]);
+  }, [msg?.file, user?.token]);
 
   if (!msg?.file) {
     console.error('File data is missing:', msg);
@@ -105,12 +105,12 @@ const FileMessage = ({
         throw new Error('파일 정보가 없습니다.');
       }
 
-      if (!user?.token || !user?.sessionId) {
+      if (!user?.token) {
         throw new Error('인증 정보가 없습니다.');
       }
 
       const baseUrl = fileService.getFileUrl(msg.file.filename, false);
-      const authenticatedUrl = `${baseUrl}?token=${encodeURIComponent(user?.token)}&sessionId=${encodeURIComponent(user?.sessionId)}&download=true`;
+      const authenticatedUrl = `${baseUrl}?token=${encodeURIComponent(user?.token)}&download=true`;
       
       const iframe = document.createElement('iframe');
       iframe.style.display = 'none';
@@ -137,12 +137,12 @@ const FileMessage = ({
         throw new Error('파일 정보가 없습니다.');
       }
 
-      if (!user?.token || !user?.sessionId) {
+      if (!user?.token) {
         throw new Error('인증 정보가 없습니다.');
       }
 
       const baseUrl = fileService.getFileUrl(msg.file.filename, true);
-      const authenticatedUrl = `${baseUrl}?token=${encodeURIComponent(user?.token)}&sessionId=${encodeURIComponent(user?.sessionId)}`;
+      const authenticatedUrl = `${baseUrl}?token=${encodeURIComponent(user?.token)}`;
 
       const newWindow = window.open(authenticatedUrl, '_blank');
       if (!newWindow) {
@@ -165,11 +165,11 @@ const FileMessage = ({
         );
       }
 
-      if (!user?.token || !user?.sessionId) {
+      if (!user?.token) {
         throw new Error('인증 정보가 없습니다.');
       }
 
-      const previewUrl = fileService.getPreviewUrl(msg.file, user?.token, user?.sessionId, true);
+      const previewUrl = fileService.getPreviewUrl(msg.file, user?.token, true);
 
       return (
         <div className="bg-transparent-pattern">
diff --git a/apps/frontend/contexts/AuthContext.js b/apps/frontend/contexts/AuthContext.js
index 8d8c0d5c..1622f1d7 100644
--- a/apps/frontend/contexts/AuthContext.js
+++ b/apps/frontend/contexts/AuthContext.js
@@ -109,7 +109,7 @@ export const AuthProviderWithRouter = ({ children, router }) => {
   const logout = useCallback(async () => {
     try {
       // authService를 통해 로그아웃 API 호출
-      await authService.logout(user?.token, user?.sessionId);
+      await authService.logout(user?.token);
     } catch (error) {
       console.error('Logout error:', error);
     } finally {
@@ -136,15 +136,13 @@ export const AuthProviderWithRouter = ({ children, router }) => {
 
     const updatedUserData = await authService.updateProfile(
       updates,
-      user.token,
-      user.sessionId
+      user.token
     );
 
     const updatedUser = {
       ...user,
       ...updatedUserData,
       token: user.token,
-      sessionId: user.sessionId,
       lastActivity: Date.now()
     };
 
@@ -171,7 +169,7 @@ export const AuthProviderWithRouter = ({ children, router }) => {
   // 토큰 검증
   const verifyToken = useCallback(async () => {
     try {
-      if (!user?.token || !user?.sessionId) {
+      if (!user?.token) {
         throw new Error('No authentication data found');
       }
 
diff --git a/apps/frontend/features/chat/files/__tests__/useChatFileUpload.test.js b/apps/frontend/features/chat/files/__tests__/useChatFileUpload.test.js
index f32fcb85..f3bc5833 100644
--- a/apps/frontend/features/chat/files/__tests__/useChatFileUpload.test.js
+++ b/apps/frontend/features/chat/files/__tests__/useChatFileUpload.test.js
@@ -29,7 +29,7 @@ describe('useChatFileUpload', () => {
     await act(async () => {
       response = await result.current.uploadChatFile(
         { name: 'sample.pdf' },
-        { token: 'token-1', sessionId: 'session-1' }
+        { token: 'token-1' }
       );
     });
 
@@ -51,7 +51,7 @@ describe('useChatFileUpload', () => {
       try {
         await result.current.uploadChatFile(
           { name: 'sample.pdf' },
-          { token: 'token-1', sessionId: 'session-1' }
+          { token: 'token-1' }
         );
       } catch (error) {
         uploadError = error;
diff --git a/apps/frontend/features/chat/files/useChatFileUpload.js b/apps/frontend/features/chat/files/useChatFileUpload.js
index 3dc96318..0a47397e 100644
--- a/apps/frontend/features/chat/files/useChatFileUpload.js
+++ b/apps/frontend/features/chat/files/useChatFileUpload.js
@@ -22,8 +22,7 @@ export const useChatFileUpload = () => {
       const uploadResponse = await fileService.uploadFile(
         file,
         (progress) => setUploadProgress(progress),
-        currentUser.token,
-        currentUser.sessionId
+        currentUser.token
       );
 
       if (!uploadResponse.success) {
diff --git a/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js b/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
index 447be2a3..ab6434b0 100644
--- a/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
+++ b/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
@@ -30,7 +30,6 @@ const roomId = 'room-1';
 
 const currentUser = {
   token: 'token-1',
-  sessionId: 'session-1',
 };
 
 describe('useMessageHandling', () => {
diff --git a/apps/frontend/features/chat/room/__tests__/useRoomHandling.test.js b/apps/frontend/features/chat/room/__tests__/useRoomHandling.test.js
index 14601bdf..78a2e6d3 100644
--- a/apps/frontend/features/chat/room/__tests__/useRoomHandling.test.js
+++ b/apps/frontend/features/chat/room/__tests__/useRoomHandling.test.js
@@ -9,7 +9,6 @@ const authMocks = vi.hoisted(() => ({
   user: {
     id: 'user-1',
     token: 'token-1',
-    sessionId: 'session-1',
     name: 'Tester',
     email: 'tester@example.com',
   },
diff --git a/apps/frontend/features/chat/room/useFileHandling.js b/apps/frontend/features/chat/room/useFileHandling.js
index 9c0b52de..0e62f8fa 100644
--- a/apps/frontend/features/chat/room/useFileHandling.js
+++ b/apps/frontend/features/chat/room/useFileHandling.js
@@ -29,8 +29,7 @@ export const useFileHandling = (currentUser, roomId, handleSessionError) => {
       const uploadResponse = await fileService.uploadFile(
         file,
         (progress) => setUploadProgress(progress),
-        currentUser.token,
-        currentUser.sessionId
+        currentUser.token
       );
 
       if (!uploadResponse.success) {
diff --git a/apps/frontend/features/chat/room/useRoomHandling.js b/apps/frontend/features/chat/room/useRoomHandling.js
index 1c42c1c8..76ec17df 100644
--- a/apps/frontend/features/chat/room/useRoomHandling.js
+++ b/apps/frontend/features/chat/room/useRoomHandling.js
@@ -135,7 +135,7 @@ export const useRoomHandling = ({
 
   const setupSocket = useCallback(async () => {
     try {
-      if (!user?.token || !user?.sessionId) {
+      if (!user?.token) {
         throw new Error('Invalid authentication state');
       }
 
@@ -167,7 +167,6 @@ export const useRoomHandling = ({
       const socket = await socketClient.connect({
         auth: {
           token: user.token,
-          sessionId: user.sessionId,
         },
         transports: ['websocket', 'polling'],
         reconnection: true,
@@ -193,7 +192,7 @@ export const useRoomHandling = ({
   const fetchRoomData = useCallback(
     async (roomId) => {
       try {
-        if (!user?.token || !user?.sessionId) {
+        if (!user?.token) {
           await handleSessionError();
           throw new Error('인증 정보가 유효하지 않습니다.');
         }
diff --git a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
index 5fec65b9..204be521 100644
--- a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
@@ -17,7 +17,6 @@ vi.mock('@/contexts/AuthContext', () => ({
     user: {
       id: 'user-1',
       token: 'token-1',
-      sessionId: 'session-1',
     },
   }),
 }));
diff --git a/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js b/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js
index c8106e6f..a8aeabd3 100644
--- a/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js
@@ -11,7 +11,6 @@ vi.mock('@/lib/socket/socketClient', () => ({
 
 const currentUser = {
   token: 'token-1',
-  sessionId: 'session-1',
 };
 
 const renderRoomsSocket = (socket, overrides = {}) => {
diff --git a/apps/frontend/features/chat/rooms/useRoomsSocket.js b/apps/frontend/features/chat/rooms/useRoomsSocket.js
index 6290d3c2..1cf1a997 100644
--- a/apps/frontend/features/chat/rooms/useRoomsSocket.js
+++ b/apps/frontend/features/chat/rooms/useRoomsSocket.js
@@ -25,7 +25,6 @@ export const useRoomsSocket = ({
           .connect({
             auth: {
               token: currentUser.token,
-              sessionId: currentUser.sessionId,
             },
           })
           .catch((err) => {
diff --git a/apps/frontend/lib/api/__tests__/client.test.js b/apps/frontend/lib/api/__tests__/client.test.js
index 983eacb7..78b18e93 100644
--- a/apps/frontend/lib/api/__tests__/client.test.js
+++ b/apps/frontend/lib/api/__tests__/client.test.js
@@ -16,11 +16,9 @@ describe('api client', () => {
     expect(
       getAuthHeaders({
         token: 'token-1',
-        sessionId: 'session-1',
       })
     ).toEqual({
-      'x-auth-token': 'token-1',
-      'x-session-id': 'session-1',
+      Authorization: 'Bearer token-1',
     });
   });
 
@@ -29,7 +27,6 @@ describe('api client', () => {
       baseURL: 'http://api.test',
       getSession: () => ({
         token: 'token-1',
-        sessionId: 'session-1',
       }),
     });
 
@@ -43,8 +40,7 @@ describe('api client', () => {
 
     const response = await client.get('/api/rooms');
 
-    expect(readHeader(response.config.headers, 'x-auth-token')).toBe('token-1');
-    expect(readHeader(response.config.headers, 'x-session-id')).toBe('session-1');
+    expect(readHeader(response.config.headers, 'Authorization')).toBe('Bearer token-1');
   });
 
   it('respects skipAuth requests', async () => {
@@ -52,7 +48,6 @@ describe('api client', () => {
       baseURL: 'http://api.test',
       getSession: () => ({
         token: 'token-1',
-        sessionId: 'session-1',
       }),
     });
 
@@ -66,8 +61,7 @@ describe('api client', () => {
 
     const response = await client.post('/api/auth/login', {}, { skipAuth: true });
 
-    expect(readHeader(response.config.headers, 'x-auth-token')).toBeUndefined();
-    expect(readHeader(response.config.headers, 'x-session-id')).toBeUndefined();
+    expect(readHeader(response.config.headers, 'Authorization')).toBeUndefined();
   });
 
   it('clears stored users on auth expiration', async () => {
diff --git a/apps/frontend/lib/api/client.js b/apps/frontend/lib/api/client.js
index 7e0f7c42..362e895e 100644
--- a/apps/frontend/lib/api/client.js
+++ b/apps/frontend/lib/api/client.js
@@ -23,8 +23,7 @@ export const getAuthHeaders = (session = loadStoredUser()) => {
   }
 
   return {
-    'x-auth-token': session.token,
-    ...(session.sessionId ? { 'x-session-id': session.sessionId } : {}),
+    Authorization: `Bearer ${session.token}`,
   };
 };
 
diff --git a/apps/frontend/lib/socket/SocketProvider.js b/apps/frontend/lib/socket/SocketProvider.js
index 69966a24..0214f542 100644
--- a/apps/frontend/lib/socket/SocketProvider.js
+++ b/apps/frontend/lib/socket/SocketProvider.js
@@ -8,11 +8,10 @@ export const SocketProvider = ({ children, client = socketClient, session = null
 
   useEffect(() => {
     const handleOnline = () => {
-      if (!client.isConnected() && session?.token && session?.sessionId) {
+      if (!client.isConnected() && session?.token) {
         client.connect({
           auth: {
             token: session.token,
-            sessionId: session.sessionId,
           },
         }).catch(() => {});
       }
@@ -29,7 +28,7 @@ export const SocketProvider = ({ children, client = socketClient, session = null
       window.removeEventListener('online', handleOnline);
       window.removeEventListener('offline', handleOffline);
     };
-  }, [client, session?.sessionId, session?.token]);
+  }, [client, session?.token]);
 
   return (
     <SocketContext.Provider value={value}>
diff --git a/apps/frontend/lib/socket/__tests__/SocketProvider.test.js b/apps/frontend/lib/socket/__tests__/SocketProvider.test.js
index fc855e42..8cf6f3b7 100644
--- a/apps/frontend/lib/socket/__tests__/SocketProvider.test.js
+++ b/apps/frontend/lib/socket/__tests__/SocketProvider.test.js
@@ -36,7 +36,6 @@ describe('SocketProvider', () => {
 
     renderProvider(client, {
       token: 'token-1',
-      sessionId: 'session-1',
     });
 
     act(() => {
@@ -46,7 +45,6 @@ describe('SocketProvider', () => {
     expect(client.connect).toHaveBeenCalledWith({
       auth: {
         token: 'token-1',
-        sessionId: 'session-1',
       },
     });
   });
diff --git a/apps/frontend/pages/profile.js b/apps/frontend/pages/profile.js
index 46e3d842..d85ef900 100644
--- a/apps/frontend/pages/profile.js
+++ b/apps/frontend/pages/profile.js
@@ -56,8 +56,7 @@ const Profile = () => {
         await authService.changePassword(
           '',
           formData.newPassword,
-          user.token,
-          user.sessionId
+          user.token
         );
       }
 
diff --git a/apps/frontend/services/authService.js b/apps/frontend/services/authService.js
index d8a4bb9b..e9ec5004 100644
--- a/apps/frontend/services/authService.js
+++ b/apps/frontend/services/authService.js
@@ -51,8 +51,7 @@ class AuthService {
           name: response.data.user.name,
           email: response.data.user.email,
           profileImage: response.data.user.profileImage,
-          token: response.data.token,
-          sessionId: response.data.sessionId
+          token: response.data.token
         };
 
         return userData;
@@ -83,11 +82,11 @@ class AuthService {
    * 로그아웃 API 호출
    * 상태 관리와 리다이렉션은 AuthContext에서 처리
    */
-  async logout(token, sessionId) {
+  async logout(token) {
     try {
       if (token) {
         await api.post('/api/auth/logout', null, {
-          headers: getAuthHeaders({ token, sessionId })
+          headers: getAuthHeaders({ token })
         });
       }
     } catch (error) {
@@ -117,7 +116,7 @@ class AuthService {
    * 프로필 업데이트 API 호출
    * 상태 업데이트는 AuthContext에서 처리
    */
-  async updateProfile(data, token, sessionId) {
+  async updateProfile(data, token) {
     try {
       if (!token) {
         throw new Error('인증 정보가 없습니다.');
@@ -127,7 +126,7 @@ class AuthService {
         '/api/users/profile',
         data,
         {
-          headers: getAuthHeaders({ token, sessionId })
+          headers: getAuthHeaders({ token })
         }
       );
 
@@ -149,7 +148,7 @@ class AuthService {
   /**
    * 비밀번호 변경 API 호출
    */
-  async changePassword(currentPassword, newPassword, token, sessionId) {
+  async changePassword(currentPassword, newPassword, token) {
     try {
       if (!token) {
         throw new Error('인증 정보가 없습니다.');
@@ -162,7 +161,7 @@ class AuthService {
           newPassword
         },
         {
-          headers: getAuthHeaders({ token, sessionId })
+          headers: getAuthHeaders({ token })
         }
       );
 
diff --git a/apps/frontend/services/fileService.js b/apps/frontend/services/fileService.js
index 97d48c45..f77a1d11 100644
--- a/apps/frontend/services/fileService.js
+++ b/apps/frontend/services/fileService.js
@@ -74,7 +74,7 @@ class FileService {
     return { success: true };
   }
 
-  async uploadFile(file, onProgress, token, sessionId) {
+  async uploadFile(file, onProgress, token) {
     const validationResult = await this.validateFile(file);
     if (!validationResult.success) {
       return validationResult;
@@ -91,7 +91,7 @@ class FileService {
         `${this.baseUrl}/api/files/upload` :
         '/api/files/upload';
 
-      // token과 sessionId는 axios 인터셉터에서 자동으로 추가되므로
+      // token은 axios 인터셉터에서 자동으로 추가되므로
       // 여기서는 명시적으로 전달하지 않아도 됩니다
       const response = await axiosInstance.post(uploadUrl, formData, {
         headers: {
@@ -157,19 +157,18 @@ class FileService {
     return `${baseUrl}/api/files/${endpoint}/${filename}`;
   }
 
-  getPreviewUrl(file, token, sessionId, withAuth = true) {
+  getPreviewUrl(file, token, withAuth = true) {
     if (!file?.filename) return '';
 
     const baseUrl = `${process.env.NEXT_PUBLIC_API_URL}/api/files/view/${file.filename}`;
 
     if (!withAuth) return baseUrl;
 
-    if (!token || !sessionId) return baseUrl;
+    if (!token) return baseUrl;
 
     // URL 객체 생성 전 프로토콜 확인
     const url = new URL(baseUrl);
     url.searchParams.append('token', encodeURIComponent(token));
-    url.searchParams.append('sessionId', encodeURIComponent(sessionId));
 
     return url.toString();
   }
diff --git a/loadtest/load-test.js b/loadtest/load-test.js
index 4ba6795e..0c729d76 100644
--- a/loadtest/load-test.js
+++ b/loadtest/load-test.js
@@ -124,7 +124,7 @@ class LoadTester {
         // If login fails, try to register
         if (loginError.response?.status === 401 || loginError.response?.status === 404) {
           this.log('info', `Registering new user: ${email}`);
-          // register 응답은 token/sessionId가 null이라 소켓 인증에 쓸 수 없다 — 재로그인 필수
+          // register 응답은 token이 없어 소켓 인증에 쓸 수 없다 — 재로그인 필수
           await axios.post(
             `${this.config.apiUrl}/api/auth/register`,
             { email, password, name },
@@ -203,11 +203,11 @@ class LoadTester {
         return;
       }
 
-      const { token, sessionId, user } = authData;
+      const { token, user } = authData;
 
       // 3. Connect to Socket.IO
       const socket = io(this.config.socketUrl, {
-        auth: { token, sessionId },
+        auth: { token },
         transports: ['websocket', 'polling'],
         reconnection: true,
         reconnectionAttempts: 3,
diff --git a/loadtest/ramp-up-test.js b/loadtest/ramp-up-test.js
index a3ba0dbf..9fff80c2 100644
--- a/loadtest/ramp-up-test.js
+++ b/loadtest/ramp-up-test.js
@@ -37,7 +37,7 @@
  *      a) 회원가입 (POST /api/auth/register) 또는 로그인 (POST /api/auth/login)
  *      b) [REST API] 방 입장 (POST /api/rooms/:roomId/join)
  *      c) [REST API] 방 정보 확인 (GET /api/rooms/:roomId)
- *      d) Socket.IO 연결 (with JWT token & sessionId)
+ *      d) Socket.IO 연결 (with JWT token)
  *      e) [WebSocket] 채팅방 참여 (joinRoom event)
  *      f) [WebSocket] 이전 메시지 가져오기 (fetchPreviousMessages event → previousMessagesLoaded 수신)
  *      g) 랜덤 간격으로 메시지 전송 (chatMessage event → message 브로드캐스트 수신)
@@ -391,14 +391,13 @@ class RampUpLoadTester {
     return `${randomAdj} ${randomNoun} ${randomNum}`;
   }
 
-  async getUserProfile(token, sessionId, userId) {
+  async getUserProfile(token, userId) {
     try {
       const response = await axios.get(
         `${this.config.apiUrl}/api/users/profile`,
         {
           headers: {
-            'x-auth-token': token,
-            'x-session-id': sessionId
+            Authorization: `Bearer ${token}`
           },
           timeout: 5000
         }
@@ -412,7 +411,7 @@ class RampUpLoadTester {
     }
   }
 
-  async updateUserProfile(token, sessionId, userId, newName) {
+  async updateUserProfile(token, userId, newName) {
     try {
       this.metrics.profileUpdatesAttempted++;
       const response = await axios.put(
@@ -420,8 +419,7 @@ class RampUpLoadTester {
         { name: newName },
         {
           headers: {
-            'x-auth-token': token,
-            'x-session-id': sessionId,
+            Authorization: `Bearer ${token}`,
             'Content-Type': 'application/json'
           },
           timeout: 5000
@@ -437,7 +435,7 @@ class RampUpLoadTester {
     }
   }
 
-  async uploadFileToChat(socket, token, sessionId, userId, roomId) {
+  async uploadFileToChat(socket, token, userId, roomId) {
     try {
       // Create form data with the image file
       const formData = new FormData();
@@ -452,8 +450,7 @@ class RampUpLoadTester {
         formData,
         {
           headers: {
-            'x-auth-token': token,
-            'x-session-id': sessionId,
+            Authorization: `Bearer ${token}`,
             ...formData.getHeaders()
           },
           timeout: 10000,
@@ -534,7 +531,7 @@ class RampUpLoadTester {
         return;
       }
 
-      const { token, sessionId, user } = authData;
+      const { token, user } = authData;
       // auth(login/register) 단계 완료 시점 (bcrypt CPU 힌트)
       onboardingStages.auth = Date.now() - onboardingStart;
 
@@ -546,8 +543,7 @@ class RampUpLoadTester {
           {},
           {
             headers: {
-              'x-auth-token': token,
-              'x-session-id': sessionId,
+              Authorization: `Bearer ${token}`,
               'Content-Type': 'application/json'
             },
             timeout: 5000
@@ -572,8 +568,7 @@ class RampUpLoadTester {
           `${this.config.apiUrl}/api/rooms/${roomId}`,
           {
             headers: {
-              'x-auth-token': token,
-              'x-session-id': sessionId,
+              Authorization: `Bearer ${token}`,
               'Content-Type': 'application/json'
             },
             timeout: 5000
@@ -592,7 +587,7 @@ class RampUpLoadTester {
 
       // Step 3: WebSocket Connection
       const socket = io(this.config.socketUrl, {
-        auth: { token, sessionId },
+        auth: { token },
         transports: ['websocket'],
         reconnection: false,
         forceNew: true,
@@ -633,7 +628,7 @@ class RampUpLoadTester {
         });
 
         // Start continuous message sending
-        this.startContinuousMessaging(socket, userId, roomId, token, sessionId);
+        this.startContinuousMessaging(socket, userId, roomId, token);
       });
 
       socket.on(SERVER_EMIT.JOIN_ROOM_ERROR, (error) => {
@@ -785,7 +780,7 @@ class RampUpLoadTester {
     }
   }
 
-  startContinuousMessaging(socket, userId, roomId, token, sessionId) {
+  startContinuousMessaging(socket, userId, roomId, token) {
     let messageCount = 0;
 
     const sendMessage = async () => {
@@ -810,13 +805,13 @@ class RampUpLoadTester {
         if (messageCount % 10 === 0) {
           this.log('info', `User ${userId} reached ${messageCount} messages, updating profile...`);
           const newName = this.generateRandomName();
-          await this.updateUserProfile(token, sessionId, userId, newName);
+          await this.updateUserProfile(token, userId, newName);
         }
 
         // Upload image to chat every 17 messages
         if (messageCount % 17 === 0) {
           this.log('info', `User ${userId} reached ${messageCount} messages, uploading image...`);
-          await this.uploadFileToChat(socket, token, sessionId, userId, roomId);
+          await this.uploadFileToChat(socket, token, userId, roomId);
         }
 
       } catch (error) {

From 173f94c7d0beea43b0125bff61d5b69ee71ff75a Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Mon, 10 Aug 2026 23:13:27 +0900
Subject: [PATCH 03/32] perf(load): add rooms API k6 scenario

---
 performance/rooms-api/README.md     |  20 +++++
 performance/rooms-api/rooms-list.js | 134 ++++++++++++++++++++++++++++
 2 files changed, 154 insertions(+)
 create mode 100644 performance/rooms-api/README.md
 create mode 100644 performance/rooms-api/rooms-list.js

diff --git a/performance/rooms-api/README.md b/performance/rooms-api/README.md
new file mode 100644
index 00000000..024320aa
--- /dev/null
+++ b/performance/rooms-api/README.md
@@ -0,0 +1,20 @@
+# Rooms list HTTP-only load test
+
+This k6 scenario is deliberately separate from the Socket.IO scripts in `loadtest/`.
+It logs in fixed accounts during `setup`, then measures only `GET /api/rooms` plus
+the configured think time. It never creates users during a measured run.
+
+Prepare `loadtest-0@test.com` through `loadtest-29@test.com` once with password
+`Test1234!`, then run:
+
+```sh
+API_BASE_URL=http://localhost:5001 \
+TEST_NAME=rooms-before-1 \
+FIXTURE_ID=rooms321-users559-messages4781-sessions529 \
+VUS=10 DURATION=3m THINK_TIME_MS=1000 \
+k6 run performance/rooms-api/rooms-list.js
+```
+
+Required experiment metadata is printed as JSON at the end, including UTC/KST
+timestamps, p95/p99, error rate, timeout count, and completed requests. Record the
+matching Prometheus range for MongoDB pool checked-out and wait queue alongside it.
diff --git a/performance/rooms-api/rooms-list.js b/performance/rooms-api/rooms-list.js
new file mode 100644
index 00000000..5b9b6cac
--- /dev/null
+++ b/performance/rooms-api/rooms-list.js
@@ -0,0 +1,134 @@
+import http from 'k6/http';
+import { check, sleep } from 'k6';
+import { Counter, Rate, Trend } from 'k6/metrics';
+
+const API_BASE_URL = __ENV.API_BASE_URL || 'http://localhost:5001';
+const TEST_NAME = __ENV.TEST_NAME || 'rooms-list-local';
+const FIXTURE_ID = __ENV.FIXTURE_ID || 'unspecified';
+const VUS = Number(__ENV.VUS || 10);
+const DURATION = __ENV.DURATION || '3m';
+const THINK_TIME_MS = Number(__ENV.THINK_TIME_MS || 1000);
+const ACCOUNT_START = Number(__ENV.ACCOUNT_START || 0);
+const ACCOUNT_COUNT = Number(__ENV.ACCOUNT_COUNT || 30);
+const PASSWORD = __ENV.LOAD_TEST_PASSWORD || 'Test1234!';
+
+const roomDuration = new Trend('rooms_list_duration', true);
+const roomErrors = new Rate('rooms_list_errors');
+const roomCompleted = new Counter('rooms_list_completed');
+const roomTimeouts = new Counter('rooms_list_timeouts');
+
+export const options = {
+  vus: VUS,
+  duration: DURATION,
+  summaryTrendStats: ['avg', 'min', 'med', 'max', 'p(90)', 'p(95)', 'p(99)'],
+  thresholds: {
+    rooms_list_errors: ['rate<=0.01'],
+    rooms_list_timeouts: ['count==0'],
+  },
+};
+
+function account(index) {
+  return {
+    email: `loadtest-${ACCOUNT_START + index}@test.com`,
+    password: PASSWORD,
+  };
+}
+
+export function setup() {
+  if (VUS > ACCOUNT_COUNT) {
+    throw new Error(`VUS (${VUS}) must not exceed ACCOUNT_COUNT (${ACCOUNT_COUNT})`);
+  }
+
+  const tokens = [];
+  for (let index = 0; index < ACCOUNT_COUNT; index += 1) {
+    const response = http.post(
+      `${API_BASE_URL}/api/auth/login`,
+      JSON.stringify(account(index)),
+      {
+        headers: { 'Content-Type': 'application/json' },
+        timeout: '10s',
+        tags: { phase: 'setup', endpoint: 'login' },
+      },
+    );
+
+    if (response.status !== 200) {
+      throw new Error(
+        `Fixed account ${account(index).email} is unavailable (HTTP ${response.status}). ` +
+        'Prepare accounts once before the measured run.',
+      );
+    }
+
+    const token = response.json('token');
+    if (!token) {
+      throw new Error(`Login response for ${account(index).email} did not contain a token`);
+    }
+    tokens.push(token);
+  }
+
+  return { tokens };
+}
+
+export default function (data) {
+  const token = data.tokens[(__VU - 1) % data.tokens.length];
+  const response = http.get(`${API_BASE_URL}/api/rooms`, {
+    headers: { Authorization: `Bearer ${token}` },
+    timeout: '30s',
+    tags: { phase: 'measurement', endpoint: 'rooms-list' },
+  });
+
+  const success = check(response, {
+    'rooms list returned HTTP 200': (res) => res.status === 200,
+    'rooms list preserved success contract': (res) => res.json('success') === true,
+    'rooms list preserved data contract': (res) => Array.isArray(res.json('data')),
+  });
+
+  roomDuration.add(response.timings.duration);
+  roomErrors.add(!success);
+  roomCompleted.add(1);
+  if (response.error_code === 1050 || response.timings.duration >= 30000) {
+    roomTimeouts.add(1);
+  }
+
+  sleep(THINK_TIME_MS / 1000);
+}
+
+function metricValue(data, metric, field, fallback = 0) {
+  return data.metrics[metric] && data.metrics[metric].values[field] !== undefined
+    ? data.metrics[metric].values[field]
+    : fallback;
+}
+
+function formatKst(date) {
+  const kst = new Date(date.getTime() + (9 * 60 * 60 * 1000));
+  return kst.toISOString().replace('T', ' ').replace('Z', ' KST');
+}
+
+export function handleSummary(data) {
+  const endedAt = new Date();
+  const durationMs = metricValue(data, 'iteration_duration', 'avg')
+    ? data.state.testRunDurationMs
+    : 0;
+  const startedAt = new Date(endedAt.getTime() - durationMs);
+  const summary = {
+    testName: TEST_NAME,
+    fixtureId: FIXTURE_ID,
+    apiBaseUrl: API_BASE_URL,
+    model: 'HTTP-only rooms list',
+    vus: VUS,
+    duration: DURATION,
+    thinkTimeMs: THINK_TIME_MS,
+    startedAt: startedAt.toISOString(),
+    endedAt: endedAt.toISOString(),
+    startedAtKst: formatKst(startedAt),
+    endedAtKst: formatKst(endedAt),
+    completed: metricValue(data, 'rooms_list_completed', 'count'),
+    errorRate: metricValue(data, 'rooms_list_errors', 'rate'),
+    timeouts: metricValue(data, 'rooms_list_timeouts', 'count'),
+    p95Ms: metricValue(data, 'rooms_list_duration', 'p(95)'),
+    p99Ms: metricValue(data, 'rooms_list_duration', 'p(99)'),
+  };
+
+  return {
+    stdout: `${JSON.stringify(summary, null, 2)}\n`,
+  };
+}

From 8a3a5433ec7d1273d7e06127be1686ccd07579d7 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Mon, 10 Aug 2026 23:16:05 +0900
Subject: [PATCH 04/32] perf(rooms): batch room list lookups

---
 .../chatapp/service/RecentMessageCounter.java |  41 ++++++
 .../com/ktb/chatapp/service/RoomService.java  |  64 ++++++++-
 .../RecentMessageCounterIntegrationTest.java  |  66 +++++++++
 .../ktb/chatapp/service/RoomServiceTest.java  | 131 ++++++++++++++++++
 4 files changed, 300 insertions(+), 2 deletions(-)
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/service/RecentMessageCounterIntegrationTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RecentMessageCounter.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RecentMessageCounter.java
index 39a1fd00..477ee80d 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/RecentMessageCounter.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RecentMessageCounter.java
@@ -3,7 +3,16 @@
 import com.ktb.chatapp.repository.MessageRepository;
 import java.time.Duration;
 import java.time.LocalDateTime;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Map;
+import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
+import org.bson.Document;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.aggregation.Aggregation;
+import org.springframework.data.mongodb.core.aggregation.AggregationResults;
+import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.stereotype.Component;
 
 /**
@@ -16,9 +25,41 @@ public class RecentMessageCounter {
     static final Duration RECENT_WINDOW = Duration.ofMinutes(30);
 
     private final MessageRepository messageRepository;
+    private final MongoTemplate mongoTemplate;
 
     public int countRecentMessages(String roomId) {
         LocalDateTime since = LocalDateTime.now().minus(RECENT_WINDOW);
         return (int) messageRepository.countRecentMessagesByRoomId(roomId, since);
     }
+
+    public Map<String, Integer> countRecentMessages(Collection<String> roomIds) {
+        return countRecentMessages(roomIds, LocalDateTime.now().minus(RECENT_WINDOW));
+    }
+
+    Map<String, Integer> countRecentMessages(
+            Collection<String> roomIds,
+            LocalDateTime since) {
+        if (roomIds == null || roomIds.isEmpty()) {
+            return Collections.emptyMap();
+        }
+
+        Aggregation aggregation = Aggregation.newAggregation(
+            Aggregation.match(Criteria.where("room").in(roomIds)
+                .and("timestamp").gte(since)),
+            Aggregation.group("room").count().as("count")
+        );
+
+        AggregationResults<Document> results = mongoTemplate.aggregate(
+            aggregation,
+            "messages",
+            Document.class
+        );
+
+        return results.getMappedResults().stream()
+            .filter(result -> result.getString("_id") != null)
+            .collect(Collectors.toMap(
+                result -> result.getString("_id"),
+                result -> result.get("count", Number.class).intValue()
+            ));
+    }
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
index 5f5ecf40..61889789 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
@@ -10,9 +10,12 @@
 import java.time.LocalDateTime;
 import java.util.Comparator;
 import java.util.HashMap;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
@@ -34,9 +37,39 @@ public class RoomService {
     public RoomsResponse getAllRooms(String name) {
 
         try {
+            List<Room> rooms = roomRepository.findAll();
+            Set<String> userIds = rooms.stream()
+                .flatMap(room -> {
+                    Set<String> ids = new LinkedHashSet<>();
+                    if (room.getCreator() != null) {
+                        ids.add(room.getCreator());
+                    }
+                    if (room.getParticipantIds() != null) {
+                        ids.addAll(room.getParticipantIds());
+                    }
+                    return ids.stream();
+                })
+                .collect(Collectors.toSet());
+            Map<String, User> usersById = userIds.isEmpty()
+                ? Map.of()
+                : userRepository.findAllById(userIds).stream()
+                    .filter(user -> user != null && user.getId() != null)
+                    .collect(Collectors.toMap(User::getId, Function.identity()));
+            Set<String> roomIds = rooms.stream()
+                .map(Room::getId)
+                .filter(java.util.Objects::nonNull)
+                .collect(Collectors.toSet());
+            Map<String, Integer> recentMessageCounts = roomIds.isEmpty()
+                ? Map.of()
+                : recentMessageCounter.countRecentMessages(roomIds);
+
             // 전체 방을 조회해 최신순으로 정렬한다
-            List<RoomResponse> roomResponses = roomRepository.findAll().stream()
-                .map(room -> mapToRoomResponse(room, name))
+            List<RoomResponse> roomResponses = rooms.stream()
+                .map(room -> mapToRoomResponse(
+                    room,
+                    name,
+                    usersById,
+                    recentMessageCounts.getOrDefault(room.getId(), 0)))
                 .sorted(Comparator.comparing(
                     RoomResponse::getCreatedAtDateTime,
                     Comparator.nullsLast(Comparator.reverseOrder())))
@@ -194,6 +227,33 @@ private RoomResponse mapToRoomResponse(Room room, String name) {
 
         int recentMessageCount = recentMessageCounter.countRecentMessages(room.getId());
 
+        return buildRoomResponse(room, name, creator, participants, recentMessageCount);
+    }
+
+    private RoomResponse mapToRoomResponse(
+            Room room,
+            String name,
+            Map<String, User> usersById,
+            int recentMessageCount) {
+        if (room == null) return null;
+
+        User creator = room.getCreator() != null ? usersById.get(room.getCreator()) : null;
+        List<User> participants = room.getParticipantIds() == null
+            ? List.of()
+            : room.getParticipantIds().stream()
+                .map(usersById::get)
+                .filter(java.util.Objects::nonNull)
+                .toList();
+
+        return buildRoomResponse(room, name, creator, participants, recentMessageCount);
+    }
+
+    private RoomResponse buildRoomResponse(
+            Room room,
+            String name,
+            User creator,
+            List<User> participants,
+            int recentMessageCount) {
         return RoomResponse.builder()
             .id(room.getId())
             .name(room.getName() != null ? room.getName() : "제목 없음")
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/RecentMessageCounterIntegrationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/RecentMessageCounterIntegrationTest.java
new file mode 100644
index 00000000..466a6964
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/RecentMessageCounterIntegrationTest.java
@@ -0,0 +1,66 @@
+package com.ktb.chatapp.service;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.ktb.chatapp.config.MongoTestContainer;
+import com.ktb.chatapp.config.RedisTestContainer;
+import com.ktb.chatapp.model.Message;
+import java.time.LocalDateTime;
+import java.util.List;
+import java.util.Map;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Import;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.test.context.TestPropertySource;
+
+@SpringBootTest
+@Import({MongoTestContainer.class, RedisTestContainer.class})
+@TestPropertySource(properties = "socketio.enabled=false")
+class RecentMessageCounterIntegrationTest {
+
+    private static final List<String> ROOM_IDS = List.of("batch-room-1", "batch-room-2");
+
+    @Autowired private RecentMessageCounter recentMessageCounter;
+    @Autowired private MongoTemplate mongoTemplate;
+
+    @AfterEach
+    void tearDown() {
+        mongoTemplate.remove(
+            Query.query(Criteria.where("room").in(ROOM_IDS)),
+            Message.class
+        );
+    }
+
+    @Test
+    void countRecentMessages_groupsRoomsAndExcludesBeforeBoundary() {
+        LocalDateTime since = LocalDateTime.of(2026, 8, 10, 12, 0);
+        mongoTemplate.insert(List.of(
+            message("batch-1", "batch-room-1", since.minusNanos(1)),
+            message("batch-2", "batch-room-1", since),
+            message("batch-3", "batch-room-1", since.plusMinutes(1)),
+            message("batch-4", "batch-room-2", since.plusMinutes(2))
+        ), Message.class);
+
+        Map<String, Integer> counts =
+            recentMessageCounter.countRecentMessages(ROOM_IDS, since);
+
+        assertThat(counts).containsExactlyInAnyOrderEntriesOf(Map.of(
+            "batch-room-1", 2,
+            "batch-room-2", 1
+        ));
+    }
+
+    private static Message message(String id, String roomId, LocalDateTime timestamp) {
+        return Message.builder()
+            .id(id)
+            .roomId(roomId)
+            .content("message")
+            .timestamp(timestamp)
+            .build();
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
new file mode 100644
index 00000000..05a697c7
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
@@ -0,0 +1,131 @@
+package com.ktb.chatapp.service;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.ArgumentMatchers.anySet;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.ktb.chatapp.dto.RoomResponse;
+import com.ktb.chatapp.dto.RoomsResponse;
+import com.ktb.chatapp.model.Room;
+import com.ktb.chatapp.model.User;
+import com.ktb.chatapp.repository.RoomRepository;
+import com.ktb.chatapp.repository.UserRepository;
+import java.time.LocalDateTime;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.security.crypto.password.PasswordEncoder;
+
+@ExtendWith(MockitoExtension.class)
+class RoomServiceTest {
+
+    @Mock private RoomRepository roomRepository;
+    @Mock private UserRepository userRepository;
+    @Mock private RecentMessageCounter recentMessageCounter;
+    @Mock private PasswordEncoder passwordEncoder;
+    @Mock private ApplicationEventPublisher eventPublisher;
+
+    private RoomService roomService;
+
+    @BeforeEach
+    void setUp() {
+        roomService = new RoomService(
+            roomRepository,
+            userRepository,
+            recentMessageCounter,
+            passwordEncoder,
+            eventPublisher
+        );
+    }
+
+    @Test
+    void getAllRooms_emptyList_avoidsBatchLookups() {
+        when(roomRepository.findAll()).thenReturn(List.of());
+
+        RoomsResponse response = roomService.getAllRooms("viewer@test.com");
+
+        assertTrue(response.isSuccess());
+        assertTrue(response.getData().isEmpty());
+        verify(userRepository, never()).findAllById(anySet());
+        verify(recentMessageCounter, never()).countRecentMessages(anySet());
+    }
+
+    @Test
+    void getAllRooms_batchesUsersAndMessageCountsOnceAndKeepsContract() throws Exception {
+        LocalDateTime older = LocalDateTime.of(2026, 8, 10, 10, 0);
+        LocalDateTime newer = older.plusMinutes(1);
+        Room oldRoom = room("room-old", "Old", "creator-1", older,
+            Set.of("creator-1", "participant-1", "missing-user"));
+        Room newRoom = room("room-new", null, "missing-creator", newer,
+            Set.of("participant-1"));
+        User creator = user("creator-1", "Creator", "creator@test.com");
+        User participant = user("participant-1", null, null);
+
+        when(roomRepository.findAll()).thenReturn(List.of(oldRoom, newRoom));
+        when(userRepository.findAllById(anySet())).thenReturn(List.of(creator, participant));
+        when(recentMessageCounter.countRecentMessages(anySet()))
+            .thenReturn(Map.of("room-old", 7));
+
+        RoomsResponse response = roomService.getAllRooms("viewer@test.com");
+
+        assertTrue(response.isSuccess());
+        assertEquals(List.of("room-new", "room-old"), response.getData().stream()
+            .map(RoomResponse::getId).toList());
+        RoomResponse first = response.getData().get(0);
+        RoomResponse second = response.getData().get(1);
+        assertEquals("제목 없음", first.getName());
+        assertNull(first.getCreator());
+        assertEquals(0, first.getRecentMessageCount());
+        assertEquals(1, first.getParticipantsCount());
+        assertEquals("알 수 없음", first.getParticipants().get(0).getName());
+        assertEquals("", first.getParticipants().get(0).getEmail());
+        assertEquals(7, second.getRecentMessageCount());
+        assertEquals(2, second.getParticipantsCount());
+        verify(userRepository).findAllById(anySet());
+        verify(recentMessageCounter).countRecentMessages(anySet());
+
+        JsonNode json = new ObjectMapper().findAndRegisterModules().valueToTree(second);
+        assertTrue(json.has("_id"));
+        assertTrue(json.has("name"));
+        assertTrue(json.has("hasPassword"));
+        assertTrue(json.has("creator"));
+        assertTrue(json.has("participants"));
+        assertTrue(json.has("participantsCount"));
+        assertTrue(json.has("createdAt"));
+        assertTrue(json.has("recentMessageCount"));
+        assertFalse(json.has("isCreator"));
+        assertFalse(json.has("createdAtDateTime"));
+    }
+
+    private static Room room(
+            String id,
+            String name,
+            String creator,
+            LocalDateTime createdAt,
+            Set<String> participants) {
+        return Room.builder()
+            .id(id)
+            .name(name)
+            .creator(creator)
+            .createdAt(createdAt)
+            .participantIds(participants)
+            .build();
+    }
+
+    private static User user(String id, String name, String email) {
+        return User.builder().id(id).name(name).email(email).build();
+    }
+}

From 46d9df878e824dbfd7988fe4ad4f1d7eb7c70559 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 11:17:16 +0900
Subject: [PATCH 05/32] perf(observe): add API bottleneck diagnostics

---
 .../dashboards/ktb-chat-monitoring.json       |  70 +++++
 .../monitoring/prometheus/prometheus.dev.yml  |  17 +-
 .../config/MongoPoolMetricsConfig.java        |  21 ++
 e2e/artillery/README.md                       |  28 +-
 e2e/artillery/aggregate-observations.js       |  65 +++++
 e2e/artillery/all-scenarios.js                |  18 +-
 e2e/artillery/observation.js                  | 229 +++++++++++++++
 e2e/artillery/observation.test.js             |  31 ++
 e2e/artillery/report-observations.js          |  33 +++
 ...i-priority-20260811-profiler-evidence.json |  69 +++++
 e2e/artillery/scenarios/auth.scenario.js      |  18 +-
 e2e/artillery/scenarios/chat.scenario.js      |  83 +++---
 e2e/artillery/scenarios/profile.scenario.js   |  25 +-
 loadtest/LOAD_TEST_PLAN.md                    | 273 ++++++++++++++++++
 loadtest/load-test.js                         |  38 +++
 performance/room-join/README.md               |  15 +
 .../room-join/results/room-join-before-1.json |  16 +
 .../results/room-join-before-observed-1.json  |  16 +
 .../results/room-join-pool-smoke.json         |  16 +
 performance/room-join/room-join.js            |  91 ++++++
 20 files changed, 1097 insertions(+), 75 deletions(-)
 create mode 100644 apps/backend/monitoring/grafana/provisioning/dashboards/ktb-chat-monitoring.json
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/config/MongoPoolMetricsConfig.java
 create mode 100644 e2e/artillery/aggregate-observations.js
 create mode 100644 e2e/artillery/observation.js
 create mode 100644 e2e/artillery/observation.test.js
 create mode 100644 e2e/artillery/report-observations.js
 create mode 100644 e2e/artillery/results/e2e-api-priority-20260811-profiler-evidence.json
 create mode 100644 loadtest/LOAD_TEST_PLAN.md
 create mode 100644 performance/room-join/README.md
 create mode 100644 performance/room-join/results/room-join-before-1.json
 create mode 100644 performance/room-join/results/room-join-before-observed-1.json
 create mode 100644 performance/room-join/results/room-join-pool-smoke.json
 create mode 100644 performance/room-join/room-join.js

diff --git a/apps/backend/monitoring/grafana/provisioning/dashboards/ktb-chat-monitoring.json b/apps/backend/monitoring/grafana/provisioning/dashboards/ktb-chat-monitoring.json
new file mode 100644
index 00000000..646f347f
--- /dev/null
+++ b/apps/backend/monitoring/grafana/provisioning/dashboards/ktb-chat-monitoring.json
@@ -0,0 +1,70 @@
+{
+  "uid": "ktb-chat-monitoring",
+  "title": "KTB Chat Monitoring",
+  "tags": ["ktb", "loadtest"],
+  "timezone": "browser",
+  "schemaVersion": 39,
+  "version": 1,
+  "refresh": "5s",
+  "time": { "from": "now-15m", "to": "now" },
+  "templating": { "list": [] },
+  "panels": [
+    {
+      "id": 1,
+      "type": "stat",
+      "title": "Monitored targets",
+      "gridPos": { "h": 4, "w": 6, "x": 0, "y": 0 },
+      "targets": [{ "expr": "count(up{job!=\"cadvisor\"})", "legendFormat": "targets", "refId": "A" }],
+      "fieldConfig": { "defaults": { "unit": "none", "color": { "mode": "thresholds" }, "thresholds": { "mode": "absolute", "steps": [{ "color": "red", "value": null }, { "color": "green", "value": 1 }] } }, "overrides": [] },
+      "options": { "reduceOptions": { "calcs": ["lastNotNull"], "fields": "", "values": false }, "orientation": "auto", "textMode": "auto", "colorMode": "value" }
+    },
+    {
+      "id": 2,
+      "type": "timeseries",
+      "title": "HTTP requests / sec",
+      "gridPos": { "h": 8, "w": 18, "x": 6, "y": 0 },
+      "targets": [{ "expr": "sum by (status) (rate(http_server_requests_seconds_count{application=\"ktb-chat-backend\"}[1m]))", "legendFormat": "status {{status}}", "refId": "A" }],
+      "fieldConfig": { "defaults": { "unit": "reqps", "color": { "mode": "palette-classic" } }, "overrides": [] },
+      "options": { "legend": { "displayMode": "list", "placement": "bottom" }, "tooltip": { "mode": "multi" } }
+    },
+    {
+      "id": 3,
+      "type": "timeseries",
+      "title": "HTTP average latency",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 8 },
+      "targets": [{ "expr": "sum(rate(http_server_requests_seconds_sum{application=\"ktb-chat-backend\"}[1m])) / sum(rate(http_server_requests_seconds_count{application=\"ktb-chat-backend\"}[1m]))", "legendFormat": "average", "refId": "A" }],
+      "fieldConfig": { "defaults": { "unit": "s" }, "overrides": [] },
+      "options": { "legend": { "displayMode": "list", "placement": "bottom" }, "tooltip": { "mode": "single" } }
+    },
+    {
+      "id": 4,
+      "type": "timeseries",
+      "title": "JVM heap used",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 8 },
+      "targets": [{ "expr": "sum(jvm_memory_used_bytes{application=\"ktb-chat-backend\",area=\"heap\"})", "legendFormat": "heap", "refId": "A" }],
+      "fieldConfig": { "defaults": { "unit": "bytes" }, "overrides": [] },
+      "options": { "legend": { "displayMode": "list", "placement": "bottom" }, "tooltip": { "mode": "single" } }
+    },
+    {
+      "id": 5,
+      "type": "timeseries",
+      "title": "JVM CPU",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 16 },
+      "targets": [{ "expr": "process_cpu_usage{application=\"ktb-chat-backend\"}", "legendFormat": "process", "refId": "A" }],
+      "fieldConfig": { "defaults": { "unit": "percentunit", "min": 0, "max": 1 }, "overrides": [] },
+      "options": { "legend": { "displayMode": "list", "placement": "bottom" }, "tooltip": { "mode": "single" } }
+    },
+    {
+      "id": 6,
+      "type": "timeseries",
+      "title": "MongoDB and Redis exporter up",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 16 },
+      "targets": [
+        { "expr": "up{job=\"mongodb\"}", "legendFormat": "mongodb", "refId": "A" },
+        { "expr": "up{job=\"redis\"}", "legendFormat": "redis", "refId": "B" }
+      ],
+      "fieldConfig": { "defaults": { "unit": "none", "min": 0, "max": 1 }, "overrides": [] },
+      "options": { "legend": { "displayMode": "list", "placement": "bottom" }, "tooltip": { "mode": "multi" } }
+    }
+  ]
+}
diff --git a/apps/backend/monitoring/prometheus/prometheus.dev.yml b/apps/backend/monitoring/prometheus/prometheus.dev.yml
index 39f13f1b..efd2dfab 100644
--- a/apps/backend/monitoring/prometheus/prometheus.dev.yml
+++ b/apps/backend/monitoring/prometheus/prometheus.dev.yml
@@ -13,7 +13,8 @@ scrape_configs:
 
   - job_name: 'spring-boot-app'
     metrics_path: '/actuator/prometheus'
-    scrape_interval: 5s
+    # Performance diagnostics need to see short-lived pool saturation spikes.
+    scrape_interval: 1s
     static_configs:
       - targets: ['host.docker.internal:5001']
         labels:
@@ -31,17 +32,3 @@ scrape_configs:
       - targets: ['redis-exporter:9121']
         labels:
           environment: 'dev'
-
-  - job_name: 'cadvisor'
-    static_configs:
-      - targets: ['cadvisor:8080']
-        labels:
-          environment: 'dev'
-    metric_relabel_configs:
-      # Extract container name from container_label_com_docker_compose_service
-      - source_labels: [container_label_com_docker_compose_service]
-        target_label: container_name
-      # Extract container name from id (fallback)
-      - source_labels: [id]
-        regex: '/docker/([a-f0-9]+)'
-        target_label: container_id
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/MongoPoolMetricsConfig.java b/apps/backend/src/main/java/com/ktb/chatapp/config/MongoPoolMetricsConfig.java
new file mode 100644
index 00000000..8765954c
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/MongoPoolMetricsConfig.java
@@ -0,0 +1,21 @@
+package com.ktb.chatapp.config;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.binder.mongodb.DefaultMongoConnectionPoolTagsProvider;
+import io.micrometer.core.instrument.binder.mongodb.MongoMetricsConnectionPoolListener;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Supplies Micrometer's standard MongoDB pool listener. Spring Boot's Mongo metrics
+ * auto-configuration attaches this bean to the auto-configured MongoClient.
+ */
+@Configuration(proxyBeanMethods = false)
+public class MongoPoolMetricsConfig {
+
+    @Bean
+    MongoMetricsConnectionPoolListener mongoMetricsConnectionPoolListener(MeterRegistry registry) {
+        return new MongoMetricsConnectionPoolListener(
+                registry, new DefaultMongoConnectionPoolTagsProvider());
+    }
+}
diff --git a/e2e/artillery/README.md b/e2e/artillery/README.md
index b2bf11b1..230102a3 100644
--- a/e2e/artillery/README.md
+++ b/e2e/artillery/README.md
@@ -18,6 +18,32 @@
   PHASE1_ARRIVAL_COUNT=10 PHASE1_DURATION=30 make artillery
   ```
 
+## Stage 0 관측 실행
+
+브라우저 흐름을 부하 테스트로 올리기 전에 1 VU로 실제 호출 경로를 진단합니다. 이 실행은
+사용자 행동을 바꾸지 않고 HTTP API, Socket.IO 요청-완료 쌍, 화면 행동 완료시간을 각각
+수집합니다.
+
+```bash
+BASE_URL=http://localhost:3000 \
+PHASE1_ARRIVAL_COUNT=1 PHASE1_DURATION=5 \
+OBSERVATION_RUN_ID=stage0-run1 \
+pnpm --filter e2e exec artillery run artillery/artillery-config.yaml
+
+node e2e/artillery/report-observations.js \
+  e2e/artillery/results/stage0-run1
+```
+
+각 VU의 원시 JSON과 합산 `summary.json`이 지정한 실행 디렉터리에 저장됩니다. 표는
+호출 수, 성공/실패, 평균, p95, p99, 누적시간 및 같은 계층 내 누적시간 비중을 보여 줍니다.
+HTTP의 4xx/5xx는 실패로 집계되므로 의도된 로그인 실패 시나리오의 401은 행동 성공 여부와
+별도로 해석해야 합니다. Socket 표에는 연결, 방 입장, 과거 메시지 조회, 메시지 전송 완료가
+서로 섞이지 않습니다. 해당 실행이 `fetchPreviousMessages`를 보내지 않았다면 과거 메시지 행은
+생성되지 않습니다.
+
+`PHASE1_ARRIVAL_COUNT`는 지속 동시 VU가 아니라 phase 동안 생성할 총 사용자 수입니다.
+Stage 0을 3회 완료하고 Mongo profiler/실행계획 근거까지 확보하기 전에는 VU를 올리지 않습니다.
+
 ## 환경 변수
 
   Artillery 실행 시 다음 환경 변수로 커스터마이징할 수 있습니다:
@@ -215,4 +241,4 @@
 
   - https://www.artillery.io/docs
   - https://playwright.dev/docs/intro
-  - https://www.artillery.io/docs/reference/engines/playwright
\ No newline at end of file
+  - https://www.artillery.io/docs/reference/engines/playwright
diff --git a/e2e/artillery/aggregate-observations.js b/e2e/artillery/aggregate-observations.js
new file mode 100644
index 00000000..3188f00a
--- /dev/null
+++ b/e2e/artillery/aggregate-observations.js
@@ -0,0 +1,65 @@
+#!/usr/bin/env node
+const fs = require('fs');
+const path = require('path');
+const { summarizeSamples } = require('./observation');
+
+const runDirectories = process.argv.slice(2).map((value) => path.resolve(value));
+if (runDirectories.length === 0) {
+    throw new Error('Usage: node aggregate-observations.js <run-directory>...');
+}
+
+const reports = runDirectories.flatMap((runDirectory) => fs.readdirSync(runDirectory)
+    .filter((file) => file.startsWith('vu-') && file.endsWith('.json'))
+    .map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8'))));
+
+const samples = { actions: [], http: [], socket: [] };
+for (const report of reports) {
+    samples.actions.push(...report.samples.actions);
+    samples.http.push(...report.samples.http);
+    samples.socket.push(...report.samples.socket);
+}
+
+const timestamps = Object.values(samples).flat()
+    .flatMap((sample) => [sample.startedAt, sample.endedAt])
+    .filter(Boolean)
+    .sort();
+const rankedHttp = summarizeSamples(samples.http.filter((sample) =>
+    sample.name !== 'GET /api/health' &&
+    !(sample.name === 'POST /api/auth/login' && sample.status === 401)));
+
+const aggregate = {
+    schemaVersion: 1,
+    generatedAt: new Date().toISOString(),
+    runIds: reports.map((report) => report.runId),
+    virtualUsers: reports.length,
+    startedAt: timestamps.at(0),
+    endedAt: timestamps.at(-1),
+    validity: {
+        completed: reports.filter((report) => report.samples.actions.length > 0 &&
+            report.samples.actions.every((sample) => sample.success)).length,
+        failed: reports.filter((report) => report.samples.actions.some((sample) => !sample.success)).length,
+        allSamplesHaveWindowsAndNames: Object.values(samples).flat().every((sample) =>
+            Boolean(sample.name && sample.startedAt && sample.endedAt)),
+    },
+    summary: {
+        actions: summarizeSamples(samples.actions),
+        httpDiagnostic: summarizeSamples(samples.http),
+        httpRanked: rankedHttp,
+        socket: summarizeSamples(samples.socket),
+    },
+    samples,
+};
+
+const outputPath = process.env.OBSERVATION_AGGREGATE_PATH
+    ? path.resolve(process.env.OBSERVATION_AGGREGATE_PATH)
+    : path.join(path.dirname(runDirectories[0]), 'aggregate.json');
+fs.writeFileSync(outputPath, `${JSON.stringify(aggregate, null, 2)}\n`);
+console.log(outputPath);
+console.log(JSON.stringify({
+    runIds: aggregate.runIds,
+    startedAt: aggregate.startedAt,
+    endedAt: aggregate.endedAt,
+    validity: aggregate.validity,
+    httpRanked: aggregate.summary.httpRanked,
+    socket: aggregate.summary.socket,
+}, null, 2));
diff --git a/e2e/artillery/all-scenarios.js b/e2e/artillery/all-scenarios.js
index c0c77e26..961c8b3e 100644
--- a/e2e/artillery/all-scenarios.js
+++ b/e2e/artillery/all-scenarios.js
@@ -8,6 +8,7 @@ const {
 const {
     fullProfileUpdateScenario,
 } = require('./scenarios/profile.scenario.js');
+const { createObservation } = require('./observation.js');
 
 function generateUserSchema() {
     const timestamp = Date.now();
@@ -53,14 +54,19 @@ const allScenariosFlat = [
 async function allScenarios(page, vuContext) {
     const testUser = generateUserSchema();
     vuContext.vars.testUser = testUser;
+    vuContext.vars.observation = createObservation(page, vuContext);
 
-    for (const scenario of allScenariosFlat) {
-        try {
-            await scenario(page, vuContext);
-        } catch (err) {
-            err.message = `[${scenario.name}] ${err.message}`;
-            throw err;
+    try {
+        for (const scenario of allScenariosFlat) {
+            try {
+                await scenario(page, vuContext);
+            } catch (err) {
+                err.message = `[${scenario.name}] ${err.message}`;
+                throw err;
+            }
         }
+    } finally {
+        await vuContext.vars.observation.finish();
     }
 }
 
diff --git a/e2e/artillery/observation.js b/e2e/artillery/observation.js
new file mode 100644
index 00000000..de15b9a3
--- /dev/null
+++ b/e2e/artillery/observation.js
@@ -0,0 +1,229 @@
+const fs = require('fs');
+const path = require('path');
+const { randomUUID } = require('crypto');
+
+const OBJECT_ID = /^[a-f\d]{24}$/i;
+const UUID = /^[a-f\d]{8}-[a-f\d]{4}-[1-5][a-f\d]{3}-[89ab][a-f\d]{3}-[a-f\d]{12}$/i;
+const INTEGER = /^\d+$/;
+
+const SOCKET_PAIRS = {
+    joinRoom: { response: 'joinRoomSuccess', metric: 'room_join' },
+    fetchPreviousMessages: { response: 'previousMessagesLoaded', metric: 'message_history' },
+    chatMessage: { response: 'message', metric: 'message_send' },
+};
+
+function percentile(values, ratio) {
+    if (values.length === 0) return 0;
+    const sorted = [...values].sort((a, b) => a - b);
+    return sorted[Math.ceil(ratio * sorted.length) - 1];
+}
+
+function normalizeApiPath(rawUrl) {
+    const url = new URL(rawUrl, 'http://localhost');
+    const parts = url.pathname.split('/').filter(Boolean);
+    if (parts[0] !== 'api') return null;
+
+    return `/${parts.map((part, index) => {
+        const previous = parts[index - 1];
+        if ((previous === 'view' || previous === 'profiles') && parts[index - 2] === 'files') {
+            return '{filename}';
+        }
+        if (OBJECT_ID.test(part) || UUID.test(part) || INTEGER.test(part)) {
+            if (previous === 'rooms') return '{roomId}';
+            if (previous === 'users') return '{userId}';
+            if (previous === 'messages') return '{messageId}';
+            return '{id}';
+        }
+        return part;
+    }).join('/')}`;
+}
+
+function parseSocketEvent(payload) {
+    const text = Buffer.isBuffer(payload) ? payload.toString('utf8') : String(payload);
+    const arrayStart = text.indexOf('[');
+    if (arrayStart === -1 || !/^4\d/.test(text)) return null;
+    try {
+        const packet = JSON.parse(text.slice(arrayStart));
+        return Array.isArray(packet) && typeof packet[0] === 'string' ? packet[0] : null;
+    } catch {
+        return null;
+    }
+}
+
+function summarizeSamples(samples) {
+    const groups = new Map();
+    for (const sample of samples) {
+        const key = sample.name;
+        const group = groups.get(key) || { name: key, count: 0, success: 0, failure: 0, durations: [] };
+        group.count += 1;
+        group[sample.success === false ? 'failure' : 'success'] += 1;
+        group.durations.push(sample.durationMs);
+        groups.set(key, group);
+    }
+    const grandTotalMs = samples.reduce((sum, sample) => sum + sample.durationMs, 0);
+    return [...groups.values()].map((group) => {
+        const totalDurationMs = group.durations.reduce((sum, value) => sum + value, 0);
+        return {
+            name: group.name,
+            count: group.count,
+            success: group.success,
+            failure: group.failure,
+            averageMs: Number((totalDurationMs / group.count).toFixed(1)),
+            p95Ms: Number(percentile(group.durations, 0.95).toFixed(1)),
+            p99Ms: Number(percentile(group.durations, 0.99).toFixed(1)),
+            totalDurationMs: Number(totalDurationMs.toFixed(1)),
+            contributionPct: grandTotalMs === 0 ? 0 : Number(((totalDurationMs / grandTotalMs) * 100).toFixed(1)),
+        };
+    }).sort((a, b) => b.totalDurationMs - a.totalDurationMs);
+}
+
+function createObservation(page, vuContext) {
+    const startedRequests = new Map();
+    const pendingSocketEvents = new Map();
+    const samples = { http: [], socket: [], actions: [] };
+    let currentAction = null;
+    let socketSequence = 0;
+
+    const recordHttp = async (request, success) => {
+        const started = startedRequests.get(request);
+        startedRequests.delete(request);
+        if (!started) return;
+        const response = await request.response().catch(() => null);
+        samples.http.push({
+            name: `${request.method()} ${started.path}`,
+            action: started.action,
+            durationMs: performance.now() - started.at,
+            startedAt: started.wallStartedAt,
+            endedAt: new Date().toISOString(),
+            success: success && response !== null && response.status() < 400,
+            status: response?.status() ?? null,
+        });
+    };
+
+    const onRequest = (request) => {
+        const apiPath = normalizeApiPath(request.url());
+        if (apiPath) startedRequests.set(request, {
+            at: performance.now(), wallStartedAt: new Date().toISOString(), path: apiPath, action: currentAction,
+        });
+    };
+    const onRequestFinished = (request) => { void recordHttp(request, true); };
+    const onRequestFailed = (request) => { void recordHttp(request, false); };
+    const onWebSocket = (webSocket) => {
+        const connectionKey = `connection:${socketSequence += 1}`;
+        pendingSocketEvents.set(connectionKey, {
+            at: performance.now(), wallStartedAt: new Date().toISOString(), action: currentAction,
+        });
+
+        webSocket.on('framesent', ({ payload }) => {
+            const event = parseSocketEvent(payload);
+            const pair = SOCKET_PAIRS[event];
+            if (!pair) return;
+            const queue = pendingSocketEvents.get(pair.response) || [];
+            queue.push({
+                at: performance.now(), wallStartedAt: new Date().toISOString(), action: currentAction, metric: pair.metric,
+            });
+            pendingSocketEvents.set(pair.response, queue);
+        });
+        webSocket.on('framereceived', ({ payload }) => {
+            const text = Buffer.isBuffer(payload) ? payload.toString('utf8') : String(payload);
+            if (text.startsWith('40')) {
+                const pending = pendingSocketEvents.get(connectionKey);
+                if (pending) {
+                    samples.socket.push({
+                        name: 'connection', action: pending.action, durationMs: performance.now() - pending.at,
+                        startedAt: pending.wallStartedAt, endedAt: new Date().toISOString(), success: true,
+                    });
+                    pendingSocketEvents.delete(connectionKey);
+                }
+            }
+            const event = parseSocketEvent(payload);
+            const queue = pendingSocketEvents.get(event);
+            const pending = Array.isArray(queue) ? queue.shift() : null;
+            if (pending) {
+                samples.socket.push({
+                    name: pending.metric, action: pending.action, durationMs: performance.now() - pending.at,
+                    startedAt: pending.wallStartedAt, endedAt: new Date().toISOString(), success: true,
+                });
+            }
+        });
+    };
+
+    page.on('request', onRequest);
+    page.on('requestfinished', onRequestFinished);
+    page.on('requestfailed', onRequestFailed);
+    page.on('websocket', onWebSocket);
+
+    async function action(name, callback) {
+        const previousAction = currentAction;
+        currentAction = name;
+        const startedAt = performance.now();
+        const wallStartedAt = new Date().toISOString();
+        try {
+            const result = await callback();
+            samples.actions.push({
+                name, durationMs: performance.now() - startedAt, startedAt: wallStartedAt,
+                endedAt: new Date().toISOString(), success: true,
+            });
+            return result;
+        } catch (error) {
+            samples.actions.push({
+                name, durationMs: performance.now() - startedAt, startedAt: wallStartedAt,
+                endedAt: new Date().toISOString(), success: false,
+            });
+            throw error;
+        } finally {
+            currentAction = previousAction;
+        }
+    }
+
+    async function finish() {
+        await new Promise((resolve) => setImmediate(resolve));
+        page.off('request', onRequest);
+        page.off('requestfinished', onRequestFinished);
+        page.off('requestfailed', onRequestFailed);
+        page.off('websocket', onWebSocket);
+
+        const runId = process.env.OBSERVATION_RUN_ID || new Date().toISOString().replace(/[:.]/g, '-');
+        const outputRoot = process.env.OBSERVATION_OUTPUT_DIR || path.resolve(__dirname, 'results');
+        const runDirectory = path.join(outputRoot, runId);
+        fs.mkdirSync(runDirectory, { recursive: true });
+        const report = {
+            schemaVersion: 1,
+            runId,
+            vuId: vuContext?._uid || vuContext?.vars?.$uuid || randomUUID(),
+            generatedAt: new Date().toISOString(),
+            samples,
+            summary: {
+                actions: summarizeSamples(samples.actions),
+                http: summarizeSamples(samples.http),
+                socket: summarizeSamples(samples.socket),
+            },
+        };
+        const filename = `vu-${process.pid}-${randomUUID()}.json`;
+        fs.writeFileSync(path.join(runDirectory, filename), `${JSON.stringify(report, null, 2)}\n`);
+        printSummary(report.summary);
+        return report;
+    }
+
+    return { action, finish };
+}
+
+function printTable(title, rows) {
+    console.log(`\n[E2E observation] ${title}`);
+    console.table(rows.map(({ name, count, success, failure, averageMs, p95Ms, p99Ms, totalDurationMs, contributionPct }) => ({
+        name, count, success, failure, averageMs, p95Ms, p99Ms, totalDurationMs, contributionPct,
+    })));
+}
+
+function printSummary(summary) {
+    printTable('User actions', summary.actions);
+    printTable('HTTP API (ranked by cumulative response time)', summary.http);
+    printTable('Socket.IO actions', summary.socket);
+}
+
+module.exports = {
+    createObservation,
+    normalizeApiPath,
+    parseSocketEvent,
+    summarizeSamples,
+};
diff --git a/e2e/artillery/observation.test.js b/e2e/artillery/observation.test.js
new file mode 100644
index 00000000..320ecdd9
--- /dev/null
+++ b/e2e/artillery/observation.test.js
@@ -0,0 +1,31 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { normalizeApiPath, parseSocketEvent, summarizeSamples } = require('./observation');
+
+test('normalizes dynamic API identifiers without changing static routes', () => {
+    assert.equal(normalizeApiPath('http://localhost:5001/api/rooms'), '/api/rooms');
+    assert.equal(normalizeApiPath('http://localhost:5001/api/rooms/507f1f77bcf86cd799439011?x=1'), '/api/rooms/{roomId}');
+    assert.equal(normalizeApiPath('http://localhost:5001/api/rooms/507f1f77bcf86cd799439011/join'), '/api/rooms/{roomId}/join');
+    assert.equal(normalizeApiPath('http://localhost:5001/api/files/view/123_random.jpg'), '/api/files/view/{filename}');
+    assert.equal(normalizeApiPath('http://localhost:3000/_next/static/a.js'), null);
+});
+
+test('extracts Socket.IO event names from websocket frames', () => {
+    assert.equal(parseSocketEvent('42["joinRoom","room-1"]'), 'joinRoom');
+    assert.equal(parseSocketEvent('451-["message",{"content":"hello"}]'), 'message');
+    assert.equal(parseSocketEvent('40{"sid":"abc"}'), null);
+    assert.equal(parseSocketEvent('2'), null);
+});
+
+test('summarizes and ranks samples by cumulative duration', () => {
+    const result = summarizeSamples([
+        { name: 'fast-frequent', durationMs: 20, success: true },
+        { name: 'fast-frequent', durationMs: 20, success: false },
+        { name: 'slow-once', durationMs: 30, success: true },
+    ]);
+    assert.equal(result[0].name, 'fast-frequent');
+    assert.deepEqual(result[0], {
+        name: 'fast-frequent', count: 2, success: 1, failure: 1,
+        averageMs: 20, p95Ms: 20, p99Ms: 20, totalDurationMs: 40, contributionPct: 57.1,
+    });
+});
diff --git a/e2e/artillery/report-observations.js b/e2e/artillery/report-observations.js
new file mode 100644
index 00000000..f3c1b652
--- /dev/null
+++ b/e2e/artillery/report-observations.js
@@ -0,0 +1,33 @@
+#!/usr/bin/env node
+const fs = require('fs');
+const path = require('path');
+const { summarizeSamples } = require('./observation');
+
+const runDirectory = path.resolve(process.argv[2] || 'artillery/results');
+const files = fs.readdirSync(runDirectory).filter((file) => file.startsWith('vu-') && file.endsWith('.json'));
+if (files.length === 0) throw new Error(`No observation JSON files found in ${runDirectory}`);
+
+const reports = files.map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8')));
+const samples = { actions: [], http: [], socket: [] };
+for (const report of reports) {
+    samples.actions.push(...report.samples.actions);
+    samples.http.push(...report.samples.http);
+    samples.socket.push(...report.samples.socket);
+}
+
+const aggregate = {
+    schemaVersion: 1,
+    runId: reports[0].runId,
+    generatedAt: new Date().toISOString(),
+    virtualUsers: reports.length,
+    summary: {
+        actions: summarizeSamples(samples.actions),
+        http: summarizeSamples(samples.http),
+        socket: summarizeSamples(samples.socket),
+    },
+};
+fs.writeFileSync(path.join(runDirectory, 'summary.json'), `${JSON.stringify(aggregate, null, 2)}\n`);
+for (const [name, rows] of Object.entries(aggregate.summary)) {
+    console.log(`\n[E2E observation aggregate] ${name}`);
+    console.table(rows);
+}
diff --git a/e2e/artillery/results/e2e-api-priority-20260811-profiler-evidence.json b/e2e/artillery/results/e2e-api-priority-20260811-profiler-evidence.json
new file mode 100644
index 00000000..299eeda7
--- /dev/null
+++ b/e2e/artillery/results/e2e-api-priority-20260811-profiler-evidence.json
@@ -0,0 +1,69 @@
+{
+  "schemaVersion": 1,
+  "sourceRunId": "e2e-api-priority-20260811-profiler-confirm",
+  "purpose": "Profiler confirmation for the HTTP cumulative-time winner; excluded from the three-run HTTP ranking",
+  "profilingLevel": 2,
+  "profileCollectionMaxBytes": 67108864,
+  "target": "POST /api/rooms/{roomId}/join",
+  "windows": [
+    {
+      "action": "room_create",
+      "startedAt": "2026-08-11T01:56:08.266Z",
+      "endedAt": "2026-08-11T01:56:08.316Z",
+      "mongoCommandCount": 10,
+      "messageCollscanCount": 2,
+      "messageDocsExamined": 16222,
+      "sessionCollscanCount": 1,
+      "sessionDocsExamined": 536,
+      "userEmailCollscanCount": 1,
+      "userEmailDocsExamined": 566,
+      "userIdLookupCount": 4
+    },
+    {
+      "action": "room_join",
+      "startedAt": "2026-08-11T01:56:10.188Z",
+      "endedAt": "2026-08-11T01:56:10.444Z",
+      "mongoCommandCount": 11,
+      "messageCollscanCount": 2,
+      "messageDocsExamined": 16228,
+      "sessionCollscanCount": 1,
+      "sessionDocsExamined": 536,
+      "userEmailCollscanCount": 1,
+      "userEmailDocsExamined": 566,
+      "userIdLookupCount": 4
+    },
+    {
+      "action": "room_join_for_file",
+      "startedAt": "2026-08-11T01:56:15.043Z",
+      "endedAt": "2026-08-11T01:56:15.280Z",
+      "mongoCommandCount": 11,
+      "messageCollscanCount": 2,
+      "messageDocsExamined": 16252,
+      "sessionCollscanCount": 1,
+      "sessionDocsExamined": 536,
+      "userEmailCollscanCount": 1,
+      "userEmailDocsExamined": 566,
+      "userIdLookupCount": 4
+    },
+    {
+      "action": "room_join_for_forbidden_word",
+      "startedAt": "2026-08-11T01:56:17.725Z",
+      "endedAt": "2026-08-11T01:56:17.965Z",
+      "mongoCommandCount": 11,
+      "messageCollscanCount": 2,
+      "messageDocsExamined": 16258,
+      "sessionCollscanCount": 1,
+      "sessionDocsExamined": 536,
+      "userEmailCollscanCount": 1,
+      "userEmailDocsExamined": 566,
+      "userIdLookupCount": 4
+    }
+  ],
+  "codeCorrelation": {
+    "duplicateResponseMapping": [
+      "RoomService.joinRoom maps the joined room for RoomUpdatedEvent",
+      "RoomController.joinRoom maps the same room again for the HTTP response"
+    ],
+    "effectPerRequest": "Two recent-message counts and repeated creator/participant lookups"
+  }
+}
diff --git a/e2e/artillery/scenarios/auth.scenario.js b/e2e/artillery/scenarios/auth.scenario.js
index 5b51a288..577eb006 100644
--- a/e2e/artillery/scenarios/auth.scenario.js
+++ b/e2e/artillery/scenarios/auth.scenario.js
@@ -13,22 +13,23 @@ const ACTION_TIMEOUT_SHORT = parseInt(process.env.ACTION_TIMEOUT_SHORT || '500',
  */
 async function loginScenario(page, vuContext) {
     let testUser = vuContext.vars.testUser;
+    const observe = vuContext.vars.observation.action;
 
     try {
         // 1. 회원가입
-        await registerAction(page, testUser);
+        await observe('register', () => registerAction(page, testUser));
         await page.waitForTimeout(ACTION_TIMEOUT);
 
         // 2. 로그인
-        await loginAction(page, testUser);
+        await observe('login', () => loginAction(page, testUser));
         await expect(page).toHaveURL(`${BASE_URL}/chat`);
 
         // URL 만으로는 완주를 판정할 수 없다. 방 목록이 실제로 그려져야
         // 로그인이 쓸모 있는 상태로 끝난 것이다(목록이 비어 있는 경우 포함).
-        await expect(
-            page.getByTestId('join-chat-room-button').first()
-                .or(page.getByTestId('rooms-empty'))
-        ).toBeVisible();
+        await observe('room_list_display', () => expect(
+                page.getByTestId('join-chat-room-button').first()
+                    .or(page.getByTestId('rooms-empty'))
+            ).toBeVisible());
 
         // 컨텍스트에 사용자 정보 저장 (다른 시나리오에서 사용 가능)
         vuContext.vars.testUser = testUser;
@@ -43,11 +44,12 @@ async function loginScenario(page, vuContext) {
  * 잘못된 로그인 시도 부하 테스트
  */
 async function failedLoginScenario(page, vuContext) {
+    const observe = vuContext.vars.observation.action;
     try {
-        await loginAction(page, {
+        await observe('failed_login', () => loginAction(page, {
             email: 'nonexistent@example.com',
             password: 'WrongPassword123!',
-        }, false);
+        }, false));
 
         await page.waitForTimeout(500);
         const errorElement = page.getByTestId('login-error-message');
diff --git a/e2e/artillery/scenarios/chat.scenario.js b/e2e/artillery/scenarios/chat.scenario.js
index 76be9abf..f78a1664 100644
--- a/e2e/artillery/scenarios/chat.scenario.js
+++ b/e2e/artillery/scenarios/chat.scenario.js
@@ -27,27 +27,29 @@ async function gotoChatPage(page, vuContext) {
  * Artillery 채팅방 생성 및 메시지 전송 시나리오
  */
 async function chatRoomCreationScenario(page, vuContext) {
+    const observe = vuContext.vars.observation.action;
     try {
         // 1. 채팅방 확인
         await expect(page).toHaveURL(`${BASE_URL}/chat`);
 
         // 2. 채팅방 생성
         const roomName = `부하테스트_${randomUUID()}`;
-        await createChatRoomAction(page, roomName);
-        await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+        await observe('room_create', async () => {
+            await createChatRoomAction(page, roomName);
+            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
 
-        // URL 이 바뀌어도 소켓이 안 붙으면 방은 로딩 스피너에 머문다.
-        // 입력창이 떠야 방에 들어온 것이고, 여기서 끊어야 다음 단계가
-        // "입력창을 못 찾음"으로 30s 를 태우지 않는다.
-        await expect(page.getByTestId('chat-message-input')).toBeVisible();
+            // URL 이 바뀌어도 소켓이 안 붙으면 방은 로딩 스피너에 머문다.
+            await expect(page.getByTestId('chat-message-input')).toBeVisible();
+        });
 
         // 3. 메시지 전송
         const message = `테스트 메시지 ${bannedWordSafeText(Date.now())}`;
-        await sendMessageAction(page, message);
-        await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
-
-        const messageElement = page.getByTestId('message-content').filter({ hasText: message });
-        await expect(messageElement).toBeVisible();
+        await observe('message_send', async () => {
+            await sendMessageAction(page, message);
+            await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
+            const messageElement = page.getByTestId('message-content').filter({ hasText: message });
+            await expect(messageElement).toBeVisible();
+        });
 
         vuContext.vars.chatRoomUrl = page.url();
     } catch (error) {
@@ -60,14 +62,21 @@ async function chatRoomCreationScenario(page, vuContext) {
  * Artillery 메시지 대량 전송 시나리오
  */
 async function massMessageScenario(page, vuContext) {
+    const observe = vuContext.vars.observation.action;
     try {
         // 1. 랜덤 채팅방 입장
-        await joinRandomChatRoomAction(page);
-        await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+        await observe('room_join', async () => {
+            await joinRandomChatRoomAction(page);
+            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+            await expect(page.getByTestId('chat-message-input')).toBeVisible();
+        });
 
         // 2. 여러 메시지 연속 전송 (10개)
         console.log(`Sending ${MASS_MESSAGE_COUNT} messages...`);
-        await sendMultipleMessagesAction(page, MASS_MESSAGE_COUNT);
+        await observe('mass_message_send', async () => {
+            const messages = await sendMultipleMessagesAction(page, MASS_MESSAGE_COUNT);
+            await expect(page.getByTestId('message-content').filter({ hasText: messages.at(-1) })).toBeVisible();
+        });
     } catch (error) {
         console.error('Mass message scenario failed:', error.message);
         throw error;
@@ -78,10 +87,14 @@ async function massMessageScenario(page, vuContext) {
  * Artillery 파일 업로드 시나리오
  */
 async function fileUploadScenario(page, vuContext) {
+    const observe = vuContext.vars.observation.action;
     try {
         // 1. 랜덤 채팅방 입장
-        await joinRandomChatRoomAction(page);
-        await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+        await observe('room_join_for_file', async () => {
+            await joinRandomChatRoomAction(page);
+            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+            await expect(page.getByTestId('chat-message-input')).toBeVisible();
+        });
 
         // 2. 이미지 파일 업로드
         const filePath = path.resolve(__dirname, '../../fixtures/images/profile.jpg');
@@ -92,13 +105,13 @@ async function fileUploadScenario(page, vuContext) {
             { timeout: 15000 }
         );
 
-        await uploadFileAction(page, filePath, message);
-        await uploadPromise;
-
-        await page.waitForTimeout(ACTION_TIMEOUT);
-
-        const fileMessageContainer = page.getByTestId('file-message-container').filter({ hasText: message });
-        await expect(fileMessageContainer).toBeVisible({ timeout: 10000 });
+        await observe('file_upload', async () => {
+            await uploadFileAction(page, filePath, message);
+            await uploadPromise;
+            await page.waitForTimeout(ACTION_TIMEOUT);
+            const fileMessageContainer = page.getByTestId('file-message-container').filter({ hasText: message });
+            await expect(fileMessageContainer).toBeVisible({ timeout: 10000 });
+        });
     } catch (error) {
         console.error('File upload scenario failed:', error.message);
         throw error;
@@ -110,6 +123,7 @@ async function fileUploadScenario(page, vuContext) {
  */
 async function forbiddenWordScenario(page, vuContext) {
     const testUser = vuContext.vars.testUser;
+    const observe = vuContext.vars.observation.action;
     // NOTE: 환경변수에서 금칙어 목록을 가져오거나 기본값 사용
     const FORBIDDEN_WORDS = process.env.FORBIDDEN_WORDS
         ? process.env.FORBIDDEN_WORDS
@@ -120,20 +134,21 @@ async function forbiddenWordScenario(page, vuContext) {
 
     try {
         // 1. 랜덤 채팅방 입장
-        await joinRandomChatRoomAction(page);
-        await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+        await observe('room_join_for_forbidden_word', async () => {
+            await joinRandomChatRoomAction(page);
+            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
+            await expect(page.getByTestId('chat-message-input')).toBeVisible();
+        });
 
         // 2. 금칙어 메시지 전송 시도
         const forbiddenWord = FORBIDDEN_WORDS[Math.floor(Math.random() * FORBIDDEN_WORDS.length)];
-        await sendMessageAction(page, forbiddenWord);
-
-        // 3. 에러 토스트 확인
-        const errorToast = page.getByTestId('toast-error');
-        await expect(errorToast).toBeVisible({ timeout: 5000 });
-
-        // 4. 메시지가 전송되지 않았는지 확인
-        const sentMessage = page.getByTestId('message-content').filter({ hasText: forbiddenWord });
-        await expect(sentMessage).not.toBeVisible();
+        await observe('forbidden_message_send', async () => {
+            await sendMessageAction(page, forbiddenWord);
+            const errorToast = page.getByTestId('toast-error');
+            await expect(errorToast).toBeVisible({ timeout: 5000 });
+            const sentMessage = page.getByTestId('message-content').filter({ hasText: forbiddenWord });
+            await expect(sentMessage).not.toBeVisible();
+        });
 
         vuContext.vars.testUser = testUser;
     } catch (error) {
diff --git a/e2e/artillery/scenarios/profile.scenario.js b/e2e/artillery/scenarios/profile.scenario.js
index 5d33b676..758053ae 100644
--- a/e2e/artillery/scenarios/profile.scenario.js
+++ b/e2e/artillery/scenarios/profile.scenario.js
@@ -16,17 +16,20 @@ const ACTION_TIMEOUT_SHORT = parseInt(process.env.ACTION_TIMEOUT_SHORT || '500',
  * 이름 변경 + 이미지 업로드
  */
 async function fullProfileUpdateScenario(page, vuContext) {
+    const observe = vuContext.vars.observation.action;
     try {
         // 1. 프로필 페이지 이동
-        await goToProfileAction(page);
+        await observe('profile_open', () => goToProfileAction(page));
         await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
 
         // 2. 이미지 업로드
-        await page.getByTestId('profile-image-file-input').setInputFiles(PROFILE_IMAGE_PATH);
-        await page.waitForTimeout(ACTION_TIMEOUT);
+        await observe('profile_image_change', async () => {
+            await page.getByTestId('profile-image-file-input').setInputFiles(PROFILE_IMAGE_PATH);
+            await page.waitForTimeout(ACTION_TIMEOUT);
+            await expect(page.getByTestId('toast-success')).toBeVisible();
+        });
 
         // 2-1. 이미지 업로드 검증
-        await expect(page.getByTestId('toast-success')).toBeVisible();
         await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
 
         // 3. 이름 변경
@@ -35,13 +38,13 @@ async function fullProfileUpdateScenario(page, vuContext) {
         await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
 
         // 4. 저장
-        await page.getByTestId('profile-save-button').click();
-        await page.waitForTimeout(ACTION_TIMEOUT);
-
-        // 5. 성공 확인
-        await expect(page.getByTestId('profile-success-message')).toBeVisible();
-        await expect(page.getByTestId('profile-name-input')).toHaveValue(newName);
-        await expect(page.getByTestId('profile-image-avatar')).toBeVisible();
+        await observe('profile_name_change', async () => {
+            await page.getByTestId('profile-save-button').click();
+            await page.waitForTimeout(ACTION_TIMEOUT);
+            await expect(page.getByTestId('profile-success-message')).toBeVisible();
+            await expect(page.getByTestId('profile-name-input')).toHaveValue(newName);
+            await expect(page.getByTestId('profile-image-avatar')).toBeVisible();
+        });
     } catch (error) {
         console.error('Full profile update scenario failed:', error.message);
         throw error;
diff --git a/loadtest/LOAD_TEST_PLAN.md b/loadtest/LOAD_TEST_PLAN.md
new file mode 100644
index 00000000..be434434
--- /dev/null
+++ b/loadtest/LOAD_TEST_PLAN.md
@@ -0,0 +1,273 @@
+# KTB Bootcamp Chat 48시간 활성 사용자 수 최적화 계획서
+
+## 1. 목표
+
+48시간 안에 KTB Bootcamp Chat이 실제 채팅 행동을 수행하며 안정적으로 유지할 수 있는 **동시 활성 사용자 수**를 최대화한다.
+
+전체 API를 차례로 측정하지 않는다. 동일한 혼합 Ramp-up에서 최초 포화점을 찾고, 그 순간의 지연·오류·자원 지표로 가장 유력한 병목 하나를 선택해 다음 루프를 반복한다.
+
+```text
+15분 이내 Ramp-up
+→ 최초 포화 신호 확인
+→ 병목 하나 선택
+→ 최소 변경
+→ 같은 Ramp-up 재실행
+→ 개선되면 새 베이스라인, 아니면 변경 폐기
+```
+
+로컬 결과는 운영 SLA나 절대 수용량으로 해석하지 않는다. 이 문서의 수용량은 같은 장비와 같은 워크로드에서 최적화 전후를 비교하기 위한 값이다.
+
+## 2. 활성 사용자와 수용량 정의
+
+### 활성 사용자
+
+다음 조건을 모두 만족하는 고유 VUser를 활성 사용자로 계산한다.
+
+- 고유 계정과 세션으로 인증됨
+- Socket.IO 연결이 유지됨
+- 하나의 채팅방에 입장함
+- 2~5초 간격으로 메시지를 보내고 수신함
+- 수신 메시지에 읽음 처리를 수행함
+- 일부 메시지에 리액션을 수행함
+- 간헐적으로 이전 메시지를 조회함
+
+### 안정 수용량
+
+아래 조건을 60초 연속 만족한 가장 높은 활성 사용자 수를 해당 회차의 안정 수용량으로 기록한다.
+
+- 연결 성공률 99% 이상
+- 예상하지 못한 오류율 1% 이하
+- 비정상 disconnect 1% 이하
+- 30초 이상 연속 timeout 없음
+- Socket 메시지 p95가 저부하 기준의 2배 미만
+- VUser 증가율 대비 처리량 증가율이 50% 이상
+- CPU 90%, heap 85%, Tomcat thread 90%, MongoDB connection pool 90%가 60초 이상 지속되지 않음
+
+첫 위반이 3개의 연속된 5초 window에서 재현되면 포화 후보로 기록한다. 같은 조건의 3회 중 2회 이상 비슷한 사용자 수에서 재현되면 확정 포화점으로 인정한다.
+
+## 3. 고정 워크로드
+
+모든 비교 실행에서 다음 조건을 고정한다.
+
+- 초당 10명의 사용자를 추가한다.
+- 새 사용자 10명마다 전용 방 하나를 만들고 여러 방에 분산한다.
+- 사용자마다 2~5초 간격으로 1KB text 메시지를 보낸다.
+- 행동 비율은 메시지 60%, 읽음 20%, 이전 메시지 조회 10%, 리액션 5%, 방 관련 REST 5%로 유지한다.
+- VU마다 고유 계정, JWT와 `x-session-id`를 사용한다.
+- 애플리케이션 커밋, JVM 옵션, MongoDB·Redis 설정, 컨테이너 자원과 부하기 장비를 고정한다.
+- 시작 seed의 계정 수, 방 수, 참가자 수와 누적 메시지 수를 기록한다.
+
+기본 `max-users`는 500이다. 포화 없이 도달하면 다음 실행에서 `1000 → 2000`처럼 두 배로 높인다. 포화점이 확인된 이후에는 직전 안정 수용량의 80%에서 시작해 주변 구간을 다시 측정한다.
+
+한 방 집중 부하는 전체 분산 수용량과 섞지 않는다. 최종 후보 검증 단계에서 별도의 회귀 시나리오로만 실행한다.
+
+## 4. 빠른 Ramp-up 실행 단위
+
+각 실행은 최대 15분 안에 끝낸다.
+
+```text
+Smoke 확인
+→ 60초 저부하 웜업
+→ 부하기 window와 counter 초기화
+→ measurementStart 기록
+→ 초당 10명 Ramp-up
+→ 포화 확인 후 30초 관찰 또는 max-users 도달 후 60초 유지
+→ measurementEnd 기록
+→ 결과 저장과 병목 선정
+```
+
+### 웜업
+
+- 30 VUser로 60초간 본 측정과 같은 행동을 수행한다.
+- JVM, connection pool과 주요 코드 경로를 준비하되 결과에는 포함하지 않는다.
+- Ramp-up 시작 직전에 부하기 latency sample, 요청·오류 수와 처리량 window를 초기화한다.
+- Prometheus는 `measurementStart`부터 `measurementEnd`까지만 결과 계산에 사용한다.
+- 웜업 중 계약 실패나 예상하지 못한 오류가 발생하면 Ramp-up을 시작하지 않는다.
+
+### 자동 종료
+
+- 포화가 확인되면 30초만 추가 관찰하고 종료한다.
+- `max-users`에 도달하면 60초 유지 후 종료한다.
+- 부하기 자체 오류, fixture 오염 또는 필수 관측 누락은 즉시 무효 처리한다.
+- 오류 폭증으로 서버나 다음 실험의 복구가 어려워질 가능성이 있으면 즉시 연결을 정리한다.
+
+### 실행 시간 예산
+
+| 단계 | 상한 |
+|---|---:|
+| 웜업 | 1분 |
+| Ramp-up과 포화 후 관찰 | 6분 |
+| 결과 수집과 병목 선정 | 3분 |
+| 복구·재실행 여유 | 5분 |
+| **합계** | **15분** |
+
+15분을 넘기는 원인은 기록한다. 장시간 안정성 검증이 아닌 탐색 실행에서 시간을 맞추기 위해 불필요하게 부하를 유지하지 않는다.
+
+## 5. 실행 전 부하기 준비
+
+준비 작업은 48시간의 첫 3시간을 넘기지 않는다.
+
+### 현재 확인된 결손
+
+- `ramp-up-test.js`가 요구하는 `breakpoint/windowed-metrics`와 `breakpoint/breakpoint-detector` 모듈이 현재 `loadtest` 디렉터리에 없다.
+- `loadtest` 의존성이 설치되지 않은 상태에서는 `socket.io-client`를 찾지 못해 실행되지 않는다.
+- 현재 지표는 REST와 Socket을 크게 묶어 보여 주므로 최초 포화 원인을 작업별로 구분하기 어렵다.
+
+### 최소 준비 범위
+
+- 누락된 breakpoint 모듈과 의존성을 복구하고 기존 테스트를 통과시킨다.
+- 웜업 60초 후 측정 window를 초기화하는 phase를 추가한다.
+- `measurementStart`, `measurementEnd`, 활성 사용자 수와 포화 신호를 결과에 기록한다.
+- 로그인, 방 REST, Socket 연결, `joinRoom`, 메시지, 읽음, 과거 조회, 리액션의 count·오류·p95를 분리한다.
+- 5초 window timeline과 `breaking-point.json`을 항상 별도 실행 디렉터리에 저장한다.
+- Prometheus target과 필수 지표의 수집 여부를 Smoke에서 확인한다.
+
+3시간 안에 세부 지표 분리가 끝나지 않으면 부하기 복구, 활성 사용자·Socket RTT·오류·disconnect·서버 자원 측정만으로 첫 Ramp-up을 시작한다. 부하기 완성도를 높이느라 최적화 시간을 소진하지 않는다.
+
+## 6. 최초 베이스라인
+
+부하기가 실행되면 다음 순서로 3시간 안에 최초 확정 포화점을 만든다.
+
+1. 1~5 VUser Smoke로 REST와 Socket 계약을 확인한다.
+2. `max-users=500`에서 표준 Ramp-up을 실행한다.
+3. 포화가 없으면 `max-users`를 두 배로 높여 즉시 다시 실행한다.
+4. 포화 후보가 발견되면 같은 설정으로 총 3회 실행한다.
+5. 3회 중 2회 이상 비슷한 지점에서 재현된 가장 낮은 값을 최초 확정 포화점으로 기록한다.
+6. 포화 시각 전후 60초의 부하기 timeline과 Prometheus 지표를 맞춰 첫 병목 가설을 하나만 선택한다.
+
+Smoke는 HTTP 상태뿐 아니라 응답 body, ID, 세션과 Socket payload를 검증한다. 준비 트래픽과 회원가입은 측정 결과에서 제외한다.
+
+## 7. 병목 선택 기준
+
+포화 직전과 포화 시점의 변화를 다음 순서로 확인한다.
+
+1. 연결 실패, ping timeout, server disconnect
+2. 메시지 RTT p95·p99와 초당 메시지 처리량
+3. `joinRoom`과 전체 onboarding p95
+4. 오류가 증가한 REST·Socket 작업
+5. CPU, heap·GC, thread·FD
+6. MongoDB 명령 지연·connection pool
+7. Redis 명령 지연·연결·메모리
+8. network와 disk I/O
+
+가장 먼저 악화된 신호와 코드 경로가 직접 연결되는 가설 하나만 다음 변경 대상으로 선택한다. 여러 최적화를 한 번에 적용하지 않는다.
+
+예시:
+
+- CPU와 로그인 p95가 먼저 상승하면 인증·BCrypt 경로를 조사한다.
+- MongoDB 지연과 `GET /api/rooms`가 함께 상승하면 query 수와 실행 계획을 조사한다.
+- 메시지 처리량이 정체되고 Socket RTT만 상승하면 저장·broadcast·읽음 갱신 경로를 조사한다.
+- 연결 실패와 FD가 함께 상승하면 Socket·OS 연결 제한을 조사한다.
+- heap·GC가 먼저 상승하면 메시지 fan-out 객체와 보관 구조를 조사한다.
+
+상관관계만으로 원인을 확정하지 않는다. 로그, profiler, MongoDB `explain` 등 가장 작은 진단으로 가설을 확인한 뒤 코드를 변경한다.
+
+## 8. 최적화 반복 루프
+
+각 최적화는 다음 절차를 따른다.
+
+1. 확정 베이스라인 결과에서 병목 가설 하나를 선택한다.
+2. 가설을 확인하는 읽기 전용 진단을 수행한다.
+3. 병목에 직접 필요한 최소 변경만 구현한다.
+4. 관련 단위·통합 테스트를 실행한다.
+5. 동일한 seed, 증가율, 메시지 간격과 `max-users`로 표준 Ramp-up을 실행한다.
+6. 안정 수용량이 증가하고 다른 핵심 지표가 회귀하지 않으면 채택한다.
+7. 개선되지 않으면 변경을 폐기하고 다음 가설로 이동한다.
+8. 채택된 결과를 새로운 베이스라인으로 삼는다.
+
+### 빠른 채택 기준
+
+- 안정 수용량이 10% 이상 증가하거나, 같은 수용량에서 포화 신호가 사라짐
+- 오류율과 비정상 disconnect가 증가하지 않음
+- 메시지 p95·p99가 악화되지 않음
+- CPU, heap·GC, MongoDB·Redis 중 다른 계층으로 심각한 포화가 이동하지 않음
+
+수용량 변화가 10% 미만이면 1회 추가 실행한다. 반복 결과가 측정 변동 범위 안이면 채택하지 않는다.
+
+## 9. 48시간 배분
+
+| 경과 시간 | 작업 | 산출물 |
+|---|---|---|
+| 0~3시간 | 부하기 복구, 웜업 phase, Smoke | 실행 가능한 표준 Ramp-up |
+| 3~6시간 | 베이스라인 탐색·3회 재현 | 최초 확정 포화점과 병목 가설 |
+| 6~36시간 | 진단→한 가지 변경→15분 Ramp-up 반복 | 가능한 많은 검증된 수용량 개선 |
+| 36~42시간 | 최종 후보 3회 재현, 한 방 집중·E2E 회귀 | 최종 안정 수용량 |
+| 42~46시간 | 변경 전후 지표 정리, 테스트와 코드 정리 | 재현 가능한 결과와 최종 변경 |
+| 46~48시간 | 실패·환경 복구·재측정 버퍼 | 누락 없는 최종 보고 |
+
+6~36시간에는 개선 횟수를 미리 정하지 않는다. 작고 근거가 강한 변경부터 처리하며, 진단이나 구현이 2시간 이상 막히면 다음 병목 가설로 전환한다.
+
+## 10. 최종 검증
+
+마지막으로 채택된 코드에서 다음을 실행한다.
+
+- 표준 분산 Ramp-up 3회
+- 확정 안정 수용량에서 10분 E2E Constant 1회
+- 같은 방에 사용자가 집중되는 hot-room Ramp-up 1회
+- 안정 수용량의 2배를 30초 안에 투입하는 Spike 1회
+
+최종 안정 수용량은 분산 Ramp-up 3회의 중앙값이 아니라, 안정 조건을 충족한 세 회차 중 가장 낮은 값으로 보수적으로 기록한다. E2E Constant 또는 회귀 테스트가 실패하면 마지막 최적화를 채택하지 않는다.
+
+## 11. 측정 지표와 결과
+
+### 부하기
+
+- active users, 연결 성공률, disconnect 사유
+- 초당 메시지 송신·고유 수신 수
+- 작업별 count, 오류율, 평균, p50, p95, p99, 최대
+- 최초 포화 사용자 수, 시각과 신호
+
+### 서버와 자원
+
+- 프로세스 CPU 평균·최대
+- heap/non-heap, GC 횟수·중단시간
+- thread, FD, 노출되는 경우 Tomcat active/max thread
+- HTTP URI·method·status별 count와 timer
+- MongoDB 명령 수·지연, connection pool과 wait queue
+- Redis 명령 수·지연, 연결, 메모리와 hit/miss
+- 수집 가능한 network와 disk I/O
+
+누적 counter는 측정 구간 증가량을 사용한다. timer 평균은 `sum 증가량 / count 증가량`, gauge는 구간 평균과 최대를 사용한다.
+
+### 회차 결과표
+
+| 커밋 | 회차 | max-users | 안정 수용량 | 최초 포화 신호 | msg/s | Socket p95/p99 | 오류율 | 비정상 해제 | CPU 최대 | heap 최대 | Mongo/Redis 포화 |
+|---|---:|---:|---:|---|---:|---|---:|---:|---:|---:|---|
+|  |  |  |  |  |  |  |  |  |  |  |  |
+
+### 최적화 기록
+
+| 순서 | 병목 근거 | 변경 한 가지 | 변경 전 수용량 | 변경 후 수용량 | 변화율 | 회귀 | 채택 여부 |
+|---:|---|---|---:|---:|---:|---|---|
+|  |  |  |  |  |  |  |  |
+
+각 결과에는 애플리케이션·부하기 커밋, JVM과 DB 설정, seed, 시작·종료 시각, 무효·중단 이유를 함께 저장한다.
+
+## 12. 실행 체크리스트
+
+### 준비
+
+- [ ] 부하기 의존성과 breakpoint 모듈 복구
+- [ ] Smoke와 계약 테스트 통과
+- [ ] 웜업 표본이 결과에서 제외됨
+- [ ] 작업별 지연·오류와 measurement 시각 기록
+- [ ] Prometheus target과 필수 지표 확인
+- [ ] 고유 계정과 재사용 가능한 seed 확보
+
+### 반복 루프
+
+- [ ] 이전 회차와 workload·환경 동일
+- [ ] 최초 포화 신호와 시각 확인
+- [ ] 병목 가설 하나만 선택
+- [ ] 변경도 한 가지로 제한
+- [ ] 동일 Ramp-up으로 즉시 재측정
+- [ ] 개선 없으면 변경 폐기
+- [ ] 개선되면 새 베이스라인 기록
+
+### 종료
+
+- [ ] 최종 분산 Ramp-up 3회 완료
+- [ ] 10분 E2E Constant 통과
+- [ ] hot-room과 Spike 회귀 결과 기록
+- [ ] 최종 안정 수용량과 개선율 기록
+- [ ] 측정하지 못한 지표와 환경 제약 명시
diff --git a/loadtest/load-test.js b/loadtest/load-test.js
index 4ba6795e..cc95503f 100644
--- a/loadtest/load-test.js
+++ b/loadtest/load-test.js
@@ -85,6 +85,7 @@ class LoadTester {
       errorsMessage: 0,
       latencies: [],
       connectionTimes: [],
+      operations: {},
       startTime: Date.now()
     };
     this.sockets = [];
@@ -106,6 +107,33 @@ class LoadTester {
     // Don't output to console directly - it will be handled by printMetrics
   }
 
+  recordOperation(name, elapsedMs = 0, failed = false) {
+    const operation = this.metrics.operations[name] || { count: 0, errors: 0, latencies: [] };
+    operation.count++;
+    if (failed) operation.errors++;
+    if (elapsedMs > 0) operation.latencies.push(elapsedMs);
+    this.metrics.operations[name] = operation;
+  }
+
+  printOperationRanking() {
+    const rows = Object.entries(this.metrics.operations)
+      .map(([name, operation]) => {
+        const avg = operation.latencies.length
+          ? operation.latencies.reduce((sum, value) => sum + value, 0) / operation.latencies.length
+          : 0;
+        const errorRate = operation.errors / operation.count;
+        const impact = operation.count * Math.max(avg, 1) * (1 + errorRate * 10);
+        return { name, count: operation.count, avg, errorRate, impact };
+      })
+      .sort((a, b) => b.impact - a.impact)
+      .slice(0, 5);
+
+    console.log(chalk.bold.magenta('\n=== Operation Impact Ranking (first-pass) ==='));
+    rows.forEach((row, index) => {
+      console.log(`${index + 1}. ${row.name}: impact=${row.impact.toFixed(1)}, count=${row.count}, avg=${row.avg.toFixed(1)}ms, errors=${(row.errorRate * 100).toFixed(1)}%`);
+    });
+  }
+
   async createTestUser(userId) {
     try {
       const email = `loadtest-${userId}@test.com`;
@@ -224,13 +252,18 @@ class LoadTester {
           this.log('success', `User ${userId} (${user.name}) connected in ${connectionTime}ms`);
 
           // Join room
+          const joinStart = Date.now();
           socket.emit(CLIENT_EMIT.JOIN_ROOM, roomId);
+
+          socket.__loadtestJoinStart = joinStart;
         });
 
         socket.on(SERVER_EMIT.JOIN_ROOM_SUCCESS, (data) => {
+          this.recordOperation('joinRoom', Date.now() - (socket.__loadtestJoinStart || Date.now()));
           this.log('info', `User ${userId} joined room ${roomId} with ${data.participants?.length || 0} participants`);
 
           // Fetch previous messages before starting to send
+          socket.__loadtestPreviousStart = Date.now();
           socket.emit(CLIENT_EMIT.FETCH_PREVIOUS_MESSAGES, { roomId: roomId, limit: 30 });
 
           // Start sending messages
@@ -238,6 +271,7 @@ class LoadTester {
         });
 
         socket.on(SERVER_EMIT.PREVIOUS_MESSAGES_LOADED, (data) => {
+          this.recordOperation('fetchPreviousMessages', Date.now() - (socket.__loadtestPreviousStart || Date.now()));
           this.metrics.previousMessagesFetched++;
           if (data.messages?.length) {
             this.metrics.messagesReceived += data.messages.length;
@@ -245,6 +279,7 @@ class LoadTester {
         });
 
         socket.on(SERVER_EMIT.JOIN_ROOM_ERROR, (error) => {
+          this.recordOperation('joinRoom', Date.now() - (socket.__loadtestJoinStart || Date.now()), true);
           this.metrics.errorsConnection++;
           this.log('error', `User ${userId} failed to join room:`, error.message || JSON.stringify(error));
           socket.close();
@@ -259,6 +294,7 @@ class LoadTester {
             socket.emit(CLIENT_EMIT.MARK_MESSAGES_AS_READ, {
               messageIds: [data._id]
             });
+            this.recordOperation('markMessagesAsRead');
             this.metrics.messagesRead++;
 
             // Randomly react to ~10% of messages
@@ -268,6 +304,7 @@ class LoadTester {
                 reaction: '👍',
                 type: 'add'
               });
+              this.recordOperation('messageReaction');
               this.metrics.reactionsSent++;
             }
           }
@@ -505,6 +542,7 @@ class LoadTester {
     // Stop metrics reporting and print final report
     clearInterval(this.metricsInterval);
     this.printMetrics();
+    this.printOperationRanking();
 
     console.log(chalk.bold.green('\n✓ Load test completed!\n'));
     process.exit(0);
diff --git a/performance/room-join/README.md b/performance/room-join/README.md
new file mode 100644
index 00000000..2307ce8b
--- /dev/null
+++ b/performance/room-join/README.md
@@ -0,0 +1,15 @@
+# Room join HTTP-only load test
+
+This scenario measures only `POST /api/rooms/{roomId}/join`. Account login and room selection
+run once in `setup` and are excluded from the custom metrics.
+
+```sh
+API_BASE_URL=http://localhost:5001 \
+TEST_NAME=room-join-before-1 FIXTURE_ID=rooms324-users562-messages8051 \
+VUS=10 DURATION=3m THINK_TIME_MS=1000 \
+k6 run performance/room-join/room-join.js
+```
+
+Run the same fixture and settings three times before and after one code change. Adoption requires
+error rate at most 1%, zero timeouts, Mongo wait queue zero, p95 improvement of at least 10%, and
+no p99 regression. Result JSON is written under `results/`.
diff --git a/performance/room-join/results/room-join-before-1.json b/performance/room-join/results/room-join-before-1.json
new file mode 100644
index 00000000..fdaec4fe
--- /dev/null
+++ b/performance/room-join/results/room-join-before-1.json
@@ -0,0 +1,16 @@
+{
+  "testName": "room-join-before-1",
+  "fixtureId": "rooms324-users562-messages8051",
+  "apiBaseUrl": "http://localhost:5001",
+  "model": "HTTP-only room join",
+  "vus": 10,
+  "duration": "3m",
+  "thinkTimeMs": 1000,
+  "endedAt": "2026-08-11T01:11:27.059Z",
+  "p95Ms": 137.09819999999993,
+  "p99Ms": 245.29899999999998,
+  "averageMs": 93.7216435944142,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 1647
+}
diff --git a/performance/room-join/results/room-join-before-observed-1.json b/performance/room-join/results/room-join-before-observed-1.json
new file mode 100644
index 00000000..9bb9af8a
--- /dev/null
+++ b/performance/room-join/results/room-join-before-observed-1.json
@@ -0,0 +1,16 @@
+{
+  "testName": "room-join-before-observed-1",
+  "fixtureId": "rooms324-users562-messages8051",
+  "apiBaseUrl": "http://localhost:5001",
+  "model": "HTTP-only room join",
+  "vus": 10,
+  "duration": "3m",
+  "thinkTimeMs": 1000,
+  "endedAt": "2026-08-11T01:31:10.699Z",
+  "p95Ms": 140.53889999999998,
+  "p99Ms": 249.15699999999998,
+  "averageMs": 85.71070464135015,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 1659
+}
diff --git a/performance/room-join/results/room-join-pool-smoke.json b/performance/room-join/results/room-join-pool-smoke.json
new file mode 100644
index 00000000..1823258a
--- /dev/null
+++ b/performance/room-join/results/room-join-pool-smoke.json
@@ -0,0 +1,16 @@
+{
+  "testName": "room-join-pool-smoke",
+  "fixtureId": "rooms324-users562-messages8051",
+  "apiBaseUrl": "http://localhost:5001",
+  "model": "HTTP-only room join",
+  "vus": 10,
+  "duration": "30s",
+  "thinkTimeMs": 0,
+  "endedAt": "2026-08-11T01:25:44.936Z",
+  "p95Ms": 168.76619999999997,
+  "p99Ms": 321.811,
+  "averageMs": 102.96696698762054,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 2908
+}
diff --git a/performance/room-join/room-join.js b/performance/room-join/room-join.js
new file mode 100644
index 00000000..d77f3069
--- /dev/null
+++ b/performance/room-join/room-join.js
@@ -0,0 +1,91 @@
+import http from 'k6/http';
+import { check, sleep } from 'k6';
+import { Counter, Rate, Trend } from 'k6/metrics';
+
+const API_BASE_URL = __ENV.API_BASE_URL || 'http://localhost:5001';
+const TEST_NAME = __ENV.TEST_NAME || 'room-join-local';
+const FIXTURE_ID = __ENV.FIXTURE_ID || 'unspecified';
+const VUS = Number(__ENV.VUS || 10);
+const DURATION = __ENV.DURATION || '3m';
+const THINK_TIME_MS = Number(__ENV.THINK_TIME_MS || 1000);
+const ACCOUNT_COUNT = Number(__ENV.ACCOUNT_COUNT || 30);
+const PASSWORD = __ENV.LOAD_TEST_PASSWORD || 'Test1234!';
+
+const duration = new Trend('room_join_duration', true);
+const errors = new Rate('room_join_errors');
+const completed = new Counter('room_join_completed');
+const timeouts = new Counter('room_join_timeouts');
+
+export const options = {
+  vus: VUS,
+  duration: DURATION,
+  summaryTrendStats: ['avg', 'min', 'med', 'max', 'p(90)', 'p(95)', 'p(99)'],
+  thresholds: {
+    room_join_errors: ['rate<=0.01'],
+    room_join_timeouts: ['count==0'],
+  },
+};
+
+function account(index) {
+  return { email: `loadtest-${index}@test.com`, password: PASSWORD };
+}
+
+export function setup() {
+  if (VUS > ACCOUNT_COUNT) throw new Error(`VUS (${VUS}) exceeds ACCOUNT_COUNT (${ACCOUNT_COUNT})`);
+  const tokens = [];
+  for (let index = 0; index < ACCOUNT_COUNT; index += 1) {
+    const response = http.post(`${API_BASE_URL}/api/auth/login`, JSON.stringify(account(index)), {
+      headers: { 'Content-Type': 'application/json' }, timeout: '10s', tags: { phase: 'setup' },
+    });
+    if (response.status !== 200 || !response.json('token')) {
+      throw new Error(`Fixed account ${account(index).email} unavailable (HTTP ${response.status})`);
+    }
+    tokens.push(response.json('token'));
+  }
+  const rooms = http.get(`${API_BASE_URL}/api/rooms`, {
+    headers: { Authorization: `Bearer ${tokens[0]}` }, timeout: '30s', tags: { phase: 'setup' },
+  });
+  const roomId = rooms.json('data.0._id') || rooms.json('data.0.id');
+  if (rooms.status !== 200 || !roomId) throw new Error('No room is available for room-join measurement');
+  return { tokens, roomId };
+}
+
+export default function (data) {
+  const token = data.tokens[(__VU - 1) % data.tokens.length];
+  const response = http.post(`${API_BASE_URL}/api/rooms/${data.roomId}/join`, '{}', {
+    headers: { Authorization: `Bearer ${token}`, 'Content-Type': 'application/json' },
+    timeout: '30s', tags: { phase: 'measurement', endpoint: 'room-join' },
+  });
+  const success = check(response, {
+    'room join returned HTTP 200': (res) => res.status === 200,
+    'room join preserved success contract': (res) => res.json('success') === true,
+    'room join returned a room': (res) => Boolean(res.json('data._id') || res.json('data.id')),
+  });
+  duration.add(response.timings.duration);
+  errors.add(!success);
+  completed.add(1);
+  if (response.error_code === 1050 || response.timings.duration >= 30000) timeouts.add(1);
+  sleep(THINK_TIME_MS / 1000);
+}
+
+function value(data, metric, field, fallback = 0) {
+  return data.metrics[metric]?.values[field] ?? fallback;
+}
+
+export function handleSummary(data) {
+  const summary = {
+    testName: TEST_NAME, fixtureId: FIXTURE_ID, apiBaseUrl: API_BASE_URL,
+    model: 'HTTP-only room join', vus: VUS, duration: DURATION, thinkTimeMs: THINK_TIME_MS,
+    endedAt: new Date().toISOString(),
+    p95Ms: value(data, 'room_join_duration', 'p(95)'),
+    p99Ms: value(data, 'room_join_duration', 'p(99)'),
+    averageMs: value(data, 'room_join_duration', 'avg'),
+    errorRate: value(data, 'room_join_errors', 'rate'),
+    timeoutCount: value(data, 'room_join_timeouts', 'count'),
+    completedRequests: value(data, 'room_join_completed', 'count'),
+  };
+  return {
+    stdout: `${JSON.stringify(summary, null, 2)}\n`,
+    [`performance/room-join/results/${TEST_NAME}.json`]: `${JSON.stringify(summary, null, 2)}\n`,
+  };
+}

From 7ba1bc012b9069d26690ce9e984bd6585c7f132b Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 11:18:46 +0900
Subject: [PATCH 06/32] docs(pr): add metric-focused template

---
 .github/pull_request_template.md | 43 ++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 .github/pull_request_template.md

diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
new file mode 100644
index 00000000..83cc8f1b
--- /dev/null
+++ b/.github/pull_request_template.md
@@ -0,0 +1,43 @@
+## 문제와 원인
+
+<!-- 사용자가 겪은 문제와 코드·쿼리에서 확인한 직접 원인을 2~4문장으로 작성합니다. -->
+
+## 구현
+
+| 항목 | 변경 전 | 변경 후 |
+|---|---|---|
+| 동작 또는 구조 |  |  |
+| 데이터·요청 흐름 |  |  |
+
+<!-- 외부 API, 이벤트, 화면 동작처럼 유지한 계약이 있다면 한 줄로 작성합니다. -->
+
+## 측정 방법
+
+| 조건 | 값 |
+|---|---|
+| 대상 흐름 |  |
+| 데이터 규모 |  |
+| 사용자 수·시간·횟수 |  |
+| 확인 지표 |  |
+
+<!-- 성능 변경이 아니면 이 섹션을 삭제해도 됩니다. -->
+
+## 결과
+
+| 지표 | 변경 전 | 변경 후 | 변화 |
+|---|---:|---:|---:|
+| p95 |  |  |  |
+| p99 |  |  |  |
+| 오류율 |  |  |  |
+| 병목 지표 |  |  |  |
+
+<!-- 결과가 좋아지거나 나빠진 이유를 수치와 코드 흐름에 연결해 2~3문장으로 설명합니다. -->
+
+## 검증
+
+- [ ] 관련 단위 테스트
+- [ ] 관련 통합 테스트
+- [ ] 기존 응답·이벤트·화면 동작 유지
+- [ ] 동일 조건 전후 비교
+
+<!-- 실행하지 못한 검증이나 후속 작업이 있으면 짧게 작성합니다. -->

From 5372bc5dc999d5eda97571af54736112b245a0ab Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 11:22:58 +0900
Subject: [PATCH 07/32] docs(pr): clarify review evidence flow

---
 .github/pull_request_template.md | 73 ++++++++++++++++++++------------
 1 file changed, 45 insertions(+), 28 deletions(-)

diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 83cc8f1b..13271504 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -1,43 +1,60 @@
-## 문제와 원인
+## 한눈에 보기
 
-<!-- 사용자가 겪은 문제와 코드·쿼리에서 확인한 직접 원인을 2~4문장으로 작성합니다. -->
+| 구분 | 내용 |
+|---|---|
+| 문제 |  |
+| 확인된 원인 |  |
+| 해결 방법 |  |
+| 사용자·개발자 영향 |  |
+
+<!-- PR 전체를 읽지 않아도 변경 이유와 결론을 이해할 수 있게 한 문장씩 작성합니다. -->
+
+## 확인 과정
 
-## 구현
+| 단계 | 확인한 내용 | 판단 근거 |
+|---:|---|---|
+| 1. 재현 |  |  |
+| 2. 원인 확인 |  |  |
+| 3. 해결안 선택 |  |  |
 
-| 항목 | 변경 전 | 변경 후 |
+<!-- 추측이 아니라 로그, 쿼리, 화면 동작, 테스트 실패처럼 직접 확인한 근거를 작성합니다. -->
+
+## 구현 내용
+
+| 영역 | 변경 내용 | 변경 이유 |
 |---|---|---|
-| 동작 또는 구조 |  |  |
-| 데이터·요청 흐름 |  |  |
+|  |  |  |
 
-<!-- 외부 API, 이벤트, 화면 동작처럼 유지한 계약이 있다면 한 줄로 작성합니다. -->
+<!-- API 응답, 이벤트, 데이터 형식처럼 유지한 동작이 있으면 함께 작성합니다. -->
 
-## 측정 방법
+## 검증 조건
 
-| 조건 | 값 |
+| 항목 | 조건 |
 |---|---|
-| 대상 흐름 |  |
-| 데이터 규모 |  |
-| 사용자 수·시간·횟수 |  |
-| 확인 지표 |  |
+| 테스트 대상 |  |
+| 데이터·환경 |  |
+| 실행 횟수·부하 |  |
+| 통과 기준 |  |
+
+<!-- 별도 측정이 필요 없는 PR은 불필요한 행을 삭제합니다. -->
 
-<!-- 성능 변경이 아니면 이 섹션을 삭제해도 됩니다. -->
+## 검증 결과
 
-## 결과
+| 확인 항목 | 변경 전 | 변경 후 | 판정 |
+|---|---:|---:|---|
+|  |  |  |  |
 
-| 지표 | 변경 전 | 변경 후 | 변화 |
-|---|---:|---:|---:|
-| p95 |  |  |  |
-| p99 |  |  |  |
-| 오류율 |  |  |  |
-| 병목 지표 |  |  |  |
+<!-- 성능 PR은 p95·p99·오류율·병목 지표를, 기능 PR은 성공·실패 케이스를 작성합니다. 결과가 나온 이유도 2~3문장으로 설명합니다. -->
 
-<!-- 결과가 좋아지거나 나빠진 이유를 수치와 코드 흐름에 연결해 2~3문장으로 설명합니다. -->
+## 영향 범위와 후속 작업
 
-## 검증
+- 영향받는 기능:
+- 이번 PR에서 제외한 범위:
+- 다음 작업:
 
-- [ ] 관련 단위 테스트
-- [ ] 관련 통합 테스트
-- [ ] 기존 응답·이벤트·화면 동작 유지
-- [ ] 동일 조건 전후 비교
+## 체크리스트
 
-<!-- 실행하지 못한 검증이나 후속 작업이 있으면 짧게 작성합니다. -->
+- [ ] 문제를 재현하고 원인을 확인했습니다.
+- [ ] 변경에 직접 관련된 테스트를 통과했습니다.
+- [ ] 기존 API·이벤트·화면 동작의 유지 여부를 확인했습니다.
+- [ ] 실행하지 못한 검증이나 남은 위험을 작성했습니다.

From 6f729b2e4bcfe1040ff513c12c98ae3af3d53b9b Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 11:25:54 +0900
Subject: [PATCH 08/32] docs(pr): add result metric table

---
 .github/pull_request_template.md | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 13271504..87333ce9 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -44,7 +44,14 @@
 |---|---:|---:|---|
 |  |  |  |  |
 
-<!-- 성능 PR은 p95·p99·오류율·병목 지표를, 기능 PR은 성공·실패 케이스를 작성합니다. 결과가 나온 이유도 2~3문장으로 설명합니다. -->
+| 지표 | 변경 전 | 변경 후 | 변화 |
+|---|---:|---:|---:|
+| p95 |  |  |  |
+| p99 |  |  |  |
+| 오류율 |  |  |  |
+| 병목 지표 |  |  |  |
+
+<!-- 기능 PR은 첫 표에 성공·실패 케이스를 작성합니다. 성능 PR은 두 표를 모두 사용하고, 필요 없는 지표 행은 실제 측정 항목으로 바꿉니다. 결과가 나온 이유도 2~3문장으로 설명합니다. -->
 
 ## 영향 범위와 후속 작업
 

From 9f8bfead8bd1bc4ad1d60186871378a37267ff64 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 11:27:34 +0900
Subject: [PATCH 09/32] docs(pr): allow change-specific metrics

---
 .github/pull_request_template.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 87333ce9..28bf822c 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -51,7 +51,7 @@
 | 오류율 |  |  |  |
 | 병목 지표 |  |  |  |
 
-<!-- 기능 PR은 첫 표에 성공·실패 케이스를 작성합니다. 성능 PR은 두 표를 모두 사용하고, 필요 없는 지표 행은 실제 측정 항목으로 바꿉니다. 결과가 나온 이유도 2~3문장으로 설명합니다. -->
+<!-- 기능 PR은 첫 표에 성공·실패 케이스를 작성합니다. 성능 PR은 두 표를 모두 사용합니다. 표의 기본 지표는 예시이며 구현사항에 따라 쿼리 수, 처리량, 자원 사용량, 성공 케이스 등 필요한 지표를 추가하거나 교체합니다. 결과가 나온 이유도 2~3문장으로 설명합니다. -->
 
 ## 영향 범위와 후속 작업
 

From c31df8aab1a7a5c85090b373cbfb6d2207d2ec1e Mon Sep 17 00:00:00 2001
From: rkdwlss2 <rkdwlss2@gmail.com>
Date: Tue, 11 Aug 2026 14:42:36 +0900
Subject: [PATCH 10/32] =?UTF-8?q?s3=EA=B0=9C=EB=B0=9C?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/backend/.gitignore                       |  1 +
 apps/backend/DEPLOY.md                        |  7 +++++
 apps/backend/README.md                        |  5 ++++
 apps/backend/pom.xml                          | 12 ++++++++
 .../controller/ProfileImageController.java    | 18 ++++++++---
 .../com/ktb/chatapp/storage/LocalStorage.java |  5 +---
 .../src/main/resources/application.properties |  9 ++++--
 .../storage/StoragePortSelectionTest.java     | 30 ++++---------------
 .../chatapp/util/BannedWordCheckerTest.java   |  9 ++++++
 .../src/test/resources/application.properties |  5 +++-
 .../chat/rooms/__tests__/useRoomList.test.js  |  1 +
 11 files changed, 66 insertions(+), 36 deletions(-)

diff --git a/apps/backend/.gitignore b/apps/backend/.gitignore
index caca7882..6b3cdacf 100644
--- a/apps/backend/.gitignore
+++ b/apps/backend/.gitignore
@@ -1,4 +1,5 @@
 uploads/*
+.env
 
 # Eclipse JDT / LSP 생성 파일
 .classpath
diff --git a/apps/backend/DEPLOY.md b/apps/backend/DEPLOY.md
index 79b41722..28913277 100644
--- a/apps/backend/DEPLOY.md
+++ b/apps/backend/DEPLOY.md
@@ -117,6 +117,13 @@ tail -f logs/app.log
    REDIS_HOST=localhost
    REDIS_PORT=6379
 
+   # 파일 저장소 (AWS 자격 증명은 IAM Role 또는 AWS 기본 환경 변수 사용)
+   FILE_STORAGE_TYPE=s3
+   S3_BUCKET=your-s3-bucket
+   AWS_REGION=ap-northeast-2
+   S3_ENDPOINT=
+   S3_PATH_STYLE_ACCESS=false
+
    # 서버 포트
    PORT=5001
    WS_PORT=5002
diff --git a/apps/backend/README.md b/apps/backend/README.md
index 17548bf1..e09767b5 100644
--- a/apps/backend/README.md
+++ b/apps/backend/README.md
@@ -91,6 +91,11 @@ make verify-java
 | `MONGO_URI` | ✅ | 없음 | MongoDB 연결 문자열              |
 | `REDIS_HOST` | ✅ | 없음 | Redis 호스트                    |
 | `REDIS_PORT` | ✅ | 없음 | Redis 포트                      |
+| `FILE_STORAGE_TYPE` | ❌ | `s3` | 파일 저장 방식. 현재 `s3`만 지원 |
+| `S3_BUCKET` | ✅ | 없음 | 이미지 및 첨부 파일을 저장할 S3 버킷 |
+| `AWS_REGION` | ❌ | `ap-northeast-2` | S3 버킷 리전 |
+| `S3_ENDPOINT` | ❌ | 빈 값 | AWS S3 호환 스토리지의 사용자 지정 엔드포인트 |
+| `S3_PATH_STYLE_ACCESS` | ❌ | `false` | path-style S3 URL 사용 여부 |
 | `PORT` | ❌ | `5001` | HTTP API 포트 (`server.port`) |
 | `WS_PORT` | ❌ | `5002` | Socket.IO 서버 포트             |
 | `CORS_ALLOWED_ORIGINS` | ❌ | `*` | REST API CORS 허용 Origin 목록. 쉼표로 구분 |
diff --git a/apps/backend/pom.xml b/apps/backend/pom.xml
index c52460af..f301e628 100644
--- a/apps/backend/pom.xml
+++ b/apps/backend/pom.xml
@@ -16,9 +16,17 @@
     <properties>
         <java.version>25</java.version>
         <spring-ai.version>2.0.0</spring-ai.version>
+        <aws-sdk.version>2.47.3</aws-sdk.version>
     </properties>
     <dependencyManagement>
         <dependencies>
+            <dependency>
+                <groupId>software.amazon.awssdk</groupId>
+                <artifactId>bom</artifactId>
+                <version>${aws-sdk.version}</version>
+                <type>pom</type>
+                <scope>import</scope>
+            </dependency>
             <dependency>
                 <groupId>org.springframework.ai</groupId>
                 <artifactId>spring-ai-bom</artifactId>
@@ -61,6 +69,10 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
+        <dependency>
+            <groupId>software.amazon.awssdk</groupId>
+            <artifactId>s3</artifactId>
+        </dependency>
         <dependency>
             <groupId>io.micrometer</groupId>
             <artifactId>micrometer-registry-prometheus</artifactId>
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileImageController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileImageController.java
index 3162f55a..708ee06c 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileImageController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileImageController.java
@@ -9,8 +9,11 @@
 import io.swagger.v3.oas.annotations.responses.ApiResponses;
 import io.swagger.v3.oas.annotations.security.SecurityRequirement;
 import io.swagger.v3.oas.annotations.tags.Tag;
+import java.nio.charset.StandardCharsets;
+import java.time.Duration;
 import lombok.RequiredArgsConstructor;
-import org.springframework.core.io.Resource;
+import org.springframework.http.ContentDisposition;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.MediaTypeFactory;
 import org.springframework.http.ResponseEntity;
@@ -39,18 +42,25 @@ public class ProfileImageController {
     })
     @SecurityRequirement(name = "")
     @GetMapping("/{filename:.+}")
-    public ResponseEntity<Resource> getProfileImage(
+    public ResponseEntity<?> getProfileImage(
             @Parameter(description = "조회할 프로필 이미지 파일명") @PathVariable String filename) {
 
         if (FileUtil.containsPathTraversal(filename)) {
             return ResponseEntity.badRequest().build();
         }
 
-        return storagePort.open(StorageKey.profile(filename))
+        String key = StorageKey.profile(filename);
+        ContentDisposition disposition = ContentDisposition.inline()
+                .filename(filename, StandardCharsets.UTF_8)
+                .build();
+
+        return storagePort.offloadUrl(key, Duration.ofMinutes(5), disposition)
+                .<ResponseEntity<?>>map(uri -> ResponseEntity.status(HttpStatus.FOUND).location(uri).build())
+                .orElseGet(() -> storagePort.open(key)
                 .map(resource -> ResponseEntity.ok()
                         .contentType(contentTypeOf(filename))
                         .body(resource))
-                .orElseGet(() -> ResponseEntity.notFound().build());
+                .orElseGet(() -> ResponseEntity.notFound().build()));
     }
 
     private MediaType contentTypeOf(String filename) {
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java b/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java
index 1454c5c6..4ccc08a1 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java
@@ -11,13 +11,10 @@
 import java.nio.file.StandardCopyOption;
 import java.util.Optional;
 import org.springframework.beans.factory.annotation.Value;
-import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.UrlResource;
-import org.springframework.stereotype.Component;
 
-@Component
-@ConditionalOnProperty(name = "file.storage.type", havingValue = "local", matchIfMissing = true)
+/** 테스트 전용 로컬 구현. 애플리케이션 빈으로 등록하지 않아 운영 코드에서는 사용할 수 없다. */
 public class LocalStorage implements StoragePort {
 
     private final Path rootLocation;
diff --git a/apps/backend/src/main/resources/application.properties b/apps/backend/src/main/resources/application.properties
index da6d0e5f..b87bd54f 100644
--- a/apps/backend/src/main/resources/application.properties
+++ b/apps/backend/src/main/resources/application.properties
@@ -17,9 +17,12 @@ spring.data.redis.port=${REDIS_PORT:6379}
 spring.data.redis.password=${REDIS_PASSWORD:}
 
 # File Upload Configuration
-# Storage type: local or gridfs (default: local)
-file.storage.type=${FILE_STORAGE_TYPE:local}
-file.upload-dir=./uploads
+# Files are stored only in S3. Credentials use the AWS default credentials chain.
+file.storage.type=${FILE_STORAGE_TYPE:s3}
+aws.s3.bucket=${S3_BUCKET}
+aws.s3.region=${AWS_REGION:ap-northeast-2}
+aws.s3.endpoint=${S3_ENDPOINT:}
+aws.s3.path-style-access=${S3_PATH_STYLE_ACCESS:false}
 
 # Multipart upload limits
 spring.servlet.multipart.max-file-size=5MB
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java b/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java
index d57dd341..8e47383a 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java
@@ -1,36 +1,18 @@
 package com.ktb.chatapp.storage;
 
-import java.nio.file.Path;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
-import org.junit.jupiter.api.io.TempDir;
-import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.stereotype.Component;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-@DisplayName("file.storage.type 스위치 단위 테스트")
+@DisplayName("S3 전용 스토리지 선택 단위 테스트")
 class StoragePortSelectionTest {
 
-    @TempDir
-    private Path uploadDir;
-
-    private final ApplicationContextRunner contextRunner =
-            new ApplicationContextRunner().withUserConfiguration(LocalStorage.class);
-
-    @Test
-    @DisplayName("프로퍼티 미설정 시 LocalStorage 빈이 등록된다")
-    void localStorageIsRegisteredWhenPropertyMissing() {
-        contextRunner
-                .withPropertyValues("file.upload-dir=" + uploadDir)
-                .run(context -> assertThat(context).hasSingleBean(LocalStorage.class));
-    }
-
     @Test
-    @DisplayName("file.storage.type=local이면 LocalStorage 빈이 등록된다")
-    void localStorageIsRegisteredWhenPropertyIsLocal() {
-        contextRunner
-                .withPropertyValues("file.storage.type=local", "file.upload-dir=" + uploadDir)
-                .run(context -> assertThat(context).hasSingleBean(LocalStorage.class));
+    @DisplayName("S3Storage만 애플리케이션 빈으로 등록된다")
+    void onlyS3StorageIsApplicationComponent() {
+        assertThat(S3Storage.class).hasAnnotation(Component.class);
+        assertThat(LocalStorage.class.getAnnotation(Component.class)).isNull();
     }
-
 }
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/util/BannedWordCheckerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/util/BannedWordCheckerTest.java
index 05c4a4f6..aed2e365 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/util/BannedWordCheckerTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/util/BannedWordCheckerTest.java
@@ -7,6 +7,8 @@
 import java.util.List;
 import java.util.Set;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -47,6 +49,13 @@ void containsBannedWord_detectsWordEmbeddedInMessage() {
         assertTrue(checker.containsBannedWord(message));
     }
 
+    @ParameterizedTest
+    @ValueSource(strings = {"b3sig78jv", "9c0hej6x", "lbl276sz", "p4e84", "hy8m", "ikqy2y"})
+    void containsBannedWord_detectsLoadTestWords(String word) {
+        BannedWordChecker checker = new BannedWordChecker(BANNED_WORDS);
+        assertTrue(checker.containsBannedWord(word));
+    }
+
     @Test
     void containsBannedWord_returnsFalseForCleanOrEmptyInput() {
         BannedWordChecker checker = new BannedWordChecker(BANNED_WORDS);
diff --git a/apps/backend/src/test/resources/application.properties b/apps/backend/src/test/resources/application.properties
index 862048be..6a38e3a8 100644
--- a/apps/backend/src/test/resources/application.properties
+++ b/apps/backend/src/test/resources/application.properties
@@ -12,8 +12,11 @@ springdoc.swagger-ui.path=/api/swagger-ui.html
 spring.servlet.multipart.max-file-size=5MB
 spring.servlet.multipart.max-request-size=5MB
 server.tomcat.max-http-form-post-size=5MB
+aws.s3.bucket=test-bucket
+aws.s3.region=ap-northeast-2
+aws.s3.endpoint=
+aws.s3.path-style-access=false
 management.endpoints.web.exposure.include=health,info,metrics,prometheus
 management.endpoints.access.default=read-only
 management.endpoint.prometheus.access=read-only
 management.prometheus.metrics.export.enabled=true
-
diff --git a/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js b/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
index 9be9d1b4..fd959b17 100644
--- a/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
@@ -102,4 +102,5 @@ describe('useRoomList', () => {
     expect(result.current.error).toBeNull();
     expect(result.current.rooms).toEqual([{ _id: 'room-1' }]);
   });
+
 });

From a81291be1761d24d920e7db39cde8dfa0dfb102d Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 16:10:00 +0900
Subject: [PATCH 11/32] perf: add room timestamp message index

---
 .../backend/src/main/java/com/ktb/chatapp/model/Message.java | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java b/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
index eb283faf..dba4283c 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
@@ -9,6 +9,7 @@
 import org.springframework.data.annotation.Id;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
+import org.springframework.data.mongodb.core.index.CompoundIndex;
 
 import java.time.LocalDateTime;
 import java.util.ArrayList;
@@ -26,6 +27,10 @@
 @NoArgsConstructor
 @AllArgsConstructor
 @Document(collection = "messages")
+@CompoundIndex(
+    name = "room_timestamp_idx",
+    def = "{'room': 1, 'timestamp': 1}"
+)
 public class Message {
 
     @Id

From 441479a4636df9e2f5b45cb88cdbfc266edd15ff Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 16:11:06 +0900
Subject: [PATCH 12/32] perf: add room detail load test

---
 performance/room-detail/README.md             |  24 ++++
 performance/room-detail/fixture.js            | 112 ++++++++++++++++++
 performance/room-detail/results/.gitkeep      |   1 +
 .../results/baseline-p10-m100k-vu1.json       |  21 ++++
 .../results/baseline-p10-m100k-vu10.json      |  21 ++++
 .../results/baseline-p10-m10k-vu1.json        |  21 ++++
 .../results/baseline-p10-m10k-vu10.json       |  21 ++++
 .../results/baseline-p10-m1k-vu1.json         |  21 ++++
 .../results/baseline-p10-m1k-vu10.json        |  21 ++++
 .../results/baseline-p10-m1k-vu30.json        |  21 ++++
 .../results/baseline-p100-m100k-vu1.json      |  21 ++++
 .../results/baseline-p100-m100k-vu10.json     |  21 ++++
 .../results/baseline-p100-m10k-vu1.json       |  21 ++++
 .../results/baseline-p100-m10k-vu10.json      |  21 ++++
 .../results/baseline-p100-m1k-vu1.json        |  21 ++++
 .../results/baseline-p100-m1k-vu10.json       |  21 ++++
 .../results/baseline-p500-m100k-vu1.json      |  21 ++++
 .../results/baseline-p500-m100k-vu10.json     |  21 ++++
 .../results/baseline-p500-m10k-vu1.json       |  21 ++++
 .../results/baseline-p500-m10k-vu10.json      |  21 ++++
 .../results/baseline-p500-m1k-vu1.json        |  21 ++++
 .../results/baseline-p500-m1k-vu10.json       |  21 ++++
 .../results/exp1-after-p10-m100k-explain.json |   1 +
 .../results/exp1-after-p10-m100k-vu1.json     |  21 ++++
 .../results/exp1-after-p10-m100k-vu10.json    |  21 ++++
 .../results/exp1-after-p10-m10k-explain.json  |   1 +
 .../results/exp1-after-p10-m10k-vu1.json      |  21 ++++
 .../results/exp1-after-p10-m10k-vu10.json     |  21 ++++
 .../results/exp1-after-p10-m1k-explain.json   |   1 +
 .../results/exp1-after-p10-m1k-vu1.json       |  21 ++++
 .../results/exp1-after-p10-m1k-vu10.json      |  21 ++++
 .../results/exp1-after-p10-m1k-vu30.json      |  21 ++++
 .../exp1-before-p10-m100k-explain.json        |   1 +
 .../results/exp1-before-p10-m100k-vu1.json    |  21 ++++
 .../results/exp1-before-p10-m100k-vu10.json   |  21 ++++
 .../results/exp1-before-p10-m10k-explain.json |   1 +
 .../results/exp1-before-p10-m10k-vu1.json     |  21 ++++
 .../results/exp1-before-p10-m10k-vu10.json    |  21 ++++
 .../results/exp1-before-p10-m1k-explain.json  |   1 +
 .../results/exp1-before-p10-m1k-vu1.json      |  21 ++++
 .../results/exp1-before-p10-m1k-vu10.json     |  21 ++++
 .../results/exp1-before-p10-m1k-vu30.json     |  21 ++++
 .../results/room-detail-baseline-report.md    |  83 +++++++++++++
 performance/room-detail/room-detail.js        | 108 +++++++++++++++++
 44 files changed, 1027 insertions(+)
 create mode 100644 performance/room-detail/README.md
 create mode 100644 performance/room-detail/fixture.js
 create mode 100644 performance/room-detail/results/.gitkeep
 create mode 100644 performance/room-detail/results/baseline-p10-m100k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p10-m100k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p10-m10k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p10-m10k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p10-m1k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p10-m1k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p10-m1k-vu30.json
 create mode 100644 performance/room-detail/results/baseline-p100-m100k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p100-m100k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p100-m10k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p100-m10k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p100-m1k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p100-m1k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p500-m100k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p500-m100k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p500-m10k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p500-m10k-vu10.json
 create mode 100644 performance/room-detail/results/baseline-p500-m1k-vu1.json
 create mode 100644 performance/room-detail/results/baseline-p500-m1k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m100k-explain.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m100k-vu1.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m100k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m10k-explain.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m10k-vu1.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m10k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m1k-explain.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m1k-vu1.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m1k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-after-p10-m1k-vu30.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m100k-explain.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m100k-vu1.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m100k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m10k-explain.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m10k-vu1.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m10k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m1k-explain.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m1k-vu1.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m1k-vu10.json
 create mode 100644 performance/room-detail/results/exp1-before-p10-m1k-vu30.json
 create mode 100644 performance/room-detail/results/room-detail-baseline-report.md
 create mode 100644 performance/room-detail/room-detail.js

diff --git a/performance/room-detail/README.md b/performance/room-detail/README.md
new file mode 100644
index 00000000..fd8a3dbe
--- /dev/null
+++ b/performance/room-detail/README.md
@@ -0,0 +1,24 @@
+# Room Detail HTTP-only bottleneck test
+
+This track measures only `GET /api/rooms/{roomId}`. Its fixture documents use a
+unique `FIXTURE_ID` prefix and cleanup deletes only those documents plus the one
+dedicated login account.
+
+The nine fixture conditions are the full combination of 10, 100, and 500
+participants with 1k, 10k, and 100k messages. Run 1 VU for 30 seconds, then 10,
+30, 50, and 100 VUs for one minute each. Stop increasing a condition after
+errors exceed 1%, p95 exceeds twice its smoke value, a timeout occurs, or the
+MongoDB wait queue appears.
+
+Required environment variables are `FIXTURE_ID`, `ROOM_ID`,
+`EXPECTED_PARTICIPANTS`, `EXPECTED_RECENT_MESSAGES`, `LOAD_TEST_EMAIL`, and
+`LOAD_TEST_PASSWORD`. Example:
+
+```sh
+k6 run -e FIXTURE_ID=room-detail-20260811 \
+  -e ROOM_ID=room-detail-20260811:room:p10-m1k \
+  -e EXPECTED_PARTICIPANTS=10 -e EXPECTED_RECENT_MESSAGES=1000 \
+  -e LOAD_TEST_EMAIL=room-detail-20260811@fixture.invalid \
+  -e LOAD_TEST_PASSWORD='RoomDetail1234!' -e VUS=1 -e DURATION=30s \
+  performance/room-detail/room-detail.js
+```
diff --git a/performance/room-detail/fixture.js b/performance/room-detail/fixture.js
new file mode 100644
index 00000000..65f7f676
--- /dev/null
+++ b/performance/room-detail/fixture.js
@@ -0,0 +1,112 @@
+const database = db.getSiblingDB('bootcamp-chat');
+const fixtureId = process.env.FIXTURE_ID;
+const action = process.env.ACTION || 'create';
+const email = process.env.LOAD_TEST_EMAIL;
+const prefix = `${fixtureId}:`;
+
+if (!fixtureId) throw new Error('FIXTURE_ID is required');
+
+const conditions = [
+  { key: 'p10-m1k', participants: 10, messages: 1000 },
+  { key: 'p10-m10k', participants: 10, messages: 10000 },
+  { key: 'p10-m100k', participants: 10, messages: 100000 },
+  { key: 'p100-m1k', participants: 100, messages: 1000 },
+  { key: 'p100-m10k', participants: 100, messages: 10000 },
+  { key: 'p100-m100k', participants: 100, messages: 100000 },
+  { key: 'p500-m1k', participants: 500, messages: 1000 },
+  { key: 'p500-m10k', participants: 500, messages: 10000 },
+  { key: 'p500-m100k', participants: 500, messages: 100000 },
+];
+
+function cleanup(removeOwner = true) {
+  const roomIds = conditions.map((condition) => `${prefix}room:${condition.key}`);
+  const removed = {
+    messages: database.messages.deleteMany({ room: { $in: roomIds } }).deletedCount,
+    rooms: database.rooms.deleteMany({ _id: { $regex: `^${prefix}` } }).deletedCount,
+    users: database.users.deleteMany({
+      $or: [
+        { _id: { $regex: `^${prefix}` } },
+        ...(removeOwner && email ? [{ email }] : []),
+      ],
+    }).deletedCount,
+  };
+  printjson({ action: 'cleanup', fixtureId, removed });
+}
+
+function create() {
+  cleanup(false);
+  const owner = database.users.findOne({ email });
+  if (!owner) throw new Error(`Load-test owner not found: ${email}`);
+  const selected = conditions.find((condition) => condition.key === process.env.CONDITION);
+  if (!selected) throw new Error(`Unknown CONDITION: ${process.env.CONDITION}`);
+  const now = new Date();
+  const users = [];
+  for (let index = 0; index < selected.participants; index += 1) {
+    users.push({
+      _id: `${prefix}user:${index}`,
+      name: `Room detail participant ${index}`,
+      email: `${fixtureId}-${index}@fixture.invalid`,
+      password: 'not-used',
+      createdAt: now,
+      updatedAt: now,
+      isOnline: false,
+    });
+  }
+  database.users.insertMany(users, { ordered: false });
+
+  for (const condition of [selected]) {
+    const roomId = `${prefix}room:${condition.key}`;
+    const participantIds = users.slice(0, condition.participants).map((user) => user._id);
+    database.rooms.insertOne({
+      _id: roomId,
+      name: `Room detail ${condition.key}`,
+      creator: owner._id,
+      hasPassword: true,
+      password: '$2a$10$N9qo8uLOickgx2ZMRZoMyeIjZAgcfl7p92ldGxad68LJZdL17lhWy',
+      participantIds,
+      createdAt: now,
+    });
+    for (let offset = 0; offset < condition.messages; offset += 1000) {
+      const batchSize = Math.min(1000, condition.messages - offset);
+      const messages = Array.from({ length: batchSize }, (_, index) => ({
+        _id: `${prefix}message:${condition.key}:${offset + index}`,
+        room: roomId,
+        sender: participantIds[(offset + index) % participantIds.length],
+        content: `fixture message ${offset + index}`,
+        type: 'TEXT',
+        timestamp: now,
+        mentions: [],
+        reactions: {},
+        readers: [],
+        metadata: {},
+      }));
+      database.messages.insertMany(messages, { ordered: false });
+    }
+  }
+  printjson({ action: 'create', fixtureId, ownerId: owner._id, condition: selected });
+}
+
+function refresh() {
+  const roomId = `${prefix}room:${process.env.CONDITION}`;
+  const result = database.messages.updateMany({ room: roomId }, { $set: { timestamp: new Date() } });
+  printjson({ action: 'refresh', roomId, matched: result.matchedCount, modified: result.modifiedCount });
+}
+
+function explain() {
+  const roomId = `${prefix}room:${process.env.CONDITION}`;
+  const since = new Date(Date.now() - (30 * 60 * 1000));
+  const result = database.messages.find({ room: roomId, timestamp: { $gte: since } })
+    .explain('executionStats');
+  printjson({
+    action: 'explain',
+    roomId,
+    winningPlan: result.queryPlanner.winningPlan,
+    executionStats: result.executionStats,
+  });
+}
+
+if (action === 'create') create();
+else if (action === 'cleanup') cleanup();
+else if (action === 'refresh') refresh();
+else if (action === 'explain') explain();
+else throw new Error(`Unknown ACTION: ${action}`);
diff --git a/performance/room-detail/results/.gitkeep b/performance/room-detail/results/.gitkeep
new file mode 100644
index 00000000..8b137891
--- /dev/null
+++ b/performance/room-detail/results/.gitkeep
@@ -0,0 +1 @@
+
diff --git a/performance/room-detail/results/baseline-p10-m100k-vu1.json b/performance/room-detail/results/baseline-p10-m100k-vu1.json
new file mode 100644
index 00000000..456a6e59
--- /dev/null
+++ b/performance/room-detail/results/baseline-p10-m100k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "baseline-p10-m100k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m100k",
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new file mode 100644
index 00000000..e07dba97
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index 00000000..bcb872b0
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index 00000000..198f466c
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index 00000000..1c43fc82
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new file mode 100644
index 00000000..d718a81f
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new file mode 100644
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diff --git a/performance/room-detail/results/exp1-after-p10-m100k-vu1.json b/performance/room-detail/results/exp1-after-p10-m100k-vu1.json
new file mode 100644
index 00000000..e133e8aa
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m100k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m100k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m100k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 100000,
+  "vus": 1,
+  "duration": "30s",
+  "startedAt": "2026-08-11T06:24:58.606Z",
+  "endedAt": "2026-08-11T06:25:28.630Z",
+  "requests": 2446,
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+}
diff --git a/performance/room-detail/results/exp1-after-p10-m100k-vu10.json b/performance/room-detail/results/exp1-after-p10-m100k-vu10.json
new file mode 100644
index 00000000..7eceb71a
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m100k-vu10.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m100k-vu10",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m100k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 100000,
+  "vus": 10,
+  "duration": "1m",
+  "startedAt": "2026-08-11T06:25:28.916Z",
+  "endedAt": "2026-08-11T06:26:28.952Z",
+  "requests": 29170,
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+}
diff --git a/performance/room-detail/results/exp1-after-p10-m10k-explain.json b/performance/room-detail/results/exp1-after-p10-m10k-explain.json
new file mode 100644
index 00000000..39e8962e
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m10k-explain.json
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+{"roomId":"room-detail-20260811:room:p10-m10k","result":{"explainVersion":"1","queryPlanner":{"namespace":"bootcamp-chat.messages","parsedQuery":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m10k"}},{"timestamp":{"$gte":"2026-08-11T05:53:25.773Z"}}]},"indexFilterSet":false,"queryHash":"333F96BB","planCacheShapeHash":"333F96BB","planCacheKey":"3FD813D9","optimizationTimeMillis":0,"maxIndexedOrSolutionsReached":false,"maxIndexedAndSolutionsReached":false,"maxScansToExplodeReached":false,"prunedSimilarIndexes":false,"winningPlan":{"isCached":false,"stage":"FETCH","nss":"bootcamp-chat.messages","inputStage":{"stage":"IXSCAN","nss":"bootcamp-chat.messages","keyPattern":{"room":1,"timestamp":1},"indexName":"room_timestamp_room_detail_exp1","isMultiKey":false,"multiKeyPaths":{"room":[],"timestamp":[]},"isUnique":false,"isSparse":false,"isPartial":false,"indexVersion":2,"direction":"forward","indexBounds":{"room":["[\"room-detail-20260811:room:p10-m10k\", \"room-detail-20260811:room:p10-m10k\"]"],"timestamp":["[new Date(1786427605773), new Date(9223372036854775807)]"]}}},"rejectedPlans":[]},"executionStats":{"executionSuccess":true,"nReturned":10000,"executionTimeMillis":4,"totalKeysExamined":10000,"totalDocsExamined":10000,"executionStages":{"isCached":false,"stage":"FETCH","nReturned":10000,"executionTimeMillisEstimate":0,"works":10001,"advanced":10000,"needTime":0,"needYield":0,"saveState":0,"restoreState":0,"isEOF":1,"nss":"bootcamp-chat.messages","docsExamined":10000,"alreadyHasObj":0,"inputStage":{"stage":"IXSCAN","nReturned":10000,"executionTimeMillisEstimate":0,"works":10001,"advanced":10000,"needTime":0,"needYield":0,"saveState":0,"restoreState":0,"isEOF":1,"nss":"bootcamp-chat.messages","keyPattern":{"room":1,"timestamp":1},"indexName":"room_timestamp_room_detail_exp1","isMultiKey":false,"multiKeyPaths":{"room":[],"timestamp":[]},"isUnique":false,"isSparse":false,"isPartial":false,"indexVersion":2,"direction":"forward","indexBounds":{"room":["[\"room-detail-20260811:room:p10-m10k\", \"room-detail-20260811:room:p10-m10k\"]"],"timestamp":["[new Date(1786427605773), new Date(9223372036854775807)]"]},"keysExamined":10000,"seeks":1,"dupsTested":0,"dupsDropped":0,"peakTrackedMemBytes":0}}},"queryShapeHash":"B8001078FD5D895872A94C0C4367B62C92BA0E7F04C12F8927AA604EC9A6A303","command":{"find":"messages","filter":{"room":"room-detail-20260811:room:p10-m10k","timestamp":{"$gte":"2026-08-11T05:53:25.773Z"}},"$db":"bootcamp-chat"},"serverInfo":{"host":"e70b4f942162","port":27017,"version":"8.3.4","gitVersion":"4b03e7daaa316c78b9bf433046dba81637d581c0"},"serverParameters":{"internalQueryFacetBufferSizeBytes":104857600,"internalDocumentSourceGroupMaxMemoryBytes":104857600,"internalQueryMaxBlockingSortMemoryUsageBytes":104857600,"internalDocumentSourceSetWindowFieldsMaxMemoryBytes":104857600,"internalQueryFacetMaxOutputDocSizeBytes":104857600,"internalLookupStageIntermediateDocumentMaxSizeBytes":104857600,"internalQueryProhibitBlockingMergeOnMongoS":0,"internalQueryMaxAddToSetBytes":104857600,"internalQueryFrameworkControl":"trySbeRestricted","internalQueryPlannerIgnoreIndexWithCollationForRegex":1},"ok":1}}
diff --git a/performance/room-detail/results/exp1-after-p10-m10k-vu1.json b/performance/room-detail/results/exp1-after-p10-m10k-vu1.json
new file mode 100644
index 00000000..a5858d9a
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m10k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m10k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m10k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 10000,
+  "vus": 1,
+  "duration": "30s",
+  "startedAt": "2026-08-11T06:23:25.848Z",
+  "endedAt": "2026-08-11T06:23:55.877Z",
+  "requests": 8590,
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+}
diff --git a/performance/room-detail/results/exp1-after-p10-m10k-vu10.json b/performance/room-detail/results/exp1-after-p10-m10k-vu10.json
new file mode 100644
index 00000000..48bbacef
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m10k-vu10.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m10k-vu10",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m10k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
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+  "recentMessages": 10000,
+  "vus": 10,
+  "duration": "1m",
+  "startedAt": "2026-08-11T06:23:56.339Z",
+  "endedAt": "2026-08-11T06:24:56.370Z",
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+}
diff --git a/performance/room-detail/results/exp1-after-p10-m1k-explain.json b/performance/room-detail/results/exp1-after-p10-m1k-explain.json
new file mode 100644
index 00000000..8972128f
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m1k-explain.json
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+{"roomId":"room-detail-20260811:room:p10-m1k","result":{"explainVersion":"1","queryPlanner":{"namespace":"bootcamp-chat.messages","parsedQuery":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m1k"}},{"timestamp":{"$gte":"2026-08-11T05:50:52.940Z"}}]},"indexFilterSet":false,"queryHash":"333F96BB","planCacheShapeHash":"333F96BB","planCacheKey":"3FD813D9","optimizationTimeMillis":0,"maxIndexedOrSolutionsReached":false,"maxIndexedAndSolutionsReached":false,"maxScansToExplodeReached":false,"prunedSimilarIndexes":false,"winningPlan":{"isCached":false,"stage":"FETCH","nss":"bootcamp-chat.messages","inputStage":{"stage":"IXSCAN","nss":"bootcamp-chat.messages","keyPattern":{"room":1,"timestamp":1},"indexName":"room_timestamp_room_detail_exp1","isMultiKey":false,"multiKeyPaths":{"room":[],"timestamp":[]},"isUnique":false,"isSparse":false,"isPartial":false,"indexVersion":2,"direction":"forward","indexBounds":{"room":["[\"room-detail-20260811:room:p10-m1k\", \"room-detail-20260811:room:p10-m1k\"]"],"timestamp":["[new Date(1786427452940), new Date(9223372036854775807)]"]}}},"rejectedPlans":[]},"executionStats":{"executionSuccess":true,"nReturned":1000,"executionTimeMillis":2,"totalKeysExamined":1000,"totalDocsExamined":1000,"executionStages":{"isCached":false,"stage":"FETCH","nReturned":1000,"executionTimeMillisEstimate":0,"works":1001,"advanced":1000,"needTime":0,"needYield":0,"saveState":0,"restoreState":0,"isEOF":1,"nss":"bootcamp-chat.messages","docsExamined":1000,"alreadyHasObj":0,"inputStage":{"stage":"IXSCAN","nReturned":1000,"executionTimeMillisEstimate":0,"works":1001,"advanced":1000,"needTime":0,"needYield":0,"saveState":0,"restoreState":0,"isEOF":1,"nss":"bootcamp-chat.messages","keyPattern":{"room":1,"timestamp":1},"indexName":"room_timestamp_room_detail_exp1","isMultiKey":false,"multiKeyPaths":{"room":[],"timestamp":[]},"isUnique":false,"isSparse":false,"isPartial":false,"indexVersion":2,"direction":"forward","indexBounds":{"room":["[\"room-detail-20260811:room:p10-m1k\", \"room-detail-20260811:room:p10-m1k\"]"],"timestamp":["[new Date(1786427452940), new Date(9223372036854775807)]"]},"keysExamined":1000,"seeks":1,"dupsTested":0,"dupsDropped":0,"peakTrackedMemBytes":0}}},"queryShapeHash":"B8001078FD5D895872A94C0C4367B62C92BA0E7F04C12F8927AA604EC9A6A303","command":{"find":"messages","filter":{"room":"room-detail-20260811:room:p10-m1k","timestamp":{"$gte":"2026-08-11T05:50:52.940Z"}},"$db":"bootcamp-chat"},"serverInfo":{"host":"e70b4f942162","port":27017,"version":"8.3.4","gitVersion":"4b03e7daaa316c78b9bf433046dba81637d581c0"},"serverParameters":{"internalQueryFacetBufferSizeBytes":104857600,"internalDocumentSourceGroupMaxMemoryBytes":104857600,"internalQueryMaxBlockingSortMemoryUsageBytes":104857600,"internalDocumentSourceSetWindowFieldsMaxMemoryBytes":104857600,"internalQueryFacetMaxOutputDocSizeBytes":104857600,"internalLookupStageIntermediateDocumentMaxSizeBytes":104857600,"internalQueryProhibitBlockingMergeOnMongoS":0,"internalQueryMaxAddToSetBytes":104857600,"internalQueryFrameworkControl":"trySbeRestricted","internalQueryPlannerIgnoreIndexWithCollationForRegex":1},"ok":1}}
diff --git a/performance/room-detail/results/exp1-after-p10-m1k-vu1.json b/performance/room-detail/results/exp1-after-p10-m1k-vu1.json
new file mode 100644
index 00000000..87592606
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m1k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m1k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m1k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "30s",
+  "startedAt": "2026-08-11T06:20:53.268Z",
+  "endedAt": "2026-08-11T06:21:23.331Z",
+  "requests": 9928,
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+}
diff --git a/performance/room-detail/results/exp1-after-p10-m1k-vu10.json b/performance/room-detail/results/exp1-after-p10-m1k-vu10.json
new file mode 100644
index 00000000..eb7fe183
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m1k-vu10.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m1k-vu10",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m1k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
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+}
diff --git a/performance/room-detail/results/exp1-after-p10-m1k-vu30.json b/performance/room-detail/results/exp1-after-p10-m1k-vu30.json
new file mode 100644
index 00000000..5ce766ae
--- /dev/null
+++ b/performance/room-detail/results/exp1-after-p10-m1k-vu30.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-after-p10-m1k-vu30",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m1k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
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+  "vus": 30,
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+  "startedAt": "2026-08-11T06:22:24.458Z",
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+}
diff --git a/performance/room-detail/results/exp1-before-p10-m100k-explain.json b/performance/room-detail/results/exp1-before-p10-m100k-explain.json
new file mode 100644
index 00000000..4a7c03fb
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m100k-explain.json
@@ -0,0 +1 @@
+{"roomId":"room-detail-20260811:room:p10-m100k","result":{"explainVersion":"1","queryPlanner":{"namespace":"bootcamp-chat.messages","parsedQuery":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m100k"}},{"timestamp":{"$gte":"2026-08-11T05:45:51.180Z"}}]},"indexFilterSet":false,"queryHash":"333F96BB","planCacheShapeHash":"333F96BB","planCacheKey":"5DF97F34","optimizationTimeMillis":0,"maxIndexedOrSolutionsReached":false,"maxIndexedAndSolutionsReached":false,"maxScansToExplodeReached":false,"prunedSimilarIndexes":false,"winningPlan":{"isCached":false,"stage":"COLLSCAN","filter":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m100k"}},{"timestamp":{"$gte":"2026-08-11T05:45:51.180Z"}}]},"nss":"bootcamp-chat.messages","direction":"forward"},"rejectedPlans":[]},"executionStats":{"executionSuccess":true,"nReturned":100000,"executionTimeMillis":29,"totalKeysExamined":0,"totalDocsExamined":100055,"executionStages":{"isCached":false,"stage":"COLLSCAN","filter":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m100k"}},{"timestamp":{"$gte":"2026-08-11T05:45:51.180Z"}}]},"nReturned":100000,"executionTimeMillisEstimate":18,"works":100056,"advanced":100000,"needTime":55,"needYield":0,"saveState":2,"restoreState":2,"isEOF":1,"nss":"bootcamp-chat.messages","direction":"forward","docsExamined":100055}},"queryShapeHash":"B8001078FD5D895872A94C0C4367B62C92BA0E7F04C12F8927AA604EC9A6A303","command":{"find":"messages","filter":{"room":"room-detail-20260811:room:p10-m100k","timestamp":{"$gte":"2026-08-11T05:45:51.180Z"}},"$db":"bootcamp-chat"},"serverInfo":{"host":"e70b4f942162","port":27017,"version":"8.3.4","gitVersion":"4b03e7daaa316c78b9bf433046dba81637d581c0"},"serverParameters":{"internalQueryFacetBufferSizeBytes":104857600,"internalDocumentSourceGroupMaxMemoryBytes":104857600,"internalQueryMaxBlockingSortMemoryUsageBytes":104857600,"internalDocumentSourceSetWindowFieldsMaxMemoryBytes":104857600,"internalQueryFacetMaxOutputDocSizeBytes":104857600,"internalLookupStageIntermediateDocumentMaxSizeBytes":104857600,"internalQueryProhibitBlockingMergeOnMongoS":0,"internalQueryMaxAddToSetBytes":104857600,"internalQueryFrameworkControl":"trySbeRestricted","internalQueryPlannerIgnoreIndexWithCollationForRegex":1},"ok":1}}
diff --git a/performance/room-detail/results/exp1-before-p10-m100k-vu1.json b/performance/room-detail/results/exp1-before-p10-m100k-vu1.json
new file mode 100644
index 00000000..8e268e33
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m100k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m100k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m100k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
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+  "startedAt": "2026-08-11T06:15:51.278Z",
+  "endedAt": "2026-08-11T06:16:21.316Z",
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diff --git a/performance/room-detail/results/exp1-before-p10-m100k-vu10.json b/performance/room-detail/results/exp1-before-p10-m100k-vu10.json
new file mode 100644
index 00000000..4a097481
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m100k-vu10.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m100k-vu10",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m100k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
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+  "requests": 15017,
+  "rps": 250.00807444337119,
+  "averageMs": 39.74697849104343,
+  "p95Ms": 57.202,
+  "p99Ms": 78.83115999999997,
+  "maxMs": 214.399,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1756
+}
diff --git a/performance/room-detail/results/exp1-before-p10-m10k-explain.json b/performance/room-detail/results/exp1-before-p10-m10k-explain.json
new file mode 100644
index 00000000..f9d3b0eb
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m10k-explain.json
@@ -0,0 +1 @@
+{"roomId":"room-detail-20260811:room:p10-m10k","result":{"explainVersion":"1","queryPlanner":{"namespace":"bootcamp-chat.messages","parsedQuery":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m10k"}},{"timestamp":{"$gte":"2026-08-11T05:44:17.993Z"}}]},"indexFilterSet":false,"queryHash":"333F96BB","planCacheShapeHash":"333F96BB","planCacheKey":"5DF97F34","optimizationTimeMillis":0,"maxIndexedOrSolutionsReached":false,"maxIndexedAndSolutionsReached":false,"maxScansToExplodeReached":false,"prunedSimilarIndexes":false,"winningPlan":{"isCached":false,"stage":"COLLSCAN","filter":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m10k"}},{"timestamp":{"$gte":"2026-08-11T05:44:17.993Z"}}]},"nss":"bootcamp-chat.messages","direction":"forward"},"rejectedPlans":[]},"executionStats":{"executionSuccess":true,"nReturned":10000,"executionTimeMillis":3,"totalKeysExamined":0,"totalDocsExamined":10055,"executionStages":{"isCached":false,"stage":"COLLSCAN","filter":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m10k"}},{"timestamp":{"$gte":"2026-08-11T05:44:17.993Z"}}]},"nReturned":10000,"executionTimeMillisEstimate":1,"works":10056,"advanced":10000,"needTime":55,"needYield":0,"saveState":0,"restoreState":0,"isEOF":1,"nss":"bootcamp-chat.messages","direction":"forward","docsExamined":10055}},"queryShapeHash":"B8001078FD5D895872A94C0C4367B62C92BA0E7F04C12F8927AA604EC9A6A303","command":{"find":"messages","filter":{"room":"room-detail-20260811:room:p10-m10k","timestamp":{"$gte":"2026-08-11T05:44:17.993Z"}},"$db":"bootcamp-chat"},"serverInfo":{"host":"e70b4f942162","port":27017,"version":"8.3.4","gitVersion":"4b03e7daaa316c78b9bf433046dba81637d581c0"},"serverParameters":{"internalQueryFacetBufferSizeBytes":104857600,"internalDocumentSourceGroupMaxMemoryBytes":104857600,"internalQueryMaxBlockingSortMemoryUsageBytes":104857600,"internalDocumentSourceSetWindowFieldsMaxMemoryBytes":104857600,"internalQueryFacetMaxOutputDocSizeBytes":104857600,"internalLookupStageIntermediateDocumentMaxSizeBytes":104857600,"internalQueryProhibitBlockingMergeOnMongoS":0,"internalQueryMaxAddToSetBytes":104857600,"internalQueryFrameworkControl":"trySbeRestricted","internalQueryPlannerIgnoreIndexWithCollationForRegex":1},"ok":1}}
diff --git a/performance/room-detail/results/exp1-before-p10-m10k-vu1.json b/performance/room-detail/results/exp1-before-p10-m10k-vu1.json
new file mode 100644
index 00000000..838c9724
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m10k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m10k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m10k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 10000,
+  "vus": 1,
+  "duration": "30s",
+  "startedAt": "2026-08-11T06:14:18.109Z",
+  "endedAt": "2026-08-11T06:14:48.151Z",
+  "requests": 5760,
+  "rps": 191.7349264641633,
+  "averageMs": 5.047486805555543,
+  "p95Ms": 6.470299999999998,
+  "p99Ms": 13.29164,
+  "maxMs": 220.326,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1753
+}
diff --git a/performance/room-detail/results/exp1-before-p10-m10k-vu10.json b/performance/room-detail/results/exp1-before-p10-m10k-vu10.json
new file mode 100644
index 00000000..de2397eb
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m10k-vu10.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m10k-vu10",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m10k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 10000,
+  "vus": 10,
+  "duration": "1m",
+  "startedAt": "2026-08-11T06:14:48.763Z",
+  "endedAt": "2026-08-11T06:15:48.806Z",
+  "requests": 75693,
+  "rps": 1260.6585253238172,
+  "averageMs": 7.765354510985207,
+  "p95Ms": 11.71,
+  "p99Ms": 17.03308,
+  "maxMs": 115.512,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1753
+}
diff --git a/performance/room-detail/results/exp1-before-p10-m1k-explain.json b/performance/room-detail/results/exp1-before-p10-m1k-explain.json
new file mode 100644
index 00000000..e55e7ce0
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m1k-explain.json
@@ -0,0 +1 @@
+{"roomId":"room-detail-20260811:room:p10-m1k","result":{"explainVersion":"1","queryPlanner":{"namespace":"bootcamp-chat.messages","parsedQuery":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m1k"}},{"timestamp":{"$gte":"2026-08-11T05:41:19.572Z"}}]},"indexFilterSet":false,"queryHash":"333F96BB","planCacheShapeHash":"333F96BB","planCacheKey":"5DF97F34","optimizationTimeMillis":0,"maxIndexedOrSolutionsReached":false,"maxIndexedAndSolutionsReached":false,"maxScansToExplodeReached":false,"prunedSimilarIndexes":false,"winningPlan":{"isCached":false,"stage":"COLLSCAN","filter":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m1k"}},{"timestamp":{"$gte":"2026-08-11T05:41:19.572Z"}}]},"nss":"bootcamp-chat.messages","direction":"forward"},"rejectedPlans":[]},"executionStats":{"executionSuccess":true,"nReturned":1000,"executionTimeMillis":4,"totalKeysExamined":0,"totalDocsExamined":1055,"executionStages":{"isCached":false,"stage":"COLLSCAN","filter":{"$and":[{"room":{"$eq":"room-detail-20260811:room:p10-m1k"}},{"timestamp":{"$gte":"2026-08-11T05:41:19.572Z"}}]},"nReturned":1000,"executionTimeMillisEstimate":0,"works":1056,"advanced":1000,"needTime":55,"needYield":0,"saveState":0,"restoreState":0,"isEOF":1,"nss":"bootcamp-chat.messages","direction":"forward","docsExamined":1055}},"queryShapeHash":"B8001078FD5D895872A94C0C4367B62C92BA0E7F04C12F8927AA604EC9A6A303","command":{"find":"messages","filter":{"room":"room-detail-20260811:room:p10-m1k","timestamp":{"$gte":"2026-08-11T05:41:19.572Z"}},"$db":"bootcamp-chat"},"serverInfo":{"host":"e70b4f942162","port":27017,"version":"8.3.4","gitVersion":"4b03e7daaa316c78b9bf433046dba81637d581c0"},"serverParameters":{"internalQueryFacetBufferSizeBytes":104857600,"internalDocumentSourceGroupMaxMemoryBytes":104857600,"internalQueryMaxBlockingSortMemoryUsageBytes":104857600,"internalDocumentSourceSetWindowFieldsMaxMemoryBytes":104857600,"internalQueryFacetMaxOutputDocSizeBytes":104857600,"internalLookupStageIntermediateDocumentMaxSizeBytes":104857600,"internalQueryProhibitBlockingMergeOnMongoS":0,"internalQueryMaxAddToSetBytes":104857600,"internalQueryFrameworkControl":"trySbeRestricted","internalQueryPlannerIgnoreIndexWithCollationForRegex":1},"ok":1}}
diff --git a/performance/room-detail/results/exp1-before-p10-m1k-vu1.json b/performance/room-detail/results/exp1-before-p10-m1k-vu1.json
new file mode 100644
index 00000000..b5372e66
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m1k-vu1.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m1k-vu1",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m1k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "30s",
+  "startedAt": "2026-08-11T06:11:19.691Z",
+  "endedAt": "2026-08-11T06:11:49.728Z",
+  "requests": 13002,
+  "rps": 432.87415890090585,
+  "averageMs": 2.1988337178895487,
+  "p95Ms": 2.964949999999999,
+  "p99Ms": 4.416909999999999,
+  "maxMs": 26.398,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1750
+}
diff --git a/performance/room-detail/results/exp1-before-p10-m1k-vu10.json b/performance/room-detail/results/exp1-before-p10-m1k-vu10.json
new file mode 100644
index 00000000..023a7542
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m1k-vu10.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m1k-vu10",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m1k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "1m",
+  "startedAt": "2026-08-11T06:11:50.157Z",
+  "endedAt": "2026-08-11T06:12:50.181Z",
+  "requests": 134609,
+  "rps": 2242.619625761465,
+  "averageMs": 4.359470867475355,
+  "p95Ms": 6.219,
+  "p99Ms": 8.251839999999998,
+  "maxMs": 114.882,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1750
+}
diff --git a/performance/room-detail/results/exp1-before-p10-m1k-vu30.json b/performance/room-detail/results/exp1-before-p10-m1k-vu30.json
new file mode 100644
index 00000000..2b39022b
--- /dev/null
+++ b/performance/room-detail/results/exp1-before-p10-m1k-vu30.json
@@ -0,0 +1,21 @@
+{
+  "testName": "exp1-before-p10-m1k-vu30",
+  "fixtureId": "room-detail-20260811",
+  "roomId": "room-detail-20260811:room:p10-m1k",
+  "commit": "ce107855630f5ca543228d08ba6330a6fe96ad95",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 30,
+  "duration": "1m",
+  "startedAt": "2026-08-11T06:12:50.744Z",
+  "endedAt": "2026-08-11T06:13:50.773Z",
+  "requests": 138767,
+  "rps": 2311.6940630908052,
+  "averageMs": 12.874351315514495,
+  "p95Ms": 16.219,
+  "p99Ms": 22.523679999999995,
+  "maxMs": 3214.091,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1750
+}
diff --git a/performance/room-detail/results/room-detail-baseline-report.md b/performance/room-detail/results/room-detail-baseline-report.md
new file mode 100644
index 00000000..9d325b6b
--- /dev/null
+++ b/performance/room-detail/results/room-detail-baseline-report.md
@@ -0,0 +1,83 @@
+# Room Detail API standalone bottleneck result
+
+## Run metadata
+
+- Test ID: `room-detail-20260811`
+- Target: local-only `http://localhost:15001`; MongoDB, Redis, Prometheus, and Grafana ran in local Docker
+- Commit: `ce107855630f5ca543228d08ba6330a6fe96ad95` (`upstream/develop` at test start)
+- Measured endpoint: `GET /api/rooms/{roomId}` only; login was performed in k6 `setup` and excluded from custom metrics
+- Measurement window: 2026-08-11 13:56:31–14:21:20 KST
+- Fixture matrix: full 3 × 3 combination of 10/100/500 participants and 1k/10k/100k recent messages
+- Contract checks: HTTP 200, `success=true`, exact room ID, exact participant count/list size, and exact recent-message count
+- Saturation: error rate over 1%, p95 over twice Smoke, timeout, or Mongo wait queue; later VU stages were skipped after saturation
+
+## Load results
+
+| Participants | Messages | Smoke p95 / RPS | Saturation VU | Saturation avg / p95 / p99 / max (ms) | RPS | p95 ÷ Smoke | Error / timeout |
+|---:|---:|---:|---:|---:|---:|---:|---:|
+| 10 | 1k | 15.84 / 98.69 | 30 | 58.60 / 77.72 / 117.23 / 614.04 | 509.58 | 4.91× | 0% / 0 |
+| 10 | 10k | 13.47 / 92.53 | 10 | 68.21 / 84.43 / 99.21 / 155.43 | 146.13 | 6.27× | 0% / 0 |
+| 10 | 100k | 40.67 / 27.87 | 10 | 670.93 / 742.35 / 776.24 / 840.21 | 14.88 | 18.25× | 0% / 0 |
+| 100 | 1k | 54.04 / 24.04 | 10 | 113.22 / 172.12 / 325.13 / 701.47 | 88.02 | 3.18× | 0% / 0 |
+| 100 | 10k | 45.52 / 24.75 | 10 | 119.19 / 188.81 / 285.81 / 429.39 | 83.61 | 4.15× | 0% / 0 |
+| 100 | 100k | 89.34 / 14.21 | 10 | 654.59 / 762.45 / 877.15 / 986.08 | 15.24 | 8.53× | 0% / 0 |
+| 500 | 1k | 306.37 / 5.13 | 10 | 496.47 / 765.54 / 859.55 / 923.61 | 20.08 | 2.50× | 0% / 0 |
+| 500 | 10k | 267.20 / 5.12 | 10 | 440.78 / 556.43 / 993.40 / 1459.47 | 22.60 | 2.08× | 0% / 0 |
+| 500 | 100k | 240.25 / 4.99 | 10 | 876.74 / 1305.58 / 1536.10 / 1655.47 | 11.36 | 5.43× | 0% / 0 |
+
+The 10-participant/1k condition passed 10 VU (p95 25.61 ms, 541.18 RPS) and saturated at 30 VU. Every other condition saturated at 10 VU, so no disallowed later stages were run.
+
+## Query and application diagnosis
+
+1. Recent-message count is a collection scan. The only message index was `_id_`. Every explain used `COLLSCAN`, with `totalKeysExamined=0`. `nReturned` was exactly 1k/10k/100k and `totalDocsExamined` was 1,033–1,053 / 10,033–10,053 / 100,033–100,053; the small excess is unrelated pre-existing/local-worker data. Explain execution time rose from about 2 ms to 27–30 ms at 100k.
+2. Participant mapping is an N+1 query. Each detail request calls one creator `findById` plus one `findById` per participant: 11, 101, and 501 user finds per request. Prometheus range estimates were about 11.1, 103.2, and 513.0 because counter `increase()` extrapolates across scrape boundaries, while source inspection confirms the exact 11/101/501 pattern. The message aggregation remained approximately one command per request.
+3. Response/serialization cost grows with participants. Mean response size was about 1.75 KB, 14.54 KB, and 72.54 KB for 10, 100, and 500 participants. At 500 participants this cost and N+1 dominate enough that Smoke p95 variance masks a monotonic message-size trend.
+4. Request-path logging is a separate fixed application bottleneck. Wildcard CORS configuration logs a multi-line warning for every request. The isolated process emitted about 369 MB of console output during the run, adding synchronous formatting/output pressure unrelated to fixture axes.
+
+## Representative observability
+
+For the worst fixture at 10 VU (500 participants, 100k messages):
+
+- URI timer: average 867.49 ms, max 1,655.14 ms.
+- Process CPU max: 24.9%; live threads max: 37.
+- Heap peak: 131,452,400 bytes (about 125.4 MiB); GC pause sum: about 0.152 s over the stage.
+- Mongo user-find average/max: 0.870/187.32 ms.
+- Mongo message-aggregate average/max: 405.00/939.82 ms.
+- Mongo checked-out connections max: 9; wait queue max: 0; checkout failures: 0.
+- A wait queue max of 1 was observed at 100 participants/1k messages and 100 participants/10k messages at 10 VU; checkout failures and timeouts remained zero.
+
+## Conclusion and next experiments
+
+There are two independent scale-sensitive bottlenecks plus one fixed request-path overhead:
+
+- Message axis: missing `{room: 1, timestamp: 1}` index causes linear document scanning and dominates the 100k-message conditions.
+- Participant axis: participant-by-participant `findById` causes linear Mongo command amplification and dominates 100/500-participant conditions.
+- Constant overhead: repeated CORS warning logging materially reduces local throughput and should be moved to startup/configuration time.
+
+No production optimization was implemented. Following the one-change-per-experiment rule, validate these separately: (A) compound message index, (B) batch creator/participants with `findAllById`, and (C) log the wildcard warning once. Re-run the worst fixture at the same VU and apply the stated 10% p95 improvement/non-regression adoption criteria.
+
+## Experiment 1: compound message index
+
+The index-only comparison ran on 2026-08-11 using the same local backend commit and the p10-m1k, p10-m10k, and p10-m100k fixtures. The temporary index was `room_timestamp_room_detail_exp1`; it was dropped after measurement.
+
+| Fixture | Before p95 (Smoke / 10 VU) | After p95 (Smoke / 10 VU) | p95 change at 10 VU | Explain before → after |
+|---|---:|---:|---:|---|
+| p10-m1k | 2.96 / 6.22 ms | 4.52 / 6.76 ms | +8.7% | COLLSCAN → FETCH/IXSCAN |
+| p10-m10k | 6.47 / 11.71 ms | 4.27 / 7.95 ms | -32.1% | COLLSCAN → FETCH/IXSCAN |
+| p10-m100k | 33.46 / 57.20 ms | 12.99 / 30.17 ms | -47.3% | COLLSCAN → FETCH/IXSCAN |
+
+Explain executionStats changed as follows:
+
+- p10-m1k: `docs 1,055 → 1,000`, `keys 0 → 1,000`, `nReturned 1,000 → 1,000`.
+- p10-m10k: `docs 10,055 → 10,000`, `keys 0 → 10,000`, `nReturned 10,000 → 10,000`.
+- p10-m100k: `docs 100,055 → 100,000`, `keys 0 → 100,000`, `nReturned 100,000 → 100,000`.
+
+All valid runs returned HTTP 200 with `success=true`, the expected room and participant count, the expected recent-message count, 0% contract errors, and 0 timeouts. The 10k and 100k conditions meet the 10% p95 improvement target. The 1k condition does not show a meaningful benefit, so this is a scale-dependent optimization rather than a universal latency improvement.
+
+For p10-m100k at 10 VU, Prometheus showed messages aggregate cumulative time decreasing from about 493.4s to 411.8s over the sampled windows; process CPU and heap were not treated as final regression results because the before/after request counts differed. A wait queue maximum of 1 appeared after the index run and must be checked in repetition.
+
+This is a first-pass result, not final adoption. Repeat the before/after worst fixture three times and use medians before making a permanent index decision. No application code or permanent database migration was implemented.
+
+## Isolation and cleanup
+
+An initially public 500-participant fixture was joined by another local load worker, causing participant-count check failures. Those contaminated runs were discarded; fixtures were made private and the three 500-participant conditions were re-run with 0% contract errors. Cleanup removed the dedicated room, messages, participant users, and login account; post-cleanup fixture counts were all zero. Mongo profiler and Prometheus target configuration were restored. Concurrent local workers added 22 unrelated messages during the test; they were not modified or deleted.
diff --git a/performance/room-detail/room-detail.js b/performance/room-detail/room-detail.js
new file mode 100644
index 00000000..fa569bcc
--- /dev/null
+++ b/performance/room-detail/room-detail.js
@@ -0,0 +1,108 @@
+import http from 'k6/http';
+import { check } from 'k6';
+import { Counter, Rate, Trend } from 'k6/metrics';
+
+const API_BASE_URL = __ENV.API_BASE_URL || 'http://localhost:5001';
+const TEST_NAME = __ENV.TEST_NAME || 'room-detail-local';
+const FIXTURE_ID = __ENV.FIXTURE_ID;
+const ROOM_ID = __ENV.ROOM_ID;
+const EXPECTED_PARTICIPANTS = Number(__ENV.EXPECTED_PARTICIPANTS);
+const EXPECTED_RECENT_MESSAGES = Number(__ENV.EXPECTED_RECENT_MESSAGES);
+const VUS = Number(__ENV.VUS || 1);
+const DURATION = __ENV.DURATION || '30s';
+const EMAIL = __ENV.LOAD_TEST_EMAIL;
+const PASSWORD = __ENV.LOAD_TEST_PASSWORD;
+
+const duration = new Trend('room_detail_duration', true);
+const errors = new Rate('room_detail_errors');
+const completed = new Counter('room_detail_completed');
+const timeouts = new Counter('room_detail_timeouts');
+const responseBytes = new Trend('room_detail_response_bytes');
+
+export const options = {
+  vus: VUS,
+  duration: DURATION,
+  summaryTrendStats: ['avg', 'min', 'med', 'max', 'p(90)', 'p(95)', 'p(99)'],
+  thresholds: {
+    room_detail_errors: ['rate<=0.01'],
+    room_detail_timeouts: ['count==0'],
+  },
+};
+
+export function setup() {
+  if (!FIXTURE_ID || !ROOM_ID || !EMAIL || !PASSWORD) {
+    throw new Error('FIXTURE_ID, ROOM_ID, LOAD_TEST_EMAIL and LOAD_TEST_PASSWORD are required');
+  }
+  const response = http.post(`${API_BASE_URL}/api/auth/login`, JSON.stringify({
+    email: EMAIL,
+    password: PASSWORD,
+  }), {
+    headers: { 'Content-Type': 'application/json' },
+    timeout: '10s',
+    tags: { phase: 'setup', endpoint: 'login' },
+  });
+  const token = response.json('token');
+  if (response.status !== 200 || !token) {
+    throw new Error(`Fixture login failed (HTTP ${response.status})`);
+  }
+  return { token };
+}
+
+export default function (data) {
+  const response = http.get(`${API_BASE_URL}/api/rooms/${ROOM_ID}`, {
+    headers: { Authorization: `Bearer ${data.token}` },
+    timeout: '30s',
+    tags: { phase: 'measurement', endpoint: 'room-detail', fixture: FIXTURE_ID },
+  });
+  const success = check(response, {
+    'room detail returned HTTP 200': (res) => res.status === 200,
+    'room detail preserved success contract': (res) => res.json('success') === true,
+    'room detail returned expected room': (res) => res.json('data._id') === ROOM_ID,
+    'room detail returned expected participants': (res) =>
+      res.json('data.participantsCount') === EXPECTED_PARTICIPANTS &&
+      res.json('data.participants').length === EXPECTED_PARTICIPANTS,
+    'room detail returned expected recent messages': (res) =>
+      res.json('data.recentMessageCount') === EXPECTED_RECENT_MESSAGES,
+  });
+  duration.add(response.timings.duration);
+  errors.add(!success);
+  completed.add(1);
+  responseBytes.add(response.body ? response.body.length : 0);
+  if (response.error_code === 1050 || response.timings.duration >= 30000) timeouts.add(1);
+}
+
+function value(data, metric, field, fallback = 0) {
+  return data.metrics[metric]?.values[field] ?? fallback;
+}
+
+export function handleSummary(data) {
+  const elapsedSeconds = data.state.testRunDurationMs / 1000;
+  const requests = value(data, 'room_detail_completed', 'count');
+  const endedAt = new Date();
+  const startedAt = new Date(endedAt.getTime() - data.state.testRunDurationMs);
+  const summary = {
+    testName: TEST_NAME,
+    fixtureId: FIXTURE_ID,
+    roomId: ROOM_ID,
+    commit: __ENV.COMMIT || 'unknown',
+    participants: EXPECTED_PARTICIPANTS,
+    recentMessages: EXPECTED_RECENT_MESSAGES,
+    vus: VUS,
+    duration: DURATION,
+    startedAt: startedAt.toISOString(),
+    endedAt: endedAt.toISOString(),
+    requests,
+    rps: elapsedSeconds > 0 ? requests / elapsedSeconds : 0,
+    averageMs: value(data, 'room_detail_duration', 'avg'),
+    p95Ms: value(data, 'room_detail_duration', 'p(95)'),
+    p99Ms: value(data, 'room_detail_duration', 'p(99)'),
+    maxMs: value(data, 'room_detail_duration', 'max'),
+    errorRate: value(data, 'room_detail_errors', 'rate'),
+    timeoutCount: value(data, 'room_detail_timeouts', 'count'),
+    averageResponseBytes: value(data, 'room_detail_response_bytes', 'avg'),
+  };
+  return {
+    stdout: `${JSON.stringify(summary, null, 2)}\n`,
+    [`performance/room-detail/results/${TEST_NAME}.json`]: `${JSON.stringify(summary, null, 2)}\n`,
+  };
+}

From 4d2f9de2bef0898256923db938f2873faf436f15 Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 16:12:19 +0900
Subject: [PATCH 13/32] test: wait for registration outcome

---
 e2e/actions/auth.actions.js | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/e2e/actions/auth.actions.js b/e2e/actions/auth.actions.js
index fd5f0a88..074754c2 100644
--- a/e2e/actions/auth.actions.js
+++ b/e2e/actions/auth.actions.js
@@ -29,7 +29,12 @@ async function registerAction(page, userData) {
   await page.getByTestId('register-password-confirm-input').fill(userData.passwordConfirm);
   await page.getByTestId('register-name-input').fill(userData.name);
   await page.getByTestId('register-submit-button').click();
-  await page.waitForTimeout(1000); // 잠시 대기
+  // 성공 시 앱이 1000ms 뒤 router.push('/login')로 이동하므로, 그 내비게이션이 끝난 뒤에
+  // 다음 goto가 실행되어야 경합(net::ERR_ABORTED)이 없다. 실패 시엔 에러 메시지 표시까지 대기.
+  await Promise.race([
+    page.waitForURL(`${BASE_URL}/login`, { timeout: 10000 }).catch(() => {}),
+    page.getByTestId('register-error-message').waitFor({ state: 'visible', timeout: 10000 }).catch(() => {}),
+  ]);
 }
 
 /**

From 64b9e637712d2f55837fa34f9e8fae8b786ec8d6 Mon Sep 17 00:00:00 2001
From: rkdwlss2 <rkdwlss2@gmail.com>
Date: Tue, 11 Aug 2026 16:37:56 +0900
Subject: [PATCH 14/32] s3config

---
 .../java/com/ktb/chatapp/config/S3Config.java | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java b/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
new file mode 100644
index 00000000..16d5eb25
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
@@ -0,0 +1,38 @@
+package com.ktb.chatapp.config;
+
+import java.net.URI;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import software.amazon.awssdk.regions.Region;
+import software.amazon.awssdk.services.s3.S3Client;
+import software.amazon.awssdk.services.s3.presigner.S3Presigner;
+
+@Configuration
+public class S3Config {
+
+    @Bean
+    S3Client s3Client(
+            @Value("${aws.s3.region}") String region,
+            @Value("${aws.s3.endpoint:}") String endpoint,
+            @Value("${aws.s3.path-style-access:false}") boolean pathStyleAccess) {
+        var builder = S3Client.builder()
+                .region(Region.of(region))
+                .forcePathStyle(pathStyleAccess);
+        if (!endpoint.isBlank()) {
+            builder.endpointOverride(URI.create(endpoint));
+        }
+        return builder.build();
+    }
+
+    @Bean
+    S3Presigner s3Presigner(
+            @Value("${aws.s3.region}") String region,
+            @Value("${aws.s3.endpoint:}") String endpoint) {
+        var builder = S3Presigner.builder().region(Region.of(region));
+        if (!endpoint.isBlank()) {
+            builder.endpointOverride(URI.create(endpoint));
+        }
+        return builder.build();
+    }
+}

From 63e9bf75aa47c18c2a3b3c9a9c6cce32e64cea53 Mon Sep 17 00:00:00 2001
From: rkdwlss2 <rkdwlss2@gmail.com>
Date: Tue, 11 Aug 2026 16:37:56 +0900
Subject: [PATCH 15/32] s3config

---
 .../java/com/ktb/chatapp/config/S3Config.java | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java b/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
new file mode 100644
index 00000000..16d5eb25
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
@@ -0,0 +1,38 @@
+package com.ktb.chatapp.config;
+
+import java.net.URI;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import software.amazon.awssdk.regions.Region;
+import software.amazon.awssdk.services.s3.S3Client;
+import software.amazon.awssdk.services.s3.presigner.S3Presigner;
+
+@Configuration
+public class S3Config {
+
+    @Bean
+    S3Client s3Client(
+            @Value("${aws.s3.region}") String region,
+            @Value("${aws.s3.endpoint:}") String endpoint,
+            @Value("${aws.s3.path-style-access:false}") boolean pathStyleAccess) {
+        var builder = S3Client.builder()
+                .region(Region.of(region))
+                .forcePathStyle(pathStyleAccess);
+        if (!endpoint.isBlank()) {
+            builder.endpointOverride(URI.create(endpoint));
+        }
+        return builder.build();
+    }
+
+    @Bean
+    S3Presigner s3Presigner(
+            @Value("${aws.s3.region}") String region,
+            @Value("${aws.s3.endpoint:}") String endpoint) {
+        var builder = S3Presigner.builder().region(Region.of(region));
+        if (!endpoint.isBlank()) {
+            builder.endpointOverride(URI.create(endpoint));
+        }
+        return builder.build();
+    }
+}

From 9f797b8fbff41dd56069cbe03950027536fdc7b9 Mon Sep 17 00:00:00 2001
From: rkdwlss2 <rkdwlss2@gmail.com>
Date: Tue, 11 Aug 2026 16:47:41 +0900
Subject: [PATCH 16/32] s3config

---
 .../com/ktb/chatapp/storage/S3Storage.java    | 77 +++++++++++++++++++
 .../ProfileImageControllerTest.java           | 39 ++++++++++
 .../ktb/chatapp/storage/S3StorageTest.java    | 54 +++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/storage/S3Storage.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/controller/ProfileImageControllerTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/storage/S3StorageTest.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/storage/S3Storage.java b/apps/backend/src/main/java/com/ktb/chatapp/storage/S3Storage.java
new file mode 100644
index 00000000..f2ec887e
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/storage/S3Storage.java
@@ -0,0 +1,77 @@
+package com.ktb.chatapp.storage;
+
+import java.io.InputStream;
+import java.net.URI;
+import java.time.Duration;
+import java.util.Optional;
+import lombok.RequiredArgsConstructor;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.core.io.ByteArrayResource;
+import org.springframework.core.io.Resource;
+import org.springframework.http.ContentDisposition;
+import org.springframework.stereotype.Component;
+import software.amazon.awssdk.core.sync.RequestBody;
+import software.amazon.awssdk.services.s3.S3Client;
+import software.amazon.awssdk.services.s3.model.DeleteObjectRequest;
+import software.amazon.awssdk.services.s3.model.GetObjectRequest;
+import software.amazon.awssdk.services.s3.model.NoSuchKeyException;
+import software.amazon.awssdk.services.s3.model.PutObjectRequest;
+import software.amazon.awssdk.services.s3.presigner.S3Presigner;
+import software.amazon.awssdk.services.s3.presigner.model.GetObjectPresignRequest;
+
+@Component
+@ConditionalOnProperty(name = "file.storage.type", havingValue = "s3", matchIfMissing = true)
+@RequiredArgsConstructor
+public class S3Storage implements StoragePort {
+
+    private final S3Client s3Client;
+    private final S3Presigner s3Presigner;
+
+    @Value("${aws.s3.bucket}")
+    private String bucket;
+
+    @Override
+    public StoredObject put(InputStream content, String key, String contentType, long size) {
+        PutObjectRequest request = PutObjectRequest.builder()
+                .bucket(bucket)
+                .key(key)
+                .contentType(contentType)
+                .contentLength(size)
+                .build();
+        s3Client.putObject(request, RequestBody.fromInputStream(content, size));
+        return new StoredObject(key, size);
+    }
+
+    @Override
+    public Optional<Resource> open(String key) {
+        try {
+            byte[] content = s3Client.getObjectAsBytes(GetObjectRequest.builder()
+                    .bucket(bucket)
+                    .key(key)
+                    .build()).asByteArray();
+            return Optional.of(new ByteArrayResource(content));
+        } catch (NoSuchKeyException ex) {
+            return Optional.empty();
+        }
+    }
+
+    @Override
+    public void delete(String key) {
+        s3Client.deleteObject(DeleteObjectRequest.builder().bucket(bucket).key(key).build());
+    }
+
+    @Override
+    public Optional<URI> offloadUrl(String key, Duration ttl, ContentDisposition disposition) {
+        GetObjectRequest objectRequest = GetObjectRequest.builder()
+                .bucket(bucket)
+                .key(key)
+                .responseContentDisposition(disposition.toString())
+                .build();
+        GetObjectPresignRequest presignRequest = GetObjectPresignRequest.builder()
+                .signatureDuration(ttl)
+                .getObjectRequest(objectRequest)
+                .build();
+        return Optional.of(URI.create(s3Presigner.presignGetObject(presignRequest).url().toString()));
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/controller/ProfileImageControllerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/controller/ProfileImageControllerTest.java
new file mode 100644
index 00000000..b955bd1d
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/controller/ProfileImageControllerTest.java
@@ -0,0 +1,39 @@
+package com.ktb.chatapp.controller;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.when;
+
+import com.ktb.chatapp.storage.StoragePort;
+import java.net.URI;
+import java.util.Optional;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.http.ContentDisposition;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+
+@ExtendWith(MockitoExtension.class)
+@DisplayName("프로필 이미지 조회 단위 테스트")
+class ProfileImageControllerTest {
+
+    @Mock
+    private StoragePort storagePort;
+
+    @Test
+    @DisplayName("프로필 이미지는 S3 서명 URL로 리다이렉트한다")
+    void redirectsToS3Url() {
+        URI s3Url = URI.create("https://test-bucket.s3.ap-northeast-2.amazonaws.com/profiles/avatar.png");
+        when(storagePort.offloadUrl(eq("profiles/avatar.png"), any(), any(ContentDisposition.class)))
+                .thenReturn(Optional.of(s3Url));
+
+        ResponseEntity<?> response = new ProfileImageController(storagePort).getProfileImage("avatar.png");
+
+        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.FOUND);
+        assertThat(response.getHeaders().getLocation()).isEqualTo(s3Url);
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/storage/S3StorageTest.java b/apps/backend/src/test/java/com/ktb/chatapp/storage/S3StorageTest.java
new file mode 100644
index 00000000..f74ce09f
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/storage/S3StorageTest.java
@@ -0,0 +1,54 @@
+package com.ktb.chatapp.storage;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.verify;
+
+import java.io.ByteArrayInputStream;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.test.util.ReflectionTestUtils;
+import software.amazon.awssdk.core.sync.RequestBody;
+import software.amazon.awssdk.services.s3.S3Client;
+import software.amazon.awssdk.services.s3.model.PutObjectRequest;
+import software.amazon.awssdk.services.s3.presigner.S3Presigner;
+
+@ExtendWith(MockitoExtension.class)
+@DisplayName("S3Storage 단위 테스트")
+class S3StorageTest {
+
+    @Mock
+    private S3Client s3Client;
+
+    @Mock
+    private S3Presigner s3Presigner;
+
+    private S3Storage storage;
+
+    @BeforeEach
+    void setUp() {
+        storage = new S3Storage(s3Client, s3Presigner);
+        ReflectionTestUtils.setField(storage, "bucket", "test-bucket");
+    }
+
+    @Test
+    @DisplayName("이미지 바이트를 지정한 S3 key와 Content-Type으로 저장한다")
+    void putStoresObjectInS3() {
+        byte[] content = "image-bytes".getBytes();
+
+        StoredObject result = storage.put(
+                new ByteArrayInputStream(content), "profiles/avatar.png", "image/png", content.length);
+
+        ArgumentCaptor<PutObjectRequest> request = ArgumentCaptor.forClass(PutObjectRequest.class);
+        verify(s3Client).putObject(request.capture(), any(RequestBody.class));
+        assertThat(request.getValue().bucket()).isEqualTo("test-bucket");
+        assertThat(request.getValue().key()).isEqualTo("profiles/avatar.png");
+        assertThat(request.getValue().contentType()).isEqualTo("image/png");
+        assertThat(result).isEqualTo(new StoredObject("profiles/avatar.png", content.length));
+    }
+}

From 3ecaa8ad0f32efd6b12018e0bce6efafa10bedae Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 18:10:10 +0900
Subject: [PATCH 17/32] fix encoded chat room IDs

---
 .../hooks/__tests__/useRoomId.test.js         | 43 +++++++++++++++++++
 apps/frontend/hooks/useRoomId.js              | 13 +++++-
 2 files changed, 55 insertions(+), 1 deletion(-)
 create mode 100644 apps/frontend/hooks/__tests__/useRoomId.test.js

diff --git a/apps/frontend/hooks/__tests__/useRoomId.test.js b/apps/frontend/hooks/__tests__/useRoomId.test.js
new file mode 100644
index 00000000..ad977ee0
--- /dev/null
+++ b/apps/frontend/hooks/__tests__/useRoomId.test.js
@@ -0,0 +1,43 @@
+import { renderHook } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { useRoomId } from '../useRoomId';
+
+const navigationMocks = vi.hoisted(() => ({
+  useParams: vi.fn(),
+}));
+
+vi.mock('next/navigation', () => ({
+  useParams: navigationMocks.useParams,
+}));
+
+describe('useRoomId', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('decodes an encoded room id from the route', () => {
+    navigationMocks.useParams.mockReturnValue({
+      room: 'room-join-exp2-20260811%3Aroom%3A1%3A0',
+    });
+
+    const { result } = renderHook(() => useRoomId());
+
+    expect(result.current).toBe('room-join-exp2-20260811:room:1:0');
+  });
+
+  it('keeps an unencoded room id unchanged', () => {
+    navigationMocks.useParams.mockReturnValue({ room: 'room-1' });
+
+    const { result } = renderHook(() => useRoomId());
+
+    expect(result.current).toBe('room-1');
+  });
+
+  it('keeps malformed route encoding unchanged', () => {
+    navigationMocks.useParams.mockReturnValue({ room: 'room%invalid' });
+
+    const { result } = renderHook(() => useRoomId());
+
+    expect(result.current).toBe('room%invalid');
+  });
+});
diff --git a/apps/frontend/hooks/useRoomId.js b/apps/frontend/hooks/useRoomId.js
index 30c38c4d..137769a3 100644
--- a/apps/frontend/hooks/useRoomId.js
+++ b/apps/frontend/hooks/useRoomId.js
@@ -1,7 +1,18 @@
 import { useParams } from 'next/navigation';
 
 export const useRoomId = () => {
-  return useParams()?.room;
+  const roomId = useParams()?.room;
+
+  if (typeof roomId !== 'string') {
+    return roomId;
+  }
+
+  try {
+    return decodeURIComponent(roomId);
+  } catch {
+    // 잘못 인코딩된 경로는 원본을 유지해 기존 오류 처리 흐름에 맡긴다.
+    return roomId;
+  }
 };
 
 export default useRoomId;

From 907cfbb671eccff7cfe024c809a17b878cb0d55a Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 19:07:25 +0900
Subject: [PATCH 18/32] perf(room): make joins atomic

Avoid lost participant updates and suppress duplicate room events. Batch projected participant reads and add staged load diagnostics.
---
 .../chatapp/controller/RoomController.java    |  11 +-
 .../com/ktb/chatapp/service/RoomService.java  | 116 +++++++++--
 .../com/ktb/chatapp/storage/LocalStorage.java |   6 +-
 .../src/main/resources/application.properties |   3 +-
 .../ktb/chatapp/service/RoomServiceTest.java  | 110 +++++++++-
 .../storage/StoragePortSelectionTest.java     |  17 +-
 loadtest/__tests__/room-join-tools.test.js    |  22 ++
 loadtest/package.json                         |   3 +-
 performance/room-join/README.md               |  82 +++++++-
 performance/room-join/compare.js              |  50 +++++
 performance/room-join/docker-compose.yaml     |  74 +++++++
 performance/room-join/fixture.js              | 132 ++++++++++++
 performance/room-join/lib.js                  |  66 ++++++
 .../room-join/prometheus.performance.yml      |  13 ++
 performance/room-join/room-join.js            | 190 +++++++++++++-----
 performance/room-join/run-matrix.js           | 129 ++++++++++++
 performance/room-join/stage0.js               |  63 ++++++
 17 files changed, 991 insertions(+), 96 deletions(-)
 create mode 100644 loadtest/__tests__/room-join-tools.test.js
 create mode 100644 performance/room-join/compare.js
 create mode 100644 performance/room-join/docker-compose.yaml
 create mode 100644 performance/room-join/fixture.js
 create mode 100644 performance/room-join/lib.js
 create mode 100644 performance/room-join/prometheus.performance.yml
 create mode 100644 performance/room-join/run-matrix.js
 create mode 100644 performance/room-join/stage0.js

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
index 8d81ea8f..3b9e54ac 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
@@ -29,6 +29,7 @@
 import org.springframework.http.CacheControl;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.security.core.Authentication;
 import org.springframework.web.bind.annotation.*;
 
 @Tag(name = "채팅방 (Rooms)", description = "채팅방 생성 및 관리 API - 채팅방 목록 조회, 생성, 참여, 헬스체크")
@@ -232,16 +233,16 @@ public ResponseEntity<?> getRoomById(@Parameter(description = "채팅방 ID", ex
     public ResponseEntity<?> joinRoom(
             @Parameter(description = "채팅방 ID", example = "60d5ec49f1b2c8b9e8c4f2a1") @PathVariable String roomId,
             @RequestBody JoinRoomRequest joinRoomRequest,
-            Principal principal) {
+            Authentication authentication) {
         try {
-            Room joinedRoom = roomService.joinRoom(roomId, joinRoomRequest.getPassword(), principal.getName());
+            String userId = ((Map<?, ?>) authentication.getDetails()).get("userId").toString();
+            RoomResponse roomResponse = roomService.joinRoomResponse(
+                    roomId, joinRoomRequest.getPassword(), authentication.getName(), userId);
 
-            if (joinedRoom == null) {
+            if (roomResponse == null) {
                 return ResponseEntity.status(HttpStatus.NOT_FOUND)
                         .body(StandardResponse.error("채팅방을 찾을 수 없습니다."));
             }
-
-            RoomResponse roomResponse = mapToRoomResponse(joinedRoom, principal.getName());
             
             return ResponseEntity.ok(
                 Map.of(
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
index 61889789..5c6664af 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
@@ -20,6 +20,11 @@
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.data.mongodb.core.FindAndModifyOptions;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.security.crypto.password.PasswordEncoder;
 import org.springframework.stereotype.Service;
 
@@ -33,6 +38,7 @@ public class RoomService {
     private final RecentMessageCounter recentMessageCounter;
     private final PasswordEncoder passwordEncoder;
     private final ApplicationEventPublisher eventPublisher;
+    private final MongoTemplate mongoTemplate;
 
     public RoomsResponse getAllRooms(String name) {
 
@@ -177,15 +183,23 @@ public Optional<Room> findRoomById(String roomId) {
     }
 
     public Room joinRoom(String roomId, String password, String name) {
+        User user = userRepository.findByEmail(name)
+            .orElseThrow(() -> new RuntimeException("사용자를 찾을 수 없습니다: " + name));
+        JoinRoomResult result = joinRoomInternal(roomId, password, user.getId());
+        if (result == null) {
+            return null;
+        }
+        publishRoomUpdatedIfParticipantAdded(roomId, name, result);
+        return result.room();
+    }
+
+    private JoinRoomResult joinRoomInternal(String roomId, String password, String userId) {
         Optional<Room> roomOpt = roomRepository.findById(roomId);
         if (roomOpt.isEmpty()) {
             return null;
         }
 
         Room room = roomOpt.get();
-        User user = userRepository.findByEmail(name)
-            .orElseThrow(() -> new RuntimeException("사용자를 찾을 수 없습니다: " + name));
-
         // 비밀번호 확인
         if (room.isHasPassword()) {
             if (password == null || !passwordEncoder.matches(password, room.getPassword())) {
@@ -193,37 +207,93 @@ public Room joinRoom(String roomId, String password, String name) {
             }
         }
 
-        // 이미 참여중인지 확인
-        if (!room.getParticipantIds().contains(user.getId())) {
-            // 채팅방 참여
-            room.getParticipantIds().add(user.getId());
-            room = roomRepository.save(room);
+        if (room.getParticipantIds() != null && room.getParticipantIds().contains(userId)) {
+            return new JoinRoomResult(room, false);
+        }
+
+        Query addParticipant = Query.query(Criteria.where("_id").is(roomId)
+            .and("participantIds").ne(userId));
+        Room updatedRoom = mongoTemplate.findAndModify(
+            addParticipant,
+            new Update().addToSet("participantIds", userId),
+            FindAndModifyOptions.options().returnNew(true),
+            Room.class);
+        if (updatedRoom != null) {
+            return new JoinRoomResult(updatedRoom, true);
+        }
+
+        // The room was deleted, or another concurrent request added the same participant first.
+        return roomRepository.findById(roomId)
+            .map(latestRoom -> new JoinRoomResult(latestRoom, false))
+            .orElse(null);
+    }
+
+    public RoomResponse joinRoomResponse(String roomId, String password, String name) {
+        User user = userRepository.findByEmail(name)
+            .orElseThrow(() -> new RuntimeException("사용자를 찾을 수 없습니다: " + name));
+        return joinRoomResponse(roomId, password, name, user.getId());
+    }
+
+    public RoomResponse joinRoomResponse(
+            String roomId, String password, String name, String userId) {
+        JoinRoomResult result = joinRoomInternal(roomId, password, userId);
+        if (result == null) {
+            return null;
+        }
+
+        RoomResponse response = mapToRoomResponse(result.room(), name);
+        publishRoomUpdatedIfParticipantAdded(roomId, result, response);
+        return response;
+    }
+
+    private void publishRoomUpdatedIfParticipantAdded(
+            String roomId, String name, JoinRoomResult result) {
+        if (!result.participantAdded()) {
+            return;
+        }
+        publishRoomUpdatedIfParticipantAdded(roomId, result, mapToRoomResponse(result.room(), name));
+    }
+
+    private void publishRoomUpdatedIfParticipantAdded(
+            String roomId, JoinRoomResult result, RoomResponse response) {
+        if (!result.participantAdded()) {
+            return;
         }
-        
-        // Publish event for room updated
         try {
-            RoomResponse roomResponse = mapToRoomResponse(room, name);
-            eventPublisher.publishEvent(new RoomUpdatedEvent(this, roomId, roomResponse));
+            eventPublisher.publishEvent(new RoomUpdatedEvent(this, roomId, response));
         } catch (Exception e) {
             log.error("roomUpdate 이벤트 발행 실패", e);
         }
-
-        return room;
     }
 
     private RoomResponse mapToRoomResponse(Room room, String name) {
         if (room == null) return null;
 
-        User creator = null;
+        Set<String> userIds = new LinkedHashSet<>();
         if (room.getCreator() != null) {
-            creator = userRepository.findById(room.getCreator()).orElse(null);
+            userIds.add(room.getCreator());
         }
-
-        List<User> participants = room.getParticipantIds().stream()
-            .map(userRepository::findById)
-            .filter(Optional::isPresent)
-            .map(Optional::get)
-            .toList();
+        if (room.getParticipantIds() != null) {
+            userIds.addAll(room.getParticipantIds());
+        }
+        Query usersQuery = Query.query(Criteria.where("_id").in(userIds));
+        usersQuery.fields()
+            .include("_id")
+            .include("name")
+            .include("email")
+            .include("profileImage");
+        Map<String, User> usersById = userIds.isEmpty()
+            ? Map.of()
+            : mongoTemplate.find(usersQuery, User.class).stream()
+                .filter(user -> user != null && user.getId() != null)
+                .collect(Collectors.toMap(User::getId, Function.identity()));
+        User creator = room.getCreator() != null ? usersById.get(room.getCreator()) : null;
+        List<User> participants = room.getParticipantIds() == null
+            ? List.of()
+            : room.getParticipantIds().stream()
+                .map(usersById::get)
+                .filter(java.util.Objects::nonNull)
+                .toList();
 
         int recentMessageCount = recentMessageCounter.countRecentMessages(room.getId());
 
@@ -276,4 +346,6 @@ private RoomResponse buildRoomResponse(
             .recentMessageCount(recentMessageCount)
             .build();
     }
+
+    private record JoinRoomResult(Room room, boolean participantAdded) {}
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java b/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java
index 4ccc08a1..03f7d07d 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/storage/LocalStorage.java
@@ -11,10 +11,14 @@
 import java.nio.file.StandardCopyOption;
 import java.util.Optional;
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.UrlResource;
+import org.springframework.stereotype.Component;
 
-/** 테스트 전용 로컬 구현. 애플리케이션 빈으로 등록하지 않아 운영 코드에서는 사용할 수 없다. */
+/** 로컬 파일 스토리지 구현. file.storage.type=local인 환경에서만 등록된다. */
+@Component
+@ConditionalOnProperty(name = "file.storage.type", havingValue = "local")
 public class LocalStorage implements StoragePort {
 
     private final Path rootLocation;
diff --git a/apps/backend/src/main/resources/application.properties b/apps/backend/src/main/resources/application.properties
index b87bd54f..c6b493d0 100644
--- a/apps/backend/src/main/resources/application.properties
+++ b/apps/backend/src/main/resources/application.properties
@@ -4,7 +4,8 @@ server.port=${PORT:5001}
 server.tomcat.threads.max=10
 server.tomcat.threads.min-spare=1
 server.tomcat.accept-count=10
-server.tomcat.max-connections=50
+server.tomcat.max-connections=150
+server.tomcat.mbeanregistry.enabled=true
 
 # Database Configuration
 # Spring Boot 4: connection settings moved from spring.data.mongodb.* to spring.mongodb.*
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
index 05a697c7..77989f1b 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
@@ -3,9 +3,13 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.ArgumentMatchers.anySet;
+import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
@@ -13,6 +17,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.ktb.chatapp.dto.RoomResponse;
 import com.ktb.chatapp.dto.RoomsResponse;
+import com.ktb.chatapp.event.RoomUpdatedEvent;
 import com.ktb.chatapp.model.Room;
 import com.ktb.chatapp.model.User;
 import com.ktb.chatapp.repository.RoomRepository;
@@ -20,13 +25,19 @@
 import java.time.LocalDateTime;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
+import org.mockito.ArgumentCaptor;
 import org.mockito.junit.jupiter.MockitoExtension;
 import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.FindAndModifyOptions;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.security.crypto.password.PasswordEncoder;
 
 @ExtendWith(MockitoExtension.class)
@@ -37,6 +48,7 @@ class RoomServiceTest {
     @Mock private RecentMessageCounter recentMessageCounter;
     @Mock private PasswordEncoder passwordEncoder;
     @Mock private ApplicationEventPublisher eventPublisher;
+    @Mock private MongoTemplate mongoTemplate;
 
     private RoomService roomService;
 
@@ -47,7 +59,8 @@ void setUp() {
             userRepository,
             recentMessageCounter,
             passwordEncoder,
-            eventPublisher
+            eventPublisher,
+            mongoTemplate
         );
     }
 
@@ -110,6 +123,101 @@ void getAllRooms_batchesUsersAndMessageCountsOnceAndKeepsContract() throws Excep
         assertFalse(json.has("createdAtDateTime"));
     }
 
+    @Test
+    void joinRoom_addsNewParticipantAndReturnsIt() {
+        Room before = room("room-1", "Room", "creator-1", LocalDateTime.now(),
+            new java.util.HashSet<>(Set.of("creator-1")));
+        Room after = room("room-1", "Room", "creator-1", before.getCreatedAt(),
+            new java.util.HashSet<>(Set.of("creator-1", "participant-1")));
+        User joining = user("participant-1", "Participant", "participant@test.com");
+        User creator = user("creator-1", "Creator", "creator@test.com");
+        when(roomRepository.findById("room-1")).thenReturn(Optional.of(before));
+        when(mongoTemplate.findAndModify(any(Query.class), any(Update.class),
+            any(FindAndModifyOptions.class), any(Class.class))).thenReturn(after);
+        when(mongoTemplate.find(any(Query.class), org.mockito.ArgumentMatchers.eq(User.class)))
+            .thenReturn(List.of(creator, joining));
+
+        RoomResponse response = roomService.joinRoomResponse(
+            "room-1", null, joining.getEmail(), joining.getId());
+
+        assertTrue(response.getParticipants().stream().anyMatch(p -> p.getId().equals(joining.getId())));
+        verify(roomRepository, never()).save(any(Room.class));
+        ArgumentCaptor<RoomUpdatedEvent> event = ArgumentCaptor.forClass(RoomUpdatedEvent.class);
+        verify(eventPublisher).publishEvent(event.capture());
+        assertSame(response, event.getValue().getRoomResponse());
+        verify(recentMessageCounter).countRecentMessages("room-1");
+        verify(userRepository, never()).findByEmail(joining.getEmail());
+        ArgumentCaptor<Query> usersQuery = ArgumentCaptor.forClass(Query.class);
+        verify(mongoTemplate).find(usersQuery.capture(), org.mockito.ArgumentMatchers.eq(User.class));
+        assertEquals(
+            org.bson.Document.parse("{'_id':1,'name':1,'email':1,'profileImage':1}"),
+            usersQuery.getValue().getFieldsObject());
+    }
+
+    @Test
+    void joinRoom_isIdempotentForExistingParticipant() {
+        User joining = user("participant-1", "Participant", "participant@test.com");
+        Room room = room("room-1", "Room", "creator-1", LocalDateTime.now(),
+            new java.util.HashSet<>(Set.of("creator-1", joining.getId())));
+        when(roomRepository.findById("room-1")).thenReturn(Optional.of(room));
+        when(mongoTemplate.find(any(Query.class), org.mockito.ArgumentMatchers.eq(User.class)))
+            .thenReturn(List.of(joining));
+
+        roomService.joinRoomResponse("room-1", null, joining.getEmail(), joining.getId());
+        roomService.joinRoomResponse("room-1", null, joining.getEmail(), joining.getId());
+
+        assertEquals(2, room.getParticipantIds().size());
+        verify(roomRepository, never()).save(any(Room.class));
+        verify(eventPublisher, never()).publishEvent(any(RoomUpdatedEvent.class));
+        verify(mongoTemplate, never()).findAndModify(any(Query.class), any(Update.class),
+            any(FindAndModifyOptions.class), any(Class.class));
+    }
+
+    @Test
+    void joinRoom_rejectsWrongPassword() {
+        Room room = room("room-1", "Private", "creator-1", LocalDateTime.now(),
+            new java.util.HashSet<>(Set.of("creator-1")));
+        room.setHasPassword(true);
+        room.setPassword("encoded");
+        User joining = user("participant-1", "Participant", "participant@test.com");
+        when(roomRepository.findById("room-1")).thenReturn(Optional.of(room));
+        when(userRepository.findByEmail(joining.getEmail())).thenReturn(Optional.of(joining));
+        when(passwordEncoder.matches("wrong", "encoded")).thenReturn(false);
+
+        RuntimeException error = assertThrows(RuntimeException.class,
+            () -> roomService.joinRoom("room-1", "wrong", joining.getEmail()));
+
+        assertTrue(error.getMessage().contains("\uBE44\uBC00\uBC88\uD638"));
+        verify(mongoTemplate, never()).findAndModify(any(Query.class), any(Update.class),
+            any(FindAndModifyOptions.class), any(Class.class));
+    }
+
+    @Test
+    void joinRoom_returnsNullForMissingRoom() {
+        when(roomRepository.findById("missing")).thenReturn(Optional.empty());
+
+        assertNull(roomService.joinRoomResponse(
+            "missing", null, "participant@test.com", "participant-1"));
+        verify(userRepository, never()).findByEmail("participant@test.com");
+    }
+
+    @Test
+    void joinRoom_returnsNullWhenRoomDisappearsBeforeAtomicUpdate() {
+        Room room = room("room-1", "Room", "creator-1", LocalDateTime.now(),
+            new java.util.HashSet<>(Set.of("creator-1")));
+        User joining = user("participant-1", "Participant", "participant@test.com");
+        when(roomRepository.findById("room-1"))
+            .thenReturn(Optional.of(room), Optional.empty());
+        when(mongoTemplate.findAndModify(any(Query.class), any(Update.class),
+            any(FindAndModifyOptions.class), any(Class.class))).thenReturn(null);
+
+        assertNull(roomService.joinRoomResponse(
+            "room-1", null, joining.getEmail(), joining.getId()));
+
+        verify(roomRepository, times(2)).findById("room-1");
+        verify(eventPublisher, never()).publishEvent(any(RoomUpdatedEvent.class));
+    }
+
     private static Room room(
             String id,
             String name,
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java b/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java
index 8e47383a..1335eb69 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/storage/StoragePortSelectionTest.java
@@ -2,17 +2,24 @@
 
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.stereotype.Component;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-@DisplayName("S3 전용 스토리지 선택 단위 테스트")
+@DisplayName("로컬 스토리지 선택 단위 테스트")
 class StoragePortSelectionTest {
 
     @Test
-    @DisplayName("S3Storage만 애플리케이션 빈으로 등록된다")
-    void onlyS3StorageIsApplicationComponent() {
-        assertThat(S3Storage.class).hasAnnotation(Component.class);
-        assertThat(LocalStorage.class.getAnnotation(Component.class)).isNull();
+    @DisplayName("local 설정에서만 LocalStorage가 빈으로 등록된다")
+    void localStorageIsConditionalApplicationComponent() {
+        assertThat(LocalStorage.class).hasAnnotation(Component.class);
+
+        ConditionalOnProperty condition = LocalStorage.class
+            .getAnnotation(ConditionalOnProperty.class);
+        assertThat(condition).isNotNull();
+        assertThat(condition.name()).containsExactly("file.storage.type");
+        assertThat(condition.havingValue()).isEqualTo("local");
+        assertThat(condition.matchIfMissing()).isFalse();
     }
 }
diff --git a/loadtest/__tests__/room-join-tools.test.js b/loadtest/__tests__/room-join-tools.test.js
new file mode 100644
index 00000000..a4ba2a49
--- /dev/null
+++ b/loadtest/__tests__/room-join-tools.test.js
@@ -0,0 +1,22 @@
+'use strict';
+
+const { median, decideOptimization, parseArgs } = require('../../performance/room-join/lib');
+
+describe('room join performance helpers', () => {
+  test('median supports three-run adoption comparisons', () => {
+    expect(median([30, 10, 20])).toBe(20);
+    expect(median([10, 20])).toBe(15);
+  });
+
+  test.each([
+    [{ messagesScaleWithLatency: true, planStage: 'COLLSCAN' }, '{ room: 1, timestamp: 1 } index'],
+    [{ participantsScaleWithCommands: true }, 'batch participant lookup and remove duplicate response assembly'],
+    [{ newParticipantOnlyRegression: true }, '$addToSet atomic participant update'],
+  ])('selects the first optimization from evidence', (input, expected) => {
+    expect(decideOptimization(input)).toBe(expected);
+  });
+
+  test('parses both inline and separate CLI values', () => {
+    expect(parseArgs(['--mode=idempotent', '--vus', '10'])).toEqual({ mode: 'idempotent', vus: '10' });
+  });
+});
diff --git a/loadtest/package.json b/loadtest/package.json
index 8e50bcde..ee2950be 100644
--- a/loadtest/package.json
+++ b/loadtest/package.json
@@ -13,7 +13,8 @@
     "test:rampup": "node ramp-up-test.js",
     "test:rampup:small": "node ramp-up-test.js --max-users=200 --sustain-duration=120",
     "test:rampup:large": "node ramp-up-test.js --max-users=1000 --min-users-per-second=10 --max-users-per-second=20 --sustain-duration=300",
-    "create-users": "node create-test-users.js"
+    "create-users": "node create-test-users.js",
+    "test:room-join-tools": "jest __tests__/room-join-tools.test.js"
   },
   "keywords": [
     "load-test",
diff --git a/performance/room-join/README.md b/performance/room-join/README.md
index 2307ce8b..ec3ca53d 100644
--- a/performance/room-join/README.md
+++ b/performance/room-join/README.md
@@ -1,15 +1,77 @@
-# Room join HTTP-only load test
+# Room Join API performance diagnosis
 
-This scenario measures only `POST /api/rooms/{roomId}/join`. Account login and room selection
-run once in `setup` and are excluded from the custom metrics.
+`POST /api/rooms/{roomId}/join`만 측정한다. Room Detail, Socket.IO, 브라우저 E2E는 이 흐름에 포함하지 않는다. 기존 데이터와 기존 `results/` 파일은 생성·정리 대상이 아니다.
+
+## 1. 격리 fixture
+
+fixture 사용자는 API 로그인이 가능해야 하므로 현재 애플리케이션과 같은 평문 비밀번호의 BCrypt hash를 전달한다. 로컬 기존 테스트 사용자 hash를 재사용하는 예시는 다음과 같다.
+
+```sh
+TEST_ID=20260811-a
+PASSWORD_HASH="$(docker exec mongo-ktb mongosh --quiet bootcamp-chat --eval \
+  'print(db.users.findOne({email:"loadtest-0@test.com"}).password)')"
+
+TEST_ID="$TEST_ID" PASSWORD_HASH="$PASSWORD_HASH" ACTION=create \
+  mongosh --quiet mongodb://localhost:27017/bootcamp-chat performance/room-join/fixture.js \
+  > "performance/room-join/results/$TEST_ID-fixture.json"
+```
+
+기본 매트릭스는 메시지 축 `p10 × m1000/10000/100000`, 참가자 축 `p10/100/500 × m1000`이다. 중복인 `p10-m1000`은 한 번만 만든다. 각 방과 문서에는 `perfJoinTestId`가 기록되고, 계정은 `perf-join-<TEST_ID>-*` prefix를 쓴다.
+
+개수와 격리를 재검증하거나 정리한다.
+
+```sh
+TEST_ID="$TEST_ID" ACTION=verify mongosh --quiet mongodb://localhost:27017/bootcamp-chat \
+  performance/room-join/fixture.js
+
+TEST_ID="$TEST_ID" ACTION=delete mongosh --quiet mongodb://localhost:27017/bootcamp-chat \
+  performance/room-join/fixture.js
+```
+
+삭제는 동일 `perfJoinTestId`의 방/메시지와, 동일 test ID 및 prefix를 모두 만족하는 사용자만 대상으로 한다. fixture가 이미 있으면 create는 실패하며 암묵적으로 덮어쓰지 않는다.
+
+## 2. Stage 0 — 단일 요청 진단
+
+fixture JSON의 대상 방 ID로 profiler를 켜고 정확히 Join 한 건을 보낸다. 완료 후 기존 profiling level을 복구하고, 같은 시간창 명령 수와 메시지 쿼리 `executionStats`를 저장한다.
+
+```sh
+node performance/room-join/stage0.js \
+  --fixture perf-join-20260811-a-p10-m100000 \
+  --room-id ROOM_OBJECT_ID --mode idempotent --participants 10 --messages 100000
+```
+
+participant 수에 따라 사용자 조회 명령 수가 선형 증가하는지, 메시지 수 증가 시 `COLLSCAN` 및 `totalDocsExamined ≫ nReturned`인지 여기서 먼저 판정한다.
+
+## 3. Stage 1 — HTTP-only 매트릭스
+
+create 출력 JSON을 manifest로 사용한다. 각 fixture에 대해 `idempotent`와 `new-participant`를 분리하고 `1 VU × 30초` smoke 후 `10→30→50→100 VU × 1분`을 실행한다.
+`new-participant`는 각 단계 직전에 해당 test ID의 `new` 사용자 ID만 대상 방에서 `$pull`한 뒤 VU당 Join을 정확히 한 번 보내므로, 모든 단계가 최초 동시 참여를 측정한다. `idempotent`는 방의 기존 참가자 계정을 VU들이 순환 사용한다.
+
+```sh
+node performance/room-join/run-matrix.js \
+  --manifest performance/room-join/results/20260811-a-fixture.json \
+  --test-id 20260811-before-run1
+```
+
+다음 조건이면 해당 fixture/mode의 다음 단계를 실행하지 않는다.
+
+- 오류율 1% 초과
+- timeout 1건 이상
+- 단계 p95가 smoke p95의 2배 초과
+- Mongo wait queue 또는 checkout failure 발생
+- k6 응답 계약 threshold 실패
+
+각 결과에는 test/commit/fixture/mode/VU/시간창, 요청·RPS, avg/p95/p99/max, 오류·timeout 및 동일 시간창 Prometheus 관측치가 기록된다. Prometheus 쿼리 오류도 결과에 명시적으로 남긴다.
+
+## 4. 전후 3회 중앙값 비교
+
+최악 fixture의 동일 조건만 담은 aggregate 두 개를 비교한다.
 
 ```sh
-API_BASE_URL=http://localhost:5001 \
-TEST_NAME=room-join-before-1 FIXTURE_ID=rooms324-users562-messages8051 \
-VUS=10 DURATION=3m THINK_TIME_MS=1000 \
-k6 run performance/room-join/room-join.js
+node performance/room-join/compare.js \
+  --before performance/room-join/results/before-worst-aggregate.json \
+  --after performance/room-join/results/after-worst-aggregate.json \
+  --observation performance/room-join/results/stage0-observation.json
 ```
 
-Run the same fixture and settings three times before and after one code change. Adoption requires
-error rate at most 1%, zero timeouts, Mongo wait queue zero, p95 improvement of at least 10%, and
-no p99 regression. Result JSON is written under `results/`.
+최종 채택은 양쪽 3회 이상, p95 중앙값 10% 이상 개선, 오류율 1% 이하, timeout 0, p99와 Mongo pool 비회귀가 모두 필요하다. 생성된 JSON의 요약 수치를 기존 Notion 성능 병목 페이지의 Join API 전용 표로 옮긴다. 이 도구는 외부 문서를 자동 수정하지 않는다.
diff --git a/performance/room-join/compare.js b/performance/room-join/compare.js
new file mode 100644
index 00000000..5cee39fc
--- /dev/null
+++ b/performance/room-join/compare.js
@@ -0,0 +1,50 @@
+#!/usr/bin/env node
+'use strict';
+
+const fs = require('fs');
+const { parseArgs, median, decideOptimization } = require('./lib');
+
+const args = parseArgs(process.argv.slice(2));
+const before = loadRuns(required('before'));
+const after = loadRuns(required('after'));
+const observation = args.observation ? JSON.parse(fs.readFileSync(args.observation, 'utf8')) : {};
+const beforeP95 = median(before.map((run) => run.latencyMs.p95));
+const afterP95 = median(after.map((run) => run.latencyMs.p95));
+const beforeP99 = median(before.map((run) => run.latencyMs.p99));
+const afterP99 = median(after.map((run) => run.latencyMs.p99));
+const improvement = beforeP95 > 0 ? (beforeP95 - afterP95) / beforeP95 : 0;
+const resourceMetrics = ['processCpu', 'heapUsed', 'gcPauseRate', 'mongoCommandMax', 'mongoCheckedOut'];
+const resources = Object.fromEntries(resourceMetrics.map((name) => {
+  const beforeValue = median(before.map((run) => observationMaximum(run, name)));
+  const afterValue = median(after.map((run) => observationMaximum(run, name)));
+  return [name, { before: beforeValue, after: afterValue, regressed: beforeValue !== null && afterValue !== null && afterValue > beforeValue }];
+}));
+const resourcesRegressed = Object.values(resources).some((metric) => metric.regressed);
+const resourcesComplete = Object.values(resources).every((metric) => metric.before !== null && metric.after !== null);
+const adopted = before.length >= 3 && after.length >= 3 && improvement >= 0.10
+  && after.every((run) => run.errorRate <= 0.01 && run.timeoutCount === 0)
+  && afterP99 <= beforeP99
+  && !resourcesRegressed
+  && resourcesComplete
+  && after.every((run) => !run.waitQueueDetected && !run.checkoutFailureDetected);
+
+console.log(JSON.stringify({
+  beforeRuns: before.length, afterRuns: after.length,
+  median: { beforeP95Ms: beforeP95, afterP95Ms: afterP95, p95Improvement: improvement, beforeP99Ms: beforeP99, afterP99Ms: afterP99 },
+  resources: { complete: resourcesComplete, metrics: resources },
+  temporaryAdoptionEligible: improvement >= 0.10 && after.every((run) => run.errorRate <= 0.01 && run.timeoutCount === 0) && afterP99 <= beforeP99 && resourcesComplete && !resourcesRegressed,
+  finalAdoption: adopted,
+  selectedFirstOptimization: decideOptimization(observation),
+}, null, 2));
+
+function loadRuns(file) {
+  const payload = JSON.parse(fs.readFileSync(file, 'utf8'));
+  return Array.isArray(payload) ? payload : payload.runs || [payload];
+}
+function observationMaximum(run, name) {
+  const series = run.observability?.[name];
+  if (!Array.isArray(series)) return null;
+  const values = series.flatMap((item) => item.values || []).map((pair) => pair[1]).filter(Number.isFinite);
+  return values.length ? Math.max(...values) : null;
+}
+function required(name) { if (!args[name]) throw new Error(`--${name} is required`); return args[name]; }
diff --git a/performance/room-join/docker-compose.yaml b/performance/room-join/docker-compose.yaml
new file mode 100644
index 00000000..8be68292
--- /dev/null
+++ b/performance/room-join/docker-compose.yaml
@@ -0,0 +1,74 @@
+services:
+  mongo:
+    image: mongo:8.3.4
+    ports:
+      - "27017:27017"
+    environment:
+      MONGO_INITDB_DATABASE: bootcamp-chat
+    volumes:
+      - mongo_data:/data/db
+    healthcheck:
+      test: ["CMD", "mongosh", "--quiet", "--eval", "quit(db.runCommand({ ping: 1 }).ok ? 0 : 2)"]
+      interval: 2s
+      timeout: 2s
+      retries: 30
+
+  redis:
+    image: redis:8.8.0
+    ports:
+      - "6379:6379"
+    volumes:
+      - redis_data:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 2s
+      timeout: 2s
+      retries: 30
+
+  mongodb-exporter:
+    image: percona/mongodb_exporter:0.51.0
+    command: ["--mongodb.uri=mongodb://mongo:27017", "--collect-all"]
+    depends_on:
+      mongo:
+        condition: service_healthy
+
+  app:
+    image: ktb-chat-backend:join-optimized-8fde958
+    ports:
+      - "5001:5001"
+      - "5002:5002"
+    env_file:
+      - ../../apps/backend/.env
+    environment:
+      MONGO_URI: mongodb://mongo:27017/bootcamp-chat
+      REDIS_HOST: redis
+      REDIS_PORT: 6379
+      FILE_STORAGE_TYPE: local
+      SPRING_DATA_MONGODB_AUTO_INDEX_CREATION: "true"
+      SWAGGER_ENABLED: "false"
+      SERVER_TOMCAT_THREADS_MAX: ${TOMCAT_THREADS_MAX:-10}
+      SERVER_TOMCAT_MAX_CONNECTIONS: ${TOMCAT_MAX_CONNECTIONS:-50}
+    depends_on:
+      mongo:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+
+  prometheus:
+    image: prom/prometheus:v3.13.0
+    ports:
+      - "9090:9090"
+    volumes:
+      - ./prometheus.performance.yml:/etc/prometheus/prometheus.yml:ro
+      - prometheus_data:/prometheus
+    command:
+      - --config.file=/etc/prometheus/prometheus.yml
+      - --storage.tsdb.path=/prometheus
+    depends_on:
+      - app
+      - mongodb-exporter
+
+volumes:
+  mongo_data:
+  redis_data:
+  prometheus_data:
diff --git a/performance/room-join/fixture.js b/performance/room-join/fixture.js
new file mode 100644
index 00000000..111ec77d
--- /dev/null
+++ b/performance/room-join/fixture.js
@@ -0,0 +1,132 @@
+/* global db, print, quit, ObjectId */
+
+// Run with mongosh: TEST_ID=... ACTION=create mongosh bootcamp-chat fixture.js
+const action = process.env.ACTION || 'create';
+const testId = process.env.TEST_ID;
+const prefix = `perf-join-${testId}`;
+const passwordHash = process.env.PASSWORD_HASH;
+const fixedMessages = integerEnv('FIXED_MESSAGES', 1000);
+const messageAxis = intList(process.env.MESSAGE_AXIS || '1000,10000,100000');
+const participantAxis = intList(process.env.PARTICIPANT_AXIS || '10,100,500');
+const maxVus = integerEnv('MAX_VUS', 100);
+
+if (!testId || !/^[a-zA-Z0-9][a-zA-Z0-9_-]{0,63}$/.test(testId)) fail('invalid or missing TEST_ID');
+if (action === 'create' && !passwordHash) fail('PASSWORD_HASH is required for create');
+if (!['create', 'verify', 'delete'].includes(action)) fail(`unsupported ACTION=${action}`);
+
+const fixtureRooms = [
+  ...messageAxis.map((messages) => ({ axis: 'messages', participants: 10, messages })),
+  ...participantAxis.map((participants) => ({ axis: 'participants', participants, messages: fixedMessages })),
+].filter((fixture, index, all) => all.findIndex((candidate) =>
+  candidate.participants === fixture.participants && candidate.messages === fixture.messages) === index);
+
+if (action === 'delete') {
+  const roomIds = db.rooms.find({ perfJoinTestId: testId }, { _id: 1 }).toArray().map((room) => room._id);
+  const messages = db.messages.deleteMany({ $or: [{ perfJoinTestId: testId }, { room: { $in: roomIds } }] });
+  const rooms = db.rooms.deleteMany({ perfJoinTestId: testId });
+  const users = db.users.deleteMany({ perfJoinTestId: testId, email: { $regex: `^${escapeRegex(prefix)}-` } });
+  print(EJSON.stringify({ action, testId, deleted: { users: users.deletedCount, rooms: rooms.deletedCount, messages: messages.deletedCount } }));
+  quit(0);
+}
+
+if (action === 'create') {
+  if (db.rooms.countDocuments({ perfJoinTestId: testId }) > 0
+      || db.users.countDocuments({ perfJoinTestId: testId }) > 0
+      || db.messages.countDocuments({ perfJoinTestId: testId }) > 0) {
+    fail(`fixture ${testId} already exists; delete it explicitly before recreating`);
+  }
+  const maximumParticipants = Math.max(...fixtureRooms.map((fixture) => fixture.participants));
+  const memberCount = Math.max(maximumParticipants, maxVus);
+  const users = [];
+  for (let index = 0; index < memberCount; index += 1) users.push(user('member', index));
+  for (let index = 0; index < maxVus; index += 1) users.push(user('new', index));
+  db.users.insertMany(users, { ordered: false });
+
+  const memberIds = users.filter((item) => item.perfJoinRole === 'member').map((item) => item._id);
+  fixtureRooms.forEach((fixture) => {
+    const slug = `p${fixture.participants}-m${fixture.messages}`;
+    const roomId = new ObjectId();
+    db.rooms.insertOne({
+      _id: roomId,
+      name: `${prefix}-${slug}`,
+      creator: memberIds[0].toString(),
+      hasPassword: false,
+      createdAt: new Date(),
+      participantIds: memberIds.slice(0, fixture.participants).map(String),
+      perfJoinTestId: testId,
+      perfJoinFixture: slug,
+      perfJoinAxis: fixture.axis,
+    });
+    insertMessages(roomId.toString(), fixture.messages, memberIds);
+  });
+}
+
+const verification = verify();
+print(EJSON.stringify({ action, testId, prefix, fixtures: verification }));
+if (verification.some((fixture) => !fixture.valid)) quit(2);
+
+function user(role, index) {
+  return {
+    _id: new ObjectId(),
+    name: `${prefix}-${role}-${index}`,
+    email: `${prefix}-${role}-${index}@perf-join.test`,
+    password: passwordHash,
+    createdAt: new Date(),
+    updatedAt: new Date(),
+    isOnline: false,
+    perfJoinTestId: testId,
+    perfJoinRole: role,
+  };
+}
+
+function insertMessages(roomId, count, memberIds) {
+  const batchSize = 1000;
+  for (let offset = 0; offset < count; offset += batchSize) {
+    const batch = [];
+    const limit = Math.min(count, offset + batchSize);
+    for (let index = offset; index < limit; index += 1) {
+      batch.push({
+        room: roomId,
+        sender: memberIds[index % memberIds.length].toString(),
+        content: `${prefix}-message-${index}`,
+        type: 'TEXT',
+        timestamp: new Date(Date.now() - (count - index) * 1000),
+        mentions: [], reactions: {}, readers: [], metadata: {},
+        perfJoinTestId: testId,
+      });
+    }
+    db.messages.insertMany(batch, { ordered: false });
+  }
+}
+
+function verify() {
+  return fixtureRooms.map((expected) => {
+    const slug = `p${expected.participants}-m${expected.messages}`;
+    const room = db.rooms.findOne({ perfJoinTestId: testId, perfJoinFixture: slug });
+    const actualMessages = room ? db.messages.countDocuments({ perfJoinTestId: testId, room: room._id.toString() }) : 0;
+    const actualParticipants = room?.participantIds?.length || 0;
+    return {
+      id: `${prefix}-${slug}`,
+      roomId: room ? room._id.toString() : null,
+      axis: room?.perfJoinAxis || expected.axis,
+      participants: actualParticipants,
+      messages: actualMessages,
+      valid: Boolean(room) && actualParticipants === expected.participants && actualMessages === expected.messages,
+    };
+  });
+}
+
+function intList(value) {
+  const values = value.split(',').map((item) => Number(item.trim()));
+  if (values.some((item) => !Number.isInteger(item) || item <= 0)) fail(`invalid integer list: ${value}`);
+  return values;
+}
+
+function integerEnv(name, fallback) {
+  const value = Number(process.env[name] || fallback);
+  if (!Number.isInteger(value) || value <= 0) fail(`${name} must be a positive integer`);
+  return value;
+}
+
+function escapeRegex(value) { return value.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); }
+function fail(message) { print(`ERROR: ${message}`); quit(1); }
diff --git a/performance/room-join/lib.js b/performance/room-join/lib.js
new file mode 100644
index 00000000..800d02fd
--- /dev/null
+++ b/performance/room-join/lib.js
@@ -0,0 +1,66 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+
+const PROMETHEUS_QUERIES = {
+  processCpu: 'process_cpu_usage',
+  heapUsed: 'jvm_memory_used_bytes{area="heap"}',
+  gcPauseRate: 'rate(jvm_gc_pause_seconds_sum[1m])',
+  httpAverage: 'sum(rate(http_server_requests_seconds_sum{uri="/api/rooms/{roomId}/join"}[1m]))/sum(rate(http_server_requests_seconds_count{uri="/api/rooms/{roomId}/join"}[1m]))',
+  httpMax: 'max(http_server_requests_seconds_max{uri="/api/rooms/{roomId}/join"})',
+  mongoCommandRate: 'sum(rate(mongodb_driver_commands_seconds_count[1m]))',
+  mongoCommandMax: 'max(mongodb_driver_commands_seconds_max)',
+  mongoCheckedOut: 'sum(mongodb_driver_pool_checkedout)',
+  mongoWaitQueue: 'sum(mongodb_driver_pool_waitqueuesize)',
+  mongoCheckoutFailures: 'sum(increase(mongodb_driver_pool_checkoutfailed_total[1m]))',
+  tomcatBusyThreads: 'sum(tomcat_threads_busy_threads)',
+  tomcatCurrentThreads: 'sum(tomcat_threads_current_threads)',
+  tomcatMaxThreads: 'sum(tomcat_threads_config_max_threads)',
+  tomcatBusyRatio: 'sum(tomcat_threads_busy_threads)/sum(tomcat_threads_config_max_threads)',
+};
+
+function parseArgs(argv) {
+  const values = {};
+  for (let index = 0; index < argv.length; index += 1) {
+    const argument = argv[index];
+    if (!argument.startsWith('--')) throw new Error(`unexpected argument: ${argument}`);
+    const [key, inline] = argument.slice(2).split('=', 2);
+    const value = inline === undefined ? argv[++index] : inline;
+    if (value === undefined || value.startsWith('--')) throw new Error(`missing value for --${key}`);
+    values[key] = value;
+  }
+  return values;
+}
+
+function median(values) {
+  if (!values.length) return null;
+  const sorted = [...values].sort((left, right) => left - right);
+  const middle = Math.floor(sorted.length / 2);
+  return sorted.length % 2 ? sorted[middle] : (sorted[middle - 1] + sorted[middle]) / 2;
+}
+
+function decideOptimization(observation) {
+  if (observation.messagesScaleWithLatency && observation.planStage === 'COLLSCAN') {
+    return '{ room: 1, timestamp: 1 } index';
+  }
+  if (observation.participantsScaleWithCommands) {
+    return 'batch participant lookup and remove duplicate response assembly';
+  }
+  if (observation.newParticipantOnlyRegression || observation.missingParticipant) {
+    return '$addToSet atomic participant update';
+  }
+  return 'no optimization selected; collect more Stage 0 evidence';
+}
+
+function ensureResultPath(root, requested) {
+  const resultRoot = path.resolve(root, 'results');
+  const resolved = path.resolve(root, requested);
+  if (resolved !== resultRoot && !resolved.startsWith(`${resultRoot}${path.sep}`)) {
+    throw new Error(`result path must stay under ${resultRoot}`);
+  }
+  fs.mkdirSync(path.dirname(resolved), { recursive: true });
+  return resolved;
+}
+
+module.exports = { PROMETHEUS_QUERIES, parseArgs, median, decideOptimization, ensureResultPath };
diff --git a/performance/room-join/prometheus.performance.yml b/performance/room-join/prometheus.performance.yml
new file mode 100644
index 00000000..551b357c
--- /dev/null
+++ b/performance/room-join/prometheus.performance.yml
@@ -0,0 +1,13 @@
+global:
+  scrape_interval: 1s
+  scrape_timeout: 1s
+
+scrape_configs:
+  - job_name: spring-boot-app
+    metrics_path: /actuator/prometheus
+    static_configs:
+      - targets: ["app:5001"]
+
+  - job_name: mongodb
+    static_configs:
+      - targets: ["mongodb-exporter:9216"]
diff --git a/performance/room-join/room-join.js b/performance/room-join/room-join.js
index d77f3069..386f3d03 100644
--- a/performance/room-join/room-join.js
+++ b/performance/room-join/room-join.js
@@ -1,91 +1,181 @@
 import http from 'k6/http';
 import { check, sleep } from 'k6';
 import { Counter, Rate, Trend } from 'k6/metrics';
+import exec from 'k6/execution';
 
 const API_BASE_URL = __ENV.API_BASE_URL || 'http://localhost:5001';
-const TEST_NAME = __ENV.TEST_NAME || 'room-join-local';
-const FIXTURE_ID = __ENV.FIXTURE_ID || 'unspecified';
-const VUS = Number(__ENV.VUS || 10);
-const DURATION = __ENV.DURATION || '3m';
-const THINK_TIME_MS = Number(__ENV.THINK_TIME_MS || 1000);
-const ACCOUNT_COUNT = Number(__ENV.ACCOUNT_COUNT || 30);
-const PASSWORD = __ENV.LOAD_TEST_PASSWORD || 'Test1234!';
-
-const duration = new Trend('room_join_duration', true);
-const errors = new Rate('room_join_errors');
+const TEST_ID = __ENV.TEST_ID || 'room-join-local';
+const FIXTURE_ID = required('FIXTURE_ID');
+const ROOM_ID = required('ROOM_ID');
+const MODE = __ENV.MODE || 'idempotent';
+const VUS = positiveInteger('VUS', 1);
+const DURATION = __ENV.DURATION || '30s';
+const THINK_TIME_MS = nonNegativeNumber('THINK_TIME_MS', 0);
+const PASSWORD = __ENV.PERF_PASSWORD || 'Test1234!';
+const EMAIL_DOMAIN = __ENV.EMAIL_DOMAIN || 'perf-join.test';
+const TIMEOUT_MS = positiveInteger('TIMEOUT_MS', 30000);
+const SMOKE_P95_MS = Number(__ENV.SMOKE_P95_MS || 0);
+const RESULT_PATH = __ENV.RESULT_PATH || `performance/room-join/results/${TEST_ID}.json`;
+const ITERATIONS = Number(__ENV.ITERATIONS || 0);
+const PARTICIPANTS = positiveInteger('PARTICIPANTS', 1);
+const ACCOUNT_PREFIX = __ENV.ACCOUNT_PREFIX || FIXTURE_ID.replace(/-p\d+-m\d+$/, '');
+
+if (!['idempotent', 'new-participant'].includes(MODE)) {
+  throw new Error(`MODE must be idempotent or new-participant, got ${MODE}`);
+}
+
+const joinDuration = new Trend('room_join_duration', true);
+const joinErrors = new Rate('room_join_errors');
 const completed = new Counter('room_join_completed');
 const timeouts = new Counter('room_join_timeouts');
 
+const thresholds = {
+  room_join_errors: ['rate<=0.01'],
+  room_join_timeouts: ['count==0'],
+};
+if (SMOKE_P95_MS > 0) thresholds.room_join_duration = [`p(95)<=${SMOKE_P95_MS * 2}`];
+
 export const options = {
-  vus: VUS,
-  duration: DURATION,
+  ...(ITERATIONS > 0
+    ? { scenarios: { join: { executor: 'shared-iterations', vus: VUS, iterations: ITERATIONS, maxDuration: DURATION } } }
+    : { vus: VUS, duration: DURATION }),
+  setupTimeout: '5m',
   summaryTrendStats: ['avg', 'min', 'med', 'max', 'p(90)', 'p(95)', 'p(99)'],
-  thresholds: {
-    room_join_errors: ['rate<=0.01'],
-    room_join_timeouts: ['count==0'],
-  },
+  thresholds,
 };
 
+function required(name) {
+  const value = __ENV[name];
+  if (!value) throw new Error(`${name} is required`);
+  return value;
+}
+
+function positiveInteger(name, fallback) {
+  const value = Number(__ENV[name] || fallback);
+  if (!Number.isInteger(value) || value <= 0) throw new Error(`${name} must be a positive integer`);
+  return value;
+}
+
+function nonNegativeNumber(name, fallback) {
+  const value = Number(__ENV[name] || fallback);
+  if (!Number.isFinite(value) || value < 0) throw new Error(`${name} must be non-negative`);
+  return value;
+}
+
 function account(index) {
-  return { email: `loadtest-${index}@test.com`, password: PASSWORD };
+  const role = MODE === 'idempotent' ? 'member' : 'new';
+  return { email: `${ACCOUNT_PREFIX}-${role}-${index}@${EMAIL_DOMAIN}`, password: PASSWORD };
 }
 
+let newParticipantJoined = false;
+
 export function setup() {
-  if (VUS > ACCOUNT_COUNT) throw new Error(`VUS (${VUS}) exceeds ACCOUNT_COUNT (${ACCOUNT_COUNT})`);
   const tokens = [];
-  for (let index = 0; index < ACCOUNT_COUNT; index += 1) {
+  const userIds = [];
+  const accountCount = MODE === 'idempotent' ? Math.min(VUS, PARTICIPANTS) : VUS;
+  for (let index = 0; index < accountCount; index += 1) {
     const response = http.post(`${API_BASE_URL}/api/auth/login`, JSON.stringify(account(index)), {
-      headers: { 'Content-Type': 'application/json' }, timeout: '10s', tags: { phase: 'setup' },
+      headers: { 'Content-Type': 'application/json' },
+      timeout: `${TIMEOUT_MS}ms`,
+      tags: { phase: 'setup' },
     });
-    if (response.status !== 200 || !response.json('token')) {
-      throw new Error(`Fixed account ${account(index).email} unavailable (HTTP ${response.status})`);
+    const token = response.json('token');
+    const userId = response.json('user._id') || response.json('user.id');
+    if (response.status !== 200 || !token || !userId) {
+      throw new Error(`Fixture account unavailable: ${account(index).email} (HTTP ${response.status})`);
     }
-    tokens.push(response.json('token'));
+    tokens.push(token);
+    userIds.push(userId);
   }
-  const rooms = http.get(`${API_BASE_URL}/api/rooms`, {
-    headers: { Authorization: `Bearer ${tokens[0]}` }, timeout: '30s', tags: { phase: 'setup' },
-  });
-  const roomId = rooms.json('data.0._id') || rooms.json('data.0.id');
-  if (rooms.status !== 200 || !roomId) throw new Error('No room is available for room-join measurement');
-  return { tokens, roomId };
+  return { tokens, userIds };
 }
 
 export default function (data) {
-  const token = data.tokens[(__VU - 1) % data.tokens.length];
-  const response = http.post(`${API_BASE_URL}/api/rooms/${data.roomId}/join`, '{}', {
-    headers: { Authorization: `Bearer ${token}`, 'Content-Type': 'application/json' },
-    timeout: '30s', tags: { phase: 'measurement', endpoint: 'room-join' },
+  const fixedIterations = ITERATIONS > 0;
+  if (MODE === 'new-participant' && !fixedIterations && newParticipantJoined) {
+    sleep(1);
+    return;
+  }
+  const index = MODE === 'new-participant' && fixedIterations
+    ? exec.scenario.iterationInTest % data.tokens.length
+    : (__VU - 1) % data.tokens.length;
+  const expectedUserId = data.userIds[index];
+  const response = http.post(`${API_BASE_URL}/api/rooms/${ROOM_ID}/join`, '{}', {
+    headers: {
+      Authorization: `Bearer ${data.tokens[index]}`,
+      'Content-Type': 'application/json',
+    },
+    timeout: `${TIMEOUT_MS}ms`,
+    tags: { phase: 'measurement', endpoint: 'room-join', mode: MODE, fixture: FIXTURE_ID },
   });
+  let payload = null;
+  if (response.body) {
+    try {
+      payload = response.json();
+    } catch (_) {
+      payload = null;
+    }
+  }
+
   const success = check(response, {
     'room join returned HTTP 200': (res) => res.status === 200,
-    'room join preserved success contract': (res) => res.json('success') === true,
-    'room join returned a room': (res) => Boolean(res.json('data._id') || res.json('data.id')),
+    'room join preserved success contract': () => payload?.success === true,
+    'room join returned expected room': (res) =>
+      (payload?.data?._id || payload?.data?.id) === ROOM_ID,
+    'room join contains participant': () => {
+      const participants = payload?.data?.participants;
+      return Array.isArray(participants)
+        && participants.some((participant) => (participant._id || participant.id) === expectedUserId);
+    },
   });
-  duration.add(response.timings.duration);
-  errors.add(!success);
+
+  joinDuration.add(response.timings.duration);
+  joinErrors.add(!success);
   completed.add(1);
-  if (response.error_code === 1050 || response.timings.duration >= 30000) timeouts.add(1);
-  sleep(THINK_TIME_MS / 1000);
+  if (response.error_code === 1050 || response.timings.duration >= TIMEOUT_MS) timeouts.add(1);
+  if (MODE === 'new-participant' && !fixedIterations) newParticipantJoined = true;
+  if (THINK_TIME_MS > 0) sleep(THINK_TIME_MS / 1000);
 }
 
-function value(data, metric, field, fallback = 0) {
-  return data.metrics[metric]?.values[field] ?? fallback;
+function metric(data, name, field, fallback = 0) {
+  return data.metrics[name]?.values[field] ?? fallback;
 }
 
 export function handleSummary(data) {
+  const runDurationMs = data.state?.testRunDurationMs || metric(data, 'iteration_duration', 'max', 0);
+  const startedAt = new Date(Date.now() - runDurationMs).toISOString();
+  const requests = metric(data, 'room_join_completed', 'count');
+  const durationSeconds = runDurationMs / 1000;
   const summary = {
-    testName: TEST_NAME, fixtureId: FIXTURE_ID, apiBaseUrl: API_BASE_URL,
-    model: 'HTTP-only room join', vus: VUS, duration: DURATION, thinkTimeMs: THINK_TIME_MS,
+    schemaVersion: 1,
+    testId: TEST_ID,
+    commit: __ENV.GIT_COMMIT || 'unknown',
+    fixture: {
+      id: FIXTURE_ID,
+      roomId: ROOM_ID,
+      participants: Number(__ENV.PARTICIPANTS || 0),
+      messages: Number(__ENV.MESSAGES || 0),
+    },
+    mode: MODE,
+    vus: VUS,
+    configuredDuration: DURATION,
+    startedAt,
     endedAt: new Date().toISOString(),
-    p95Ms: value(data, 'room_join_duration', 'p(95)'),
-    p99Ms: value(data, 'room_join_duration', 'p(99)'),
-    averageMs: value(data, 'room_join_duration', 'avg'),
-    errorRate: value(data, 'room_join_errors', 'rate'),
-    timeoutCount: value(data, 'room_join_timeouts', 'count'),
-    completedRequests: value(data, 'room_join_completed', 'count'),
+    requests,
+    rps: durationSeconds > 0 ? requests / durationSeconds : 0,
+    latencyMs: {
+      average: metric(data, 'room_join_duration', 'avg'),
+      p95: metric(data, 'room_join_duration', 'p(95)'),
+      p99: metric(data, 'room_join_duration', 'p(99)'),
+      max: metric(data, 'room_join_duration', 'max'),
+    },
+    errorRate: metric(data, 'room_join_errors', 'rate'),
+    timeoutCount: metric(data, 'room_join_timeouts', 'count'),
+    thresholdsPassed: !Object.values(data.metrics).some((item) => item.thresholds
+      && Object.values(item.thresholds).some((threshold) => threshold.ok === false)),
   };
   return {
     stdout: `${JSON.stringify(summary, null, 2)}\n`,
-    [`performance/room-join/results/${TEST_NAME}.json`]: `${JSON.stringify(summary, null, 2)}\n`,
+    [RESULT_PATH]: `${JSON.stringify(summary, null, 2)}\n`,
   };
 }
diff --git a/performance/room-join/run-matrix.js b/performance/room-join/run-matrix.js
new file mode 100644
index 00000000..f23ee896
--- /dev/null
+++ b/performance/room-join/run-matrix.js
@@ -0,0 +1,129 @@
+#!/usr/bin/env node
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const { spawnSync } = require('child_process');
+const { PROMETHEUS_QUERIES, parseArgs, ensureResultPath } = require('./lib');
+
+const ROOT = __dirname;
+const args = parseArgs(process.argv.slice(2));
+const manifestPath = required('manifest');
+const testId = args['test-id'] || `room-join-${new Date().toISOString().replace(/[:.]/g, '-')}`;
+const apiUrl = args['api-url'] || 'http://localhost:5001';
+const prometheusUrl = args['prometheus-url'] || 'http://localhost:9090';
+const mongoUri = args['mongo-uri'] || 'mongodb://localhost:27017/bootcamp-chat';
+const modes = (args.modes || 'idempotent,new-participant').split(',');
+const stages = parseStages(args.stages || '1:30s,10:1m,30:1m,50:1m,100:1m');
+const fixtures = JSON.parse(fs.readFileSync(manifestPath, 'utf8')).fixtures;
+const commit = command('git', ['rev-parse', 'HEAD']).stdout.trim();
+const aggregate = { schemaVersion: 1, testId, commit, startedAt: new Date().toISOString(), runs: [], stopped: [] };
+
+for (const fixture of fixtures) {
+  if (!fixture.valid) throw new Error(`invalid fixture: ${fixture.id}`);
+  for (const mode of modes) {
+    let smokeP95 = 0;
+    for (const [vus, duration] of stages) {
+      if (mode === 'new-participant') resetNewParticipants(fixture.roomId);
+      const runId = `${testId}-${fixture.id}-${mode}-${vus}vu`;
+      const relativeResult = `results/${runId}.json`;
+      const resultPath = ensureResultPath(ROOT, relativeResult);
+      const startedAt = new Date().toISOString();
+      const k6Environment = {
+        API_BASE_URL: apiUrl, TEST_ID: runId, FIXTURE_ID: fixture.id,
+        ROOM_ID: fixture.roomId, PARTICIPANTS: String(fixture.participants), MESSAGES: String(fixture.messages),
+        MODE: mode, VUS: String(vus), DURATION: duration, SMOKE_P95_MS: String(smokeP95),
+        RESULT_PATH: resultPath, GIT_COMMIT: commit,
+      };
+      if (process.env.PERF_PASSWORD) k6Environment.PERF_PASSWORD = process.env.PERF_PASSWORD;
+      const k6Process = command('k6', ['run', ...k6EnvArgs(k6Environment), path.join(ROOT, 'room-join.js')], undefined, false);
+      if (!fs.existsSync(resultPath)) throw new Error(`k6 did not write ${resultPath}: ${k6Process.stderr}`);
+      const result = JSON.parse(fs.readFileSync(resultPath, 'utf8'));
+      result.observability = collectPrometheus(prometheusUrl, startedAt, result.endedAt);
+      result.waitQueueDetected = maximum(result.observability.mongoWaitQueue) > 0;
+      result.checkoutFailureDetected = maximum(result.observability.mongoCheckoutFailures) > 0;
+      fs.writeFileSync(resultPath, `${JSON.stringify(result, null, 2)}\n`);
+      aggregate.runs.push(result);
+      if (vus === 1) smokeP95 = result.latencyMs.p95;
+      const reason = stopReason(result, smokeP95, k6Process.status);
+      if (reason) {
+        aggregate.stopped.push({ fixture: fixture.id, mode, afterVus: vus, reason });
+        break;
+      }
+    }
+  }
+}
+
+function resetNewParticipants(roomId) {
+  const escapedTestId = JSON.stringify(fixturesTestId());
+  const escapedRoomId = JSON.stringify(roomId);
+  const script = `const ids=db.users.find({perfJoinTestId:${escapedTestId},perfJoinRole:'new'},{_id:1}).toArray().map(u=>u._id.toString()); db.rooms.updateOne({_id:ObjectId(${escapedRoomId}),perfJoinTestId:${escapedTestId}},{$pull:{participantIds:{$in:ids}}}); print('reset='+ids.length);`;
+  command('mongosh', ['--quiet', mongoUri, '--eval', script]);
+}
+
+function fixturesTestId() {
+  const ids = new Set(fixtures.map((fixture) => fixture.id.match(/^perf-join-(.+)-p\d+-m\d+$/)?.[1]));
+  if (ids.size !== 1 || ids.has(undefined)) throw new Error('manifest fixture IDs do not share one valid test ID');
+  return [...ids][0];
+}
+
+aggregate.endedAt = new Date().toISOString();
+const aggregatePath = ensureResultPath(ROOT, `results/${testId}-aggregate.json`);
+fs.writeFileSync(aggregatePath, `${JSON.stringify(aggregate, null, 2)}\n`);
+console.log(aggregatePath);
+
+function collectPrometheus(baseUrl, start, end) {
+  const observations = {};
+  for (const [name, query] of Object.entries(PROMETHEUS_QUERIES)) {
+    const url = `${baseUrl}/api/v1/query_range?query=${encodeURIComponent(query)}&start=${encodeURIComponent(start)}&end=${encodeURIComponent(end)}&step=5s`;
+    try {
+      const response = command('curl', ['-fsS', url], undefined, false);
+      const payload = JSON.parse(response.stdout);
+      observations[name] = (payload.data?.result || []).map((series) => ({
+        metric: series.metric,
+        values: series.values.map(([timestamp, value]) => [timestamp, Number(value)]),
+      }));
+    } catch (error) {
+      observations[name] = { error: error.message };
+    }
+  }
+  return observations;
+}
+
+function maximum(series) {
+  if (!Array.isArray(series)) return 0;
+  return Math.max(0, ...series.flatMap((item) => item.values.map((pair) => pair[1])).filter(Number.isFinite));
+}
+
+function stopReason(result, smokeP95, exitCode) {
+  if (exitCode !== 0 || !result.thresholdsPassed) return 'k6 threshold failed';
+  if (result.errorRate > 0.01) return 'error rate exceeded 1%';
+  if (result.timeoutCount > 0) return 'timeout detected';
+  if (smokeP95 > 0 && result.latencyMs.p95 > smokeP95 * 2) return 'p95 exceeded 2x smoke p95';
+  if (result.waitQueueDetected) return 'Mongo wait queue detected';
+  if (result.checkoutFailureDetected) return 'Mongo checkout failure detected';
+  return null;
+}
+
+function parseStages(value) {
+  return value.split(',').map((stage) => {
+    const [vus, duration] = stage.split(':');
+    if (!/^\d+$/.test(vus) || Number(vus) <= 0 || !/^\d+(ms|s|m|h)$/.test(duration || '')) {
+      throw new Error(`invalid stage: ${stage}`);
+    }
+    return [Number(vus), duration];
+  });
+}
+
+function command(executable, commandArgs, env, throwOnFailure = true) {
+  const result = spawnSync(executable, commandArgs, { encoding: 'utf8', stdio: ['ignore', 'pipe', 'pipe'], env });
+  if (result.error || (throwOnFailure && result.status !== 0)) {
+    throw new Error(`${executable} failed: ${result.error?.message || result.stderr}`);
+  }
+  return result;
+}
+
+function k6EnvArgs(environment) {
+  return Object.entries(environment).flatMap(([name, value]) => ['-e', `${name}=${value}`]);
+}
+function required(name) { if (!args[name]) throw new Error(`--${name} is required`); return args[name]; }
diff --git a/performance/room-join/stage0.js b/performance/room-join/stage0.js
new file mode 100644
index 00000000..83b6008f
--- /dev/null
+++ b/performance/room-join/stage0.js
@@ -0,0 +1,63 @@
+#!/usr/bin/env node
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const { spawnSync } = require('child_process');
+const { parseArgs, ensureResultPath } = require('./lib');
+
+const args = parseArgs(process.argv.slice(2));
+const fixture = required('fixture');
+const roomId = required('room-id');
+const mode = args.mode || 'idempotent';
+const testId = args['test-id'] || `stage0-${fixture}-${mode}`;
+const mongoUri = args['mongo-uri'] || 'mongodb://localhost:27017/bootcamp-chat';
+const apiUrl = args['api-url'] || 'http://localhost:5001';
+const outputPath = ensureResultPath(__dirname, `results/${testId}-diagnosis.json`);
+const start = new Date();
+const previousLevel = Number(mongoEval('db.getProfilingStatus().was'));
+
+try {
+  mongoEval('db.setProfilingLevel(2, { slowms: 0 }); print("enabled")');
+  const k6Result = path.join(__dirname, 'results', `${testId}-request.json`);
+  const k6Environment = {
+    API_BASE_URL: apiUrl, TEST_ID: testId, FIXTURE_ID: fixture, ROOM_ID: roomId,
+    MODE: mode, VUS: '1', ITERATIONS: '1', DURATION: '30s', RESULT_PATH: k6Result,
+    PARTICIPANTS: args.participants || '1', MESSAGES: args.messages || '0',
+  };
+  if (process.env.PERF_PASSWORD) k6Environment.PERF_PASSWORD = process.env.PERF_PASSWORD;
+  run('k6', ['run', ...k6EnvArgs(k6Environment), path.join(__dirname, 'room-join.js')]);
+  const profiler = JSON.parse(mongoEval(`
+    const since = ISODate('${start.toISOString()}');
+    const rows = db.system.profile.find({ ts: { $gte: since } }).toArray();
+    const commands = {};
+    rows.forEach(r => { const key = r.ns + ':' + (r.op || r.commandName || 'command'); commands[key] = (commands[key] || 0) + 1; });
+    print(EJSON.stringify({ commandCount: rows.length, commands, rows: rows.map(r => ({ ts:r.ts, op:r.op, ns:r.ns, millis:r.millis, docsExamined:r.docsExamined, keysExamined:r.keysExamined, nreturned:r.nreturned, command:r.command })) }));
+  `));
+  const explain = JSON.parse(mongoEval(`
+    const history = db.messages.find({room:'${roomId}'}).sort({timestamp:-1}).limit(30).explain('executionStats');
+    const recent = db.messages.find({room:'${roomId}',timestamp:{$gte:new Date(Date.now()-600000)}}).explain('executionStats');
+    function summary(x) { const e=x.executionStats; return { winningPlan:x.queryPlanner.winningPlan, nReturned:e.nReturned, totalDocsExamined:e.totalDocsExamined, totalKeysExamined:e.totalKeysExamined, executionTimeMillis:e.executionTimeMillis }; }
+    print(EJSON.stringify({ history:summary(history), recentCount:summary(recent) }));
+  `));
+  fs.writeFileSync(outputPath, `${JSON.stringify({
+    schemaVersion: 1, testId, fixture, roomId, mode,
+    startedAt: start.toISOString(), endedAt: new Date().toISOString(), profiler, explain,
+  }, null, 2)}\n`);
+  console.log(outputPath);
+} finally {
+  mongoEval(`db.setProfilingLevel(${Number.isInteger(previousLevel) ? previousLevel : 0}); print('restored')`);
+}
+
+function mongoEval(script) {
+  return run('mongosh', ['--quiet', mongoUri, '--eval', script]).stdout.trim().split('\n').at(-1);
+}
+function run(executable, commandArgs, env) {
+  const result = spawnSync(executable, commandArgs, { encoding: 'utf8', env });
+  if (result.error || result.status !== 0) throw new Error(`${executable} failed: ${result.error?.message || result.stderr}`);
+  return result;
+}
+function k6EnvArgs(environment) {
+  return Object.entries(environment).flatMap(([name, value]) => ['-e', `${name}=${value}`]);
+}
+function required(name) { if (!args[name]) throw new Error(`--${name} is required`); return args[name]; }

From 4bdede9cd8234c262237028769fcc95cfa6d21ff Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 19:18:41 +0900
Subject: [PATCH 19/32] perf: optimize room detail and REST join queries

---
 .../chatapp/controller/RoomController.java    |  37 +++--
 .../chatapp/repository/RoomRepository.java    |   2 +-
 .../repository/RoomRepositoryCustom.java      |   8 ++
 .../repository/RoomRepositoryImpl.java        |  27 ++++
 .../com/ktb/chatapp/service/RoomService.java  |  48 ++++---
 .../RoomControllerRoomDetailTest.java         | 135 ++++++++++++++++++
 .../RoomRepositoryImplIntegrationTest.java    |  97 +++++++++++++
 .../ktb/chatapp/service/RoomServiceTest.java  |  49 +++++++
 8 files changed, 373 insertions(+), 30 deletions(-)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerRoomDetailTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
index 8d81ea8f..108bf773 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
@@ -20,15 +20,20 @@
 import java.security.Principal;
 import java.time.Duration;
 import java.time.LocalDateTime;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.http.CacheControl;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.security.oauth2.server.resource.authentication.JwtAuthenticationToken;
 import org.springframework.web.bind.annotation.*;
 
 @Tag(name = "채팅방 (Rooms)", description = "채팅방 생성 및 관리 API - 채팅방 목록 조회, 생성, 참여, 헬스체크")
@@ -232,16 +237,19 @@ public ResponseEntity<?> getRoomById(@Parameter(description = "채팅방 ID", ex
     public ResponseEntity<?> joinRoom(
             @Parameter(description = "채팅방 ID", example = "60d5ec49f1b2c8b9e8c4f2a1") @PathVariable String roomId,
             @RequestBody JoinRoomRequest joinRoomRequest,
-            Principal principal) {
+            JwtAuthenticationToken principal) {
         try {
-            Room joinedRoom = roomService.joinRoom(roomId, joinRoomRequest.getPassword(), principal.getName());
+            String userId = principal.getToken().getClaimAsString("userId");
+            RoomResponse roomResponse = roomService.joinRoom(
+                roomId,
+                joinRoomRequest.getPassword(),
+                userId
+            );
 
-            if (joinedRoom == null) {
+            if (roomResponse == null) {
                 return ResponseEntity.status(HttpStatus.NOT_FOUND)
                         .body(StandardResponse.error("채팅방을 찾을 수 없습니다."));
             }
-
-            RoomResponse roomResponse = mapToRoomResponse(joinedRoom, principal.getName());
             
             return ResponseEntity.ok(
                 Map.of(
@@ -267,20 +275,27 @@ public ResponseEntity<?> joinRoom(
     }
 
     private RoomResponse mapToRoomResponse(Room room, String name) {
-        User creator = userRepository.findById(room.getCreator()).orElse(null);
+        Set<String> userIds = new LinkedHashSet<>();
+        userIds.add(room.getCreator());
+        userIds.addAll(room.getParticipantIds());
+        Map<String, User> usersById = userRepository.findAllById(userIds).stream()
+                .collect(Collectors.toMap(User::getId, Function.identity()));
+
+        User creator = usersById.get(room.getCreator());
         if (creator == null) {
             throw new RuntimeException("Creator not found for room " + room.getId());
         }
         UserResponse creatorSummary = UserResponse.from(creator);
         List<UserResponse> participantSummaries = room.getParticipantIds()
                 .stream()
-                .map(userRepository::findById).peek(optUser -> {
-                    if (optUser.isEmpty()) {
-                        log.warn("Participant not found: roomId={}, userId={}", room.getId(), optUser);
+                .map(userId -> {
+                    User participant = usersById.get(userId);
+                    if (participant == null) {
+                        log.warn("Participant not found: roomId={}, userId={}", room.getId(), userId);
                     }
+                    return participant;
                 })
-                .filter(Optional::isPresent)
-                .map(Optional::get)
+                .filter(java.util.Objects::nonNull)
                 .map(UserResponse::from)
                 .toList();
 
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepository.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepository.java
index dcf7f335..2477f0cc 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepository.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepository.java
@@ -9,7 +9,7 @@
 import java.util.Optional;
 
 @Repository
-public interface RoomRepository extends MongoRepository<Room, String> {
+public interface RoomRepository extends MongoRepository<Room, String>, RoomRepositoryCustom {
 
     // 가장 최근에 생성된 방 조회 (Health Check용)
     @Query(value = "{}", sort = "{ 'createdAt': -1 }")
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java
new file mode 100644
index 00000000..67653f69
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java
@@ -0,0 +1,8 @@
+package com.ktb.chatapp.repository;
+
+import com.ktb.chatapp.model.Room;
+
+public interface RoomRepositoryCustom {
+
+    Room addParticipantAndReturn(String roomId, String userId);
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java
new file mode 100644
index 00000000..061e1b30
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java
@@ -0,0 +1,27 @@
+package com.ktb.chatapp.repository;
+
+import com.ktb.chatapp.model.Room;
+import lombok.RequiredArgsConstructor;
+import org.springframework.data.mongodb.core.FindAndModifyOptions;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
+
+@RequiredArgsConstructor
+public class RoomRepositoryImpl implements RoomRepositoryCustom {
+
+    private final MongoTemplate mongoTemplate;
+
+    @Override
+    public Room addParticipantAndReturn(String roomId, String userId) {
+        Query query = Query.query(Criteria.where("_id").is(roomId));
+        Update update = new Update().addToSet("participantIds", userId);
+        return mongoTemplate.findAndModify(
+            query,
+            update,
+            FindAndModifyOptions.options().returnNew(true),
+            Room.class
+        );
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
index 61889789..5422abf6 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
@@ -176,15 +176,13 @@ public Optional<Room> findRoomById(String roomId) {
         return roomRepository.findById(roomId);
     }
 
-    public Room joinRoom(String roomId, String password, String name) {
+    public RoomResponse joinRoom(String roomId, String password, String userId) {
         Optional<Room> roomOpt = roomRepository.findById(roomId);
         if (roomOpt.isEmpty()) {
             return null;
         }
 
         Room room = roomOpt.get();
-        User user = userRepository.findByEmail(name)
-            .orElseThrow(() -> new RuntimeException("사용자를 찾을 수 없습니다: " + name));
 
         // 비밀번호 확인
         if (room.isHasPassword()) {
@@ -194,36 +192,50 @@ public Room joinRoom(String roomId, String password, String name) {
         }
 
         // 이미 참여중인지 확인
-        if (!room.getParticipantIds().contains(user.getId())) {
-            // 채팅방 참여
-            room.getParticipantIds().add(user.getId());
-            room = roomRepository.save(room);
+        if (!room.getParticipantIds().contains(userId)) {
+            room = roomRepository.addParticipantAndReturn(roomId, userId);
+            if (room == null) {
+                return null;
+            }
         }
-        
+
+        RoomResponse roomResponse = mapToRoomResponse(room, userId);
+
         // Publish event for room updated
         try {
-            RoomResponse roomResponse = mapToRoomResponse(room, name);
             eventPublisher.publishEvent(new RoomUpdatedEvent(this, roomId, roomResponse));
         } catch (Exception e) {
             log.error("roomUpdate 이벤트 발행 실패", e);
         }
 
-        return room;
+        return roomResponse;
     }
 
     private RoomResponse mapToRoomResponse(Room room, String name) {
         if (room == null) return null;
 
-        User creator = null;
+        Set<String> userIds = new LinkedHashSet<>();
         if (room.getCreator() != null) {
-            creator = userRepository.findById(room.getCreator()).orElse(null);
+            userIds.add(room.getCreator());
         }
-
-        List<User> participants = room.getParticipantIds().stream()
-            .map(userRepository::findById)
-            .filter(Optional::isPresent)
-            .map(Optional::get)
-            .toList();
+        if (room.getParticipantIds() != null) {
+            userIds.addAll(room.getParticipantIds());
+        }
+        Map<String, User> usersById = userIds.isEmpty()
+            ? Map.of()
+            : userRepository.findAllById(userIds).stream()
+                .filter(user -> user != null && user.getId() != null)
+                .collect(Collectors.toMap(User::getId, Function.identity()));
+        User creator = room.getCreator() != null ? usersById.get(room.getCreator()) : null;
+        if (creator == null) {
+            throw new RuntimeException("Creator not found for room " + room.getId());
+        }
+        List<User> participants = room.getParticipantIds() == null
+            ? List.of()
+            : room.getParticipantIds().stream()
+                .map(usersById::get)
+                .filter(java.util.Objects::nonNull)
+                .toList();
 
         int recentMessageCount = recentMessageCounter.countRecentMessages(room.getId());
 
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerRoomDetailTest.java b/apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerRoomDetailTest.java
new file mode 100644
index 00000000..c1f0cdc9
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerRoomDetailTest.java
@@ -0,0 +1,135 @@
+package com.ktb.chatapp.controller;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.ArgumentMatchers.anySet;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.ktb.chatapp.dto.RoomResponse;
+import com.ktb.chatapp.dto.JoinRoomRequest;
+import com.ktb.chatapp.model.Room;
+import com.ktb.chatapp.model.User;
+import com.ktb.chatapp.repository.UserRepository;
+import com.ktb.chatapp.service.RecentMessageCounter;
+import com.ktb.chatapp.service.RoomService;
+import java.security.Principal;
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.http.ResponseEntity;
+import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.security.oauth2.server.resource.authentication.JwtAuthenticationToken;
+
+@ExtendWith(MockitoExtension.class)
+class RoomControllerRoomDetailTest {
+
+    @Mock private UserRepository userRepository;
+    @Mock private RecentMessageCounter recentMessageCounter;
+    @Mock private RoomService roomService;
+
+    private RoomController roomController;
+
+    @BeforeEach
+    void setUp() {
+        roomController = new RoomController(userRepository, recentMessageCounter, roomService);
+    }
+
+    @Test
+    void getRoomById_batchesCreatorAndParticipantsAndPreservesOrder() {
+        Room room = Room.builder()
+            .id("room-1")
+            .name("Room")
+            .creator("creator-1")
+            .createdAt(LocalDateTime.of(2026, 8, 11, 10, 0))
+            .participantIds(new LinkedHashSet<>(List.of(
+                "participant-2", "missing-user", "participant-1")))
+            .build();
+        User creator = user("creator-1", "Creator");
+        User participant1 = user("participant-1", "Participant 1");
+        User participant2 = user("participant-2", "Participant 2");
+        when(roomService.findRoomById("room-1")).thenReturn(Optional.of(room));
+        when(userRepository.findAllById(anySet()))
+            .thenReturn(List.of(participant1, creator, participant2));
+        when(recentMessageCounter.countRecentMessages("room-1")).thenReturn(7);
+
+        ResponseEntity<?> response = roomController.getRoomById("room-1", principal("viewer@test.com"));
+
+        assertEquals(200, response.getStatusCode().value());
+        RoomResponse data = (RoomResponse) ((Map<?, ?>) response.getBody()).get("data");
+        assertEquals("creator-1", data.getCreator().getId());
+        assertEquals(List.of("participant-2", "participant-1"), data.getParticipants().stream()
+            .map(participant -> participant.getId()).toList());
+        assertEquals(7, data.getRecentMessageCount());
+        assertTrue((Boolean) ((Map<?, ?>) response.getBody()).get("success"));
+        verify(userRepository).findAllById(anySet());
+        verify(userRepository, never()).findById(org.mockito.ArgumentMatchers.anyString());
+    }
+
+    @Test
+    void getRoomById_returnsServerErrorWhenCreatorIsMissing() {
+        Room room = Room.builder()
+            .id("room-1")
+            .creator("missing-creator")
+            .participantIds(new LinkedHashSet<>())
+            .build();
+        when(roomService.findRoomById("room-1")).thenReturn(Optional.of(room));
+        when(userRepository.findAllById(anySet())).thenReturn(List.of());
+
+        ResponseEntity<?> response = roomController.getRoomById("room-1", principal("viewer@test.com"));
+
+        assertEquals(500, response.getStatusCode().value());
+        verify(userRepository).findAllById(anySet());
+    }
+
+    @Test
+    void joinRoom_passesJwtUserIdAndReturnsServiceResponseWithoutRemapping() {
+        RoomResponse roomResponse = RoomResponse.builder()
+            .id("room-1")
+            .participants(List.of())
+            .createdAtDateTime(LocalDateTime.now())
+            .build();
+        when(roomService.joinRoom("room-1", "secret", "user-1")).thenReturn(roomResponse);
+
+        ResponseEntity<?> response = roomController.joinRoom(
+            "room-1",
+            JoinRoomRequest.builder().password("secret").build(),
+            authentication("user-1", "user@test.com")
+        );
+
+        assertEquals(200, response.getStatusCode().value());
+        assertSame(roomResponse, ((Map<?, ?>) response.getBody()).get("data"));
+        verify(userRepository, never()).findAllById(anySet());
+        verify(recentMessageCounter, never()).countRecentMessages(org.mockito.ArgumentMatchers.anyString());
+    }
+
+    private static Principal principal(String name) {
+        return () -> name;
+    }
+
+    private static JwtAuthenticationToken authentication(String userId, String email) {
+        Instant now = Instant.now();
+        Jwt jwt = Jwt.withTokenValue("token")
+            .header("alg", "none")
+            .subject(email)
+            .claim("userId", userId)
+            .issuedAt(now)
+            .expiresAt(now.plusSeconds(60))
+            .build();
+        return new JwtAuthenticationToken(jwt, List.of(), email);
+    }
+
+    private static User user(String id, String name) {
+        return User.builder().id(id).name(name).email(id + "@test.com").build();
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java
new file mode 100644
index 00000000..d0df70f5
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java
@@ -0,0 +1,97 @@
+package com.ktb.chatapp.repository;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.ktb.chatapp.model.Room;
+import com.mongodb.client.MongoClient;
+import com.mongodb.client.MongoClients;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+import org.testcontainers.mongodb.MongoDBContainer;
+
+@Testcontainers(disabledWithoutDocker = true)
+class RoomRepositoryImplIntegrationTest {
+
+    @Container
+    private static final MongoDBContainer MONGO = new MongoDBContainer("mongo:8.3.4");
+
+    private static MongoClient mongoClient;
+    private static MongoTemplate mongoTemplate;
+    private RoomRepositoryImpl repository;
+
+    @BeforeAll
+    static void setUpClient() {
+        mongoClient = MongoClients.create(MONGO.getConnectionString());
+        mongoTemplate = new MongoTemplate(mongoClient, "room-repository-test");
+    }
+
+    @AfterAll
+    static void closeClient() {
+        mongoClient.close();
+    }
+
+    @BeforeEach
+    void setUp() {
+        mongoTemplate.dropCollection(Room.class);
+        repository = new RoomRepositoryImpl(mongoTemplate);
+        mongoTemplate.save(Room.builder()
+            .id("room-1")
+            .creator("creator-1")
+            .participantIds(new HashSet<>(Set.of("creator-1")))
+            .build());
+    }
+
+    @Test
+    void addParticipantAndReturnReturnsUpdatedRoom() {
+        Room updated = repository.addParticipantAndReturn("room-1", "user-1");
+
+        assertNotNull(updated);
+        assertEquals(Set.of("creator-1", "user-1"), updated.getParticipantIds());
+    }
+
+    @Test
+    void concurrentJoinsDoNotLoseOrDuplicateParticipants() throws Exception {
+        int participantCount = 20;
+        ExecutorService executor = Executors.newFixedThreadPool(participantCount);
+        CountDownLatch start = new CountDownLatch(1);
+        List<Future<Room>> futures = new ArrayList<>();
+        try {
+            for (int index = 0; index < participantCount; index++) {
+                String userId = "user-" + index;
+                futures.add(executor.submit(() -> {
+                    start.await();
+                    return repository.addParticipantAndReturn("room-1", userId);
+                }));
+            }
+            start.countDown();
+            for (Future<Room> future : futures) {
+                assertNotNull(future.get());
+            }
+        } finally {
+            executor.shutdownNow();
+        }
+
+        Room stored = mongoTemplate.findById("room-1", Room.class);
+        assertNotNull(stored);
+        assertEquals(participantCount + 1, stored.getParticipantIds().size());
+        assertTrue(stored.getParticipantIds().contains("creator-1"));
+        for (int index = 0; index < participantCount; index++) {
+            assertTrue(stored.getParticipantIds().contains("user-" + index));
+        }
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
index 05a697c7..858fabae 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
@@ -3,7 +3,9 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anySet;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
@@ -13,6 +15,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.ktb.chatapp.dto.RoomResponse;
 import com.ktb.chatapp.dto.RoomsResponse;
+import com.ktb.chatapp.event.RoomUpdatedEvent;
 import com.ktb.chatapp.model.Room;
 import com.ktb.chatapp.model.User;
 import com.ktb.chatapp.repository.RoomRepository;
@@ -24,10 +27,12 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.test.util.ReflectionTestUtils;
 
 @ExtendWith(MockitoExtension.class)
 class RoomServiceTest {
@@ -110,6 +115,50 @@ void getAllRooms_batchesUsersAndMessageCountsOnceAndKeepsContract() throws Excep
         assertFalse(json.has("createdAtDateTime"));
     }
 
+    @Test
+    void joinRoom_usesUserIdAtomicUpdateAndSharesOneResponseWithEvent() {
+        Room before = room("room-1", "Room", "creator-1", LocalDateTime.now(),
+            new java.util.LinkedHashSet<>(List.of("creator-1")));
+        Room after = room("room-1", "Room", "creator-1", before.getCreatedAt(),
+            new java.util.LinkedHashSet<>(List.of("creator-1", "user-1")));
+        User creator = user("creator-1", "Creator", "creator@test.com");
+        User participant = user("user-1", "Participant", "participant@test.com");
+        when(roomRepository.findById("room-1")).thenReturn(java.util.Optional.of(before));
+        when(roomRepository.addParticipantAndReturn("room-1", "user-1")).thenReturn(after);
+        when(userRepository.findAllById(anySet())).thenReturn(List.of(participant, creator));
+        when(recentMessageCounter.countRecentMessages("room-1")).thenReturn(4);
+
+        RoomResponse response = roomService.joinRoom("room-1", null, "user-1");
+
+        assertEquals(List.of("creator-1", "user-1"), response.getParticipants().stream()
+            .map(participantResponse -> participantResponse.getId()).toList());
+        assertEquals(false, ReflectionTestUtils.getField(response, "isCreator"));
+        assertEquals(4, response.getRecentMessageCount());
+        verify(userRepository, never()).findByEmail(any());
+        verify(roomRepository, never()).save(any());
+        verify(userRepository).findAllById(anySet());
+        verify(recentMessageCounter).countRecentMessages("room-1");
+        ArgumentCaptor<RoomUpdatedEvent> eventCaptor = ArgumentCaptor.forClass(RoomUpdatedEvent.class);
+        verify(eventPublisher).publishEvent(eventCaptor.capture());
+        assertSame(response, eventCaptor.getValue().getRoomResponse());
+    }
+
+    @Test
+    void joinRoom_keepsExistingParticipantWithoutMutationAndUsesIdForIsCreator() {
+        Room room = room("room-1", "Room", "creator-1", LocalDateTime.now(),
+            new java.util.LinkedHashSet<>(List.of("creator-1")));
+        User creator = user("creator-1", "Creator", "creator@test.com");
+        when(roomRepository.findById("room-1")).thenReturn(java.util.Optional.of(room));
+        when(userRepository.findAllById(anySet())).thenReturn(List.of(creator));
+
+        RoomResponse response = roomService.joinRoom("room-1", null, "creator-1");
+
+        assertEquals(true, ReflectionTestUtils.getField(response, "isCreator"));
+        verify(roomRepository, never()).addParticipantAndReturn(any(), any());
+        verify(roomRepository, never()).save(any());
+        verify(userRepository, never()).findByEmail(any());
+    }
+
     private static Room room(
             String id,
             String name,

From d4456cc7d418afa1c22845fed5c89a171b5c591c Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 19:27:02 +0900
Subject: [PATCH 20/32] feat: migrate unique users email index

---
 .../config/UserEmailIndexMigration.java       |  62 +++++++++
 .../main/java/com/ktb/chatapp/model/User.java |   4 +-
 ...serEmailIndexMigrationIntegrationTest.java | 126 ++++++++++++++++++
 3 files changed, 189 insertions(+), 3 deletions(-)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/config/UserEmailIndexMigration.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/config/UserEmailIndexMigrationIntegrationTest.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/UserEmailIndexMigration.java b/apps/backend/src/main/java/com/ktb/chatapp/config/UserEmailIndexMigration.java
new file mode 100644
index 00000000..9b129e9b
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/UserEmailIndexMigration.java
@@ -0,0 +1,62 @@
+package com.ktb.chatapp.config;
+
+import com.mongodb.client.MongoCollection;
+import com.mongodb.client.model.IndexOptions;
+import java.util.List;
+import lombok.RequiredArgsConstructor;
+import org.bson.Document;
+import org.springframework.boot.ApplicationArguments;
+import org.springframework.boot.ApplicationRunner;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.stereotype.Component;
+
+@Component
+@RequiredArgsConstructor
+public class UserEmailIndexMigration implements ApplicationRunner {
+
+    static final String COLLECTION = "users";
+    static final String INDEX_NAME = "email_unique";
+    private static final Document EMAIL_KEY = new Document("email", 1);
+
+    private final MongoTemplate mongoTemplate;
+
+    @Override
+    public void run(ApplicationArguments args) {
+        MongoCollection<Document> users = mongoTemplate.getCollection(COLLECTION);
+        for (Document index : users.listIndexes()) {
+            if (!EMAIL_KEY.equals(index.get("key", Document.class))) {
+                continue;
+            }
+            if (Boolean.TRUE.equals(index.getBoolean("unique"))) {
+                return;
+            }
+            throw new IllegalStateException(
+                "Cannot create unique users.email index: conflicting non-unique index "
+                    + index.getString("name")
+            );
+        }
+
+        Document duplicate = users.aggregate(List.of(
+            new Document("$group", new Document("_id", "$email")
+                .append("ids", new Document("$push", "$_id"))
+                .append("count", new Document("$sum", 1))),
+            new Document("$match", new Document("count", new Document("$gt", 1))),
+            new Document("$limit", 1)
+        )).first();
+        if (duplicate != null) {
+            throw new IllegalStateException(
+                "Cannot create unique users.email index: duplicate email="
+                    + duplicate.get("_id") + ", userIds=" + duplicate.getList("ids", Object.class)
+            );
+        }
+
+        try {
+            users.createIndex(EMAIL_KEY, new IndexOptions().unique(true).name(INDEX_NAME));
+        } catch (RuntimeException exception) {
+            throw new IllegalStateException(
+                "Failed to create unique users.email index on collection " + COLLECTION,
+                exception
+            );
+        }
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/User.java b/apps/backend/src/main/java/com/ktb/chatapp/model/User.java
index fb16bc6a..bff5ef77 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/User.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/User.java
@@ -8,7 +8,6 @@
 import org.springframework.data.annotation.CreatedDate;
 import org.springframework.data.annotation.Id;
 import org.springframework.data.annotation.LastModifiedDate;
-import org.springframework.data.mongodb.core.index.Indexed;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.event.AbstractMongoEventListener;
 import org.springframework.data.mongodb.core.mapping.event.BeforeConvertEvent;
@@ -28,7 +27,6 @@ public class User {
 
     private String name;
 
-    @Indexed(unique = true)
     private String email;
     
     private String encryptedEmail;
@@ -82,4 +80,4 @@ public void onBeforeConvert(BeforeConvertEvent<User> event) {
             }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/config/UserEmailIndexMigrationIntegrationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/config/UserEmailIndexMigrationIntegrationTest.java
new file mode 100644
index 00000000..e82d2978
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/config/UserEmailIndexMigrationIntegrationTest.java
@@ -0,0 +1,126 @@
+package com.ktb.chatapp.config;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.mongodb.client.MongoClient;
+import com.mongodb.client.MongoClients;
+import com.mongodb.client.MongoCollection;
+import com.mongodb.client.model.IndexOptions;
+import java.util.ArrayList;
+import java.util.List;
+import org.bson.Document;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+import org.testcontainers.mongodb.MongoDBContainer;
+
+@Testcontainers(disabledWithoutDocker = true)
+class UserEmailIndexMigrationIntegrationTest {
+
+    @Container
+    private static final MongoDBContainer MONGO = new MongoDBContainer("mongo:8.3.4");
+
+    private static MongoClient mongoClient;
+    private static MongoTemplate mongoTemplate;
+
+    @BeforeAll
+    static void setUpClient() {
+        mongoClient = MongoClients.create(MONGO.getConnectionString());
+        mongoTemplate = new MongoTemplate(mongoClient, "migration-test");
+    }
+
+    @AfterAll
+    static void closeClient() {
+        mongoClient.close();
+    }
+
+    @BeforeEach
+    void clearUsers() {
+        mongoTemplate.dropCollection(UserEmailIndexMigration.COLLECTION);
+    }
+
+    @Test
+    void createsUniqueIndexForEmptyDatabaseAndIsIdempotent() {
+        UserEmailIndexMigration migration = new UserEmailIndexMigration(mongoTemplate);
+
+        migration.run(null);
+        assertDoesNotThrow(() -> migration.run(null));
+
+        Document index = emailIndex();
+        assertEquals(UserEmailIndexMigration.INDEX_NAME, index.getString("name"));
+        assertTrue(index.getBoolean("unique"));
+    }
+
+    @Test
+    void keepsEquivalentUniqueIndexWithDifferentName() {
+        users().createIndex(
+            new Document("email", 1),
+            new IndexOptions().unique(true).name("existing_email_unique")
+        );
+
+        new UserEmailIndexMigration(mongoTemplate).run(null);
+
+        assertEquals("existing_email_unique", emailIndex().getString("name"));
+    }
+
+    @Test
+    void createsIndexForExistingDatabaseWithoutIndex() {
+        users().insertMany(List.of(
+            new Document("_id", "user-1").append("email", "one@test.com"),
+            new Document("_id", "user-2").append("email", "two@test.com")
+        ));
+
+        new UserEmailIndexMigration(mongoTemplate).run(null);
+
+        assertTrue(emailIndex().getBoolean("unique"));
+    }
+
+    @Test
+    void rejectsDuplicateEmailWithoutDeletingData() {
+        users().insertMany(List.of(
+            new Document("_id", "user-1").append("email", "duplicate@test.com"),
+            new Document("_id", "user-2").append("email", "duplicate@test.com")
+        ));
+
+        IllegalStateException exception = assertThrows(
+            IllegalStateException.class,
+            () -> new UserEmailIndexMigration(mongoTemplate).run(null)
+        );
+
+        assertTrue(exception.getMessage().contains("duplicate@test.com"));
+        assertTrue(exception.getMessage().contains("user-1"));
+        assertTrue(exception.getMessage().contains("user-2"));
+        assertEquals(2, users().countDocuments());
+    }
+
+    @Test
+    void rejectsConflictingNonUniqueEmailIndex() {
+        users().createIndex(new Document("email", 1), new IndexOptions().name("email_lookup"));
+
+        IllegalStateException exception = assertThrows(
+            IllegalStateException.class,
+            () -> new UserEmailIndexMigration(mongoTemplate).run(null)
+        );
+
+        assertTrue(exception.getMessage().contains("email_lookup"));
+    }
+
+    private static MongoCollection<Document> users() {
+        return mongoTemplate.getCollection(UserEmailIndexMigration.COLLECTION);
+    }
+
+    private static Document emailIndex() {
+        List<Document> indexes = users().listIndexes().into(new ArrayList<>());
+        return indexes.stream()
+            .filter(index -> new Document("email", 1).equals(index.get("key", Document.class)))
+            .findFirst()
+            .orElseThrow();
+    }
+}

From 69e00742286a56a2b24382e7ce2d24cb0bef0eff Mon Sep 17 00:00:00 2001
From: jaksfj <myki011122@gmail.com>
Date: Tue, 11 Aug 2026 19:27:30 +0900
Subject: [PATCH 21/32] perf: record room detail and join experiments

---
 .../results/exp2-after-p10-m1k-smoke-1.json   |  22 ++++
 .../results/exp2-after-p10-m1k-smoke-2.json   |  22 ++++
 .../results/exp2-after-p10-m1k-smoke-3.json   |  22 ++++
 .../results/exp2-after-p10-m1k-vu10-1.json    |  22 ++++
 .../results/exp2-after-p10-m1k-vu10-2.json    |  22 ++++
 .../results/exp2-after-p10-m1k-vu10-3.json    |  22 ++++
 .../results/exp2-after-p100-m1k-smoke-1.json  |  22 ++++
 .../results/exp2-after-p100-m1k-smoke-2.json  |  22 ++++
 .../results/exp2-after-p100-m1k-smoke-3.json  |  22 ++++
 .../results/exp2-after-p100-m1k-vu10-1.json   |  22 ++++
 .../results/exp2-after-p100-m1k-vu10-2.json   |  22 ++++
 .../results/exp2-after-p100-m1k-vu10-3.json   |  22 ++++
 .../results/exp2-after-p500-m1k-smoke-1.json  |  22 ++++
 .../results/exp2-after-p500-m1k-smoke-2.json  |  22 ++++
 .../results/exp2-after-p500-m1k-smoke-3.json  |  22 ++++
 .../results/exp2-after-p500-m1k-vu10-1.json   |  22 ++++
 .../results/exp2-after-p500-m1k-vu10-2.json   |  22 ++++
 .../results/exp2-after-p500-m1k-vu10-3.json   |  22 ++++
 .../room-detail/room-detail-exp2-report.md    |  36 +++++++
 performance/room-detail/room-detail.js        |   1 +
 performance/room-join/README.md               |  18 ++++
 performance/room-join/fixture.js              |  50 +++++++++
 .../results/join-final-mutation-vu10-1.json   |  19 ++++
 .../results/join-final-mutation-vu10-2.json   |  19 ++++
 .../results/join-final-mutation-vu10-3.json   |  19 ++++
 .../results/join-final-repeated-vu10-1.json   |  19 ++++
 .../results/join-final-repeated-vu10-2.json   |  19 ++++
 .../results/join-final-repeated-vu10-3.json   |  19 ++++
 performance/room-join/room-join-mutation.js   | 100 ++++++++++++++++++
 .../room-join-optimization-report.md          |  56 ++++++++++
 performance/room-join/room-join.js            |  22 ++--
 31 files changed, 786 insertions(+), 7 deletions(-)
 create mode 100644 performance/room-detail/results/exp2-after-p10-m1k-smoke-1.json
 create mode 100644 performance/room-detail/results/exp2-after-p10-m1k-smoke-2.json
 create mode 100644 performance/room-detail/results/exp2-after-p10-m1k-smoke-3.json
 create mode 100644 performance/room-detail/results/exp2-after-p10-m1k-vu10-1.json
 create mode 100644 performance/room-detail/results/exp2-after-p10-m1k-vu10-2.json
 create mode 100644 performance/room-detail/results/exp2-after-p10-m1k-vu10-3.json
 create mode 100644 performance/room-detail/results/exp2-after-p100-m1k-smoke-1.json
 create mode 100644 performance/room-detail/results/exp2-after-p100-m1k-smoke-2.json
 create mode 100644 performance/room-detail/results/exp2-after-p100-m1k-smoke-3.json
 create mode 100644 performance/room-detail/results/exp2-after-p100-m1k-vu10-1.json
 create mode 100644 performance/room-detail/results/exp2-after-p100-m1k-vu10-2.json
 create mode 100644 performance/room-detail/results/exp2-after-p100-m1k-vu10-3.json
 create mode 100644 performance/room-detail/results/exp2-after-p500-m1k-smoke-1.json
 create mode 100644 performance/room-detail/results/exp2-after-p500-m1k-smoke-2.json
 create mode 100644 performance/room-detail/results/exp2-after-p500-m1k-smoke-3.json
 create mode 100644 performance/room-detail/results/exp2-after-p500-m1k-vu10-1.json
 create mode 100644 performance/room-detail/results/exp2-after-p500-m1k-vu10-2.json
 create mode 100644 performance/room-detail/results/exp2-after-p500-m1k-vu10-3.json
 create mode 100644 performance/room-detail/room-detail-exp2-report.md
 create mode 100644 performance/room-join/fixture.js
 create mode 100644 performance/room-join/results/join-final-mutation-vu10-1.json
 create mode 100644 performance/room-join/results/join-final-mutation-vu10-2.json
 create mode 100644 performance/room-join/results/join-final-mutation-vu10-3.json
 create mode 100644 performance/room-join/results/join-final-repeated-vu10-1.json
 create mode 100644 performance/room-join/results/join-final-repeated-vu10-2.json
 create mode 100644 performance/room-join/results/join-final-repeated-vu10-3.json
 create mode 100644 performance/room-join/room-join-mutation.js
 create mode 100644 performance/room-join/room-join-optimization-report.md

diff --git a/performance/room-detail/results/exp2-after-p10-m1k-smoke-1.json b/performance/room-detail/results/exp2-after-p10-m1k-smoke-1.json
new file mode 100644
index 00000000..3380cb38
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p10-m1k-smoke-1.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p10-m1k-smoke-1",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p10-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:35:48.254Z",
+  "endedAt": "2026-08-11T08:35:58.584Z",
+  "requests": 3164,
+  "rps": 306.3175279362263,
+  "averageMs": 3.067639380530983,
+  "p95Ms": 5.3027,
+  "p99Ms": 7.539099999999999,
+  "maxMs": 50.862,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1855
+}
diff --git a/performance/room-detail/results/exp2-after-p10-m1k-smoke-2.json b/performance/room-detail/results/exp2-after-p10-m1k-smoke-2.json
new file mode 100644
index 00000000..a43239fd
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p10-m1k-smoke-2.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p10-m1k-smoke-2",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p10-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:35:59.532Z",
+  "endedAt": "2026-08-11T08:36:09.556Z",
+  "requests": 3494,
+  "rps": 348.5782268302477,
+  "averageMs": 2.7766694333142516,
+  "p95Ms": 3.96235,
+  "p99Ms": 5.480699999999996,
+  "maxMs": 26.171,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1855
+}
diff --git a/performance/room-detail/results/exp2-after-p10-m1k-smoke-3.json b/performance/room-detail/results/exp2-after-p10-m1k-smoke-3.json
new file mode 100644
index 00000000..711adbcd
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p10-m1k-smoke-3.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p10-m1k-smoke-3",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p10-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:36:10.027Z",
+  "endedAt": "2026-08-11T08:36:20.044Z",
+  "requests": 2926,
+  "rps": 292.12340067180395,
+  "averageMs": 3.319289131920709,
+  "p95Ms": 5.782749999999999,
+  "p99Ms": 11.635999999999996,
+  "maxMs": 38.461,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1855
+}
diff --git a/performance/room-detail/results/exp2-after-p10-m1k-vu10-1.json b/performance/room-detail/results/exp2-after-p10-m1k-vu10-1.json
new file mode 100644
index 00000000..085a5a08
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p10-m1k-vu10-1.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p10-m1k-vu10-1",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p10-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:36:20.486Z",
+  "endedAt": "2026-08-11T08:36:30.507Z",
+  "requests": 14723,
+  "rps": 1469.238987493456,
+  "averageMs": 6.646223188208913,
+  "p95Ms": 11.088499999999996,
+  "p99Ms": 19.041559999999993,
+  "maxMs": 80.3,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1855
+}
diff --git a/performance/room-detail/results/exp2-after-p10-m1k-vu10-2.json b/performance/room-detail/results/exp2-after-p10-m1k-vu10-2.json
new file mode 100644
index 00000000..d7fa406b
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p10-m1k-vu10-2.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p10-m1k-vu10-2",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p10-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:36:30.969Z",
+  "endedAt": "2026-08-11T08:36:40.991Z",
+  "requests": 15395,
+  "rps": 1536.1478182310561,
+  "averageMs": 6.353281584930152,
+  "p95Ms": 11.227899999999998,
+  "p99Ms": 20.358739999999994,
+  "maxMs": 66.791,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1855
+}
diff --git a/performance/room-detail/results/exp2-after-p10-m1k-vu10-3.json b/performance/room-detail/results/exp2-after-p10-m1k-vu10-3.json
new file mode 100644
index 00000000..d61e4213
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p10-m1k-vu10-3.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p10-m1k-vu10-3",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p10-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 10,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:36:41.416Z",
+  "endedAt": "2026-08-11T08:36:51.443Z",
+  "requests": 17081,
+  "rps": 1703.6074168809434,
+  "averageMs": 5.720088753585877,
+  "p95Ms": 8.393,
+  "p99Ms": 15.023,
+  "maxMs": 124.176,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 1855
+}
diff --git a/performance/room-detail/results/exp2-after-p100-m1k-smoke-1.json b/performance/room-detail/results/exp2-after-p100-m1k-smoke-1.json
new file mode 100644
index 00000000..dabdd9b1
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p100-m1k-smoke-1.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p100-m1k-smoke-1",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p100-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 100,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:40:36.562Z",
+  "endedAt": "2026-08-11T08:40:46.624Z",
+  "requests": 1769,
+  "rps": 175.8175790744283,
+  "averageMs": 5.435735443753533,
+  "p95Ms": 7.898599999999998,
+  "p99Ms": 12.136759999999997,
+  "maxMs": 39.629,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 15538
+}
diff --git a/performance/room-detail/results/exp2-after-p100-m1k-smoke-2.json b/performance/room-detail/results/exp2-after-p100-m1k-smoke-2.json
new file mode 100644
index 00000000..18983f12
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p100-m1k-smoke-2.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p100-m1k-smoke-2",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p100-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 100,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:40:46.885Z",
+  "endedAt": "2026-08-11T08:40:56.904Z",
+  "requests": 1896,
+  "rps": 189.25795409955586,
+  "averageMs": 5.073205168776373,
+  "p95Ms": 6.37425,
+  "p99Ms": 10.521199999999997,
+  "maxMs": 59.529,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 15538
+}
diff --git a/performance/room-detail/results/exp2-after-p100-m1k-smoke-3.json b/performance/room-detail/results/exp2-after-p100-m1k-smoke-3.json
new file mode 100644
index 00000000..c5f2e013
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p100-m1k-smoke-3.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p100-m1k-smoke-3",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p100-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 100,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:40:57.191Z",
+  "endedAt": "2026-08-11T08:41:07.208Z",
+  "requests": 1421,
+  "rps": 141.87158674534,
+  "averageMs": 6.789782547501754,
+  "p95Ms": 12.404,
+  "p99Ms": 20.290599999999998,
+  "maxMs": 43.52,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 15538
+}
diff --git a/performance/room-detail/results/exp2-after-p100-m1k-vu10-1.json b/performance/room-detail/results/exp2-after-p100-m1k-vu10-1.json
new file mode 100644
index 00000000..b8fb61bd
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p100-m1k-vu10-1.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p100-m1k-vu10-1",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p100-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 100,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:41:07.475Z",
+  "endedAt": "2026-08-11T08:41:17.506Z",
+  "requests": 6367,
+  "rps": 634.7464408789069,
+  "averageMs": 15.410950683210281,
+  "p95Ms": 33.222699999999975,
+  "p99Ms": 78.18676,
+  "maxMs": 204.632,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 15538
+}
diff --git a/performance/room-detail/results/exp2-after-p100-m1k-vu10-2.json b/performance/room-detail/results/exp2-after-p100-m1k-vu10-2.json
new file mode 100644
index 00000000..efb04381
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p100-m1k-vu10-2.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p100-m1k-vu10-2",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p100-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 100,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:41:17.946Z",
+  "endedAt": "2026-08-11T08:41:27.997Z",
+  "requests": 2762,
+  "rps": 274.81843277475474,
+  "averageMs": 35.82023714699495,
+  "p95Ms": 91.45304999999995,
+  "p99Ms": 145.62409999999997,
+  "maxMs": 343.752,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 15538
+}
diff --git a/performance/room-detail/results/exp2-after-p100-m1k-vu10-3.json b/performance/room-detail/results/exp2-after-p100-m1k-vu10-3.json
new file mode 100644
index 00000000..53d1ca1f
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p100-m1k-vu10-3.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p100-m1k-vu10-3",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p100-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 100,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:41:28.965Z",
+  "endedAt": "2026-08-11T08:41:39.097Z",
+  "requests": 2687,
+  "rps": 265.217168147882,
+  "averageMs": 36.81774060290287,
+  "p95Ms": 89.50489999999996,
+  "p99Ms": 137.22372,
+  "maxMs": 254.646,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 15538
+}
diff --git a/performance/room-detail/results/exp2-after-p500-m1k-smoke-1.json b/performance/room-detail/results/exp2-after-p500-m1k-smoke-1.json
new file mode 100644
index 00000000..8d108ecd
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p500-m1k-smoke-1.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p500-m1k-smoke-1",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p500-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 500,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:42:11.896Z",
+  "endedAt": "2026-08-11T08:42:21.960Z",
+  "requests": 690,
+  "rps": 68.5663725467399,
+  "averageMs": 13.845437681159432,
+  "p95Ms": 19.4805,
+  "p99Ms": 43.09459999999999,
+  "maxMs": 95.292,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 77538
+}
diff --git a/performance/room-detail/results/exp2-after-p500-m1k-smoke-2.json b/performance/room-detail/results/exp2-after-p500-m1k-smoke-2.json
new file mode 100644
index 00000000..96e38d22
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p500-m1k-smoke-2.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p500-m1k-smoke-2",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p500-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 500,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:42:22.271Z",
+  "endedAt": "2026-08-11T08:42:32.304Z",
+  "requests": 668,
+  "rps": 66.58674927676623,
+  "averageMs": 14.32484281437125,
+  "p95Ms": 22.045399999999987,
+  "p99Ms": 43.93642,
+  "maxMs": 152.592,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 77538
+}
diff --git a/performance/room-detail/results/exp2-after-p500-m1k-smoke-3.json b/performance/room-detail/results/exp2-after-p500-m1k-smoke-3.json
new file mode 100644
index 00000000..ca75aca3
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p500-m1k-smoke-3.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p500-m1k-smoke-3",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p500-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 500,
+  "recentMessages": 1000,
+  "vus": 1,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:42:32.632Z",
+  "endedAt": "2026-08-11T08:42:42.663Z",
+  "requests": 780,
+  "rps": 77.75924183542915,
+  "averageMs": 12.211232051282042,
+  "p95Ms": 15.3067,
+  "p99Ms": 18.95093,
+  "maxMs": 31.664,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 77538
+}
diff --git a/performance/room-detail/results/exp2-after-p500-m1k-vu10-1.json b/performance/room-detail/results/exp2-after-p500-m1k-vu10-1.json
new file mode 100644
index 00000000..5f9ec843
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p500-m1k-vu10-1.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p500-m1k-vu10-1",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p500-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 500,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:42:42.919Z",
+  "endedAt": "2026-08-11T08:42:52.971Z",
+  "requests": 3988,
+  "rps": 396.77316364813623,
+  "averageMs": 24.162855065195547,
+  "p95Ms": 33.0296,
+  "p99Ms": 40.84985999999999,
+  "maxMs": 78.84,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 77538
+}
diff --git a/performance/room-detail/results/exp2-after-p500-m1k-vu10-2.json b/performance/room-detail/results/exp2-after-p500-m1k-vu10-2.json
new file mode 100644
index 00000000..a579dddb
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p500-m1k-vu10-2.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p500-m1k-vu10-2",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p500-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 500,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:42:53.830Z",
+  "endedAt": "2026-08-11T08:43:03.882Z",
+  "requests": 3535,
+  "rps": 351.70622789648235,
+  "averageMs": 27.33135190947665,
+  "p95Ms": 42.06889999999999,
+  "p99Ms": 72.57297999999997,
+  "maxMs": 196.539,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 77538
+}
diff --git a/performance/room-detail/results/exp2-after-p500-m1k-vu10-3.json b/performance/room-detail/results/exp2-after-p500-m1k-vu10-3.json
new file mode 100644
index 00000000..8a338e06
--- /dev/null
+++ b/performance/room-detail/results/exp2-after-p500-m1k-vu10-3.json
@@ -0,0 +1,22 @@
+{
+  "testName": "exp2-after-p500-m1k-vu10-3",
+  "fixtureId": "room-detail-exp2-20260811",
+  "roomId": "room-detail-exp2-20260811:room:p500-m1k",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "participants": 500,
+  "recentMessages": 1000,
+  "vus": 10,
+  "duration": "10s",
+  "startedAt": "2026-08-11T08:43:04.272Z",
+  "endedAt": "2026-08-11T08:43:14.316Z",
+  "requests": 2874,
+  "rps": 286.1622053806261,
+  "averageMs": 33.804678496868526,
+  "p95Ms": 75.44479999999993,
+  "p99Ms": 166.90414999999996,
+  "maxMs": 351.347,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "averageResponseBytes": 77538
+}
diff --git a/performance/room-detail/room-detail-exp2-report.md b/performance/room-detail/room-detail-exp2-report.md
new file mode 100644
index 00000000..2f86c1bc
--- /dev/null
+++ b/performance/room-detail/room-detail-exp2-report.md
@@ -0,0 +1,36 @@
+# Room Detail experiment 2: batched user lookup
+
+Date: 2026-08-11 (Asia/Seoul)  
+Base: `origin/master` (`01b725d`)  
+Fixture: `room-detail-exp2-20260811`, 1,000 recent messages  
+Backend: isolated `http://localhost:15001`
+
+The controller now combines creator and participant IDs and calls `findAllById` once. It then
+rebuilds participants in the Room's original iteration order, omits missing participants, and
+keeps the existing error when the creator is missing.
+
+## Results
+
+Each after value is the median of three 10-second runs. The checked-in master baseline is the
+available single 30/60-second run, so latency comparisons are directional rather than a replacement
+for the full-duration rerun.
+
+| Participants | VUs | Baseline p95 (ms) | After median p95 (ms) | Change | Baseline p99 (ms) | After median p99 (ms) |
+|---:|---:|---:|---:|---:|---:|---:|
+| 10 | 1 | 15.84 | 5.30 | -66.5% | 27.93 | 7.54 |
+| 10 | 10 | 25.61 | 11.09 | -56.7% | 34.91 | 19.04 |
+| 100 | 1 | 54.04 | 7.90 | -85.4% | 78.64 | 12.14 |
+| 100 | 10 | 172.12 | 89.50 | -48.0% | 325.13 | 137.22 |
+| 500 | 1 | 306.37 | 19.48 | -93.6% | 368.31 | 43.09 |
+| 500 | 10 | 765.54 | 42.07 | -94.5% | 859.55 | 72.57 |
+
+All 18 after runs had error rate 0 and zero timeouts. Across 86,220 measured requests, the isolated
+application reported 86,220 message aggregates and 86,238 User find commands; the extra 18 finds
+are the setup logins. This proves one User find per Room Detail request. The 500-participant result
+also uses Mongo cursor `getMore`, but still starts only one find command.
+
+## Decision
+
+Adopted. Every p95 comparison exceeds the 10% target and every p99 improves. The isolated backend
+was not a Prometheus scrape target during these shortened runs, so CPU/heap peak non-regression is
+not claimed; the HTTP contract, error, timeout, command-count, and latency checks passed.
diff --git a/performance/room-detail/room-detail.js b/performance/room-detail/room-detail.js
index fa569bcc..0c18db0f 100644
--- a/performance/room-detail/room-detail.js
+++ b/performance/room-detail/room-detail.js
@@ -85,6 +85,7 @@ export function handleSummary(data) {
     fixtureId: FIXTURE_ID,
     roomId: ROOM_ID,
     commit: __ENV.COMMIT || 'unknown',
+    mongoIndexes: __ENV.MONGO_INDEXES || 'unknown',
     participants: EXPECTED_PARTICIPANTS,
     recentMessages: EXPECTED_RECENT_MESSAGES,
     vus: VUS,
diff --git a/performance/room-join/README.md b/performance/room-join/README.md
index 2307ce8b..0be0d604 100644
--- a/performance/room-join/README.md
+++ b/performance/room-join/README.md
@@ -13,3 +13,21 @@ k6 run performance/room-join/room-join.js
 Run the same fixture and settings three times before and after one code change. Adoption requires
 error rate at most 1%, zero timeouts, Mongo wait queue zero, p95 improvement of at least 10%, and
 no p99 regression. Result JSON is written under `results/`.
+
+Set `ROOM_ID` to avoid room-list selection and `ACCOUNT_PREFIX` when using dedicated accounts.
+
+For a real mutation on every request, create the dedicated fixture and run the mutation scenario:
+
+```sh
+docker compose -f apps/backend/docker-compose.yaml exec -T \
+  -e ACTION=create -e FIXTURE_ID=room-join-exp2-20260811 \
+  -e ACCOUNT_COUNT=30 -e ACCOUNT_PREFIX=join-exp2-20260811 \
+  -e CREATOR_EMAIL=room-detail-exp2-20260811@fixture.invalid \
+  -e ROOMS_PER_ACCOUNT=200 mongo mongosh --quiet --file /dev/stdin \
+  < performance/room-join/fixture.js
+
+API_BASE_URL=http://localhost:15001 TEST_NAME=join-mutation-1 \
+FIXTURE_ID=room-join-exp2-20260811 ACCOUNT_PREFIX=join-exp2-20260811 \
+LOAD_TEST_PASSWORD='JoinExp21234!' VUS=10 DURATION=10s THINK_TIME_MS=100 \
+k6 run performance/room-join/room-join-mutation.js
+```
diff --git a/performance/room-join/fixture.js b/performance/room-join/fixture.js
new file mode 100644
index 00000000..8c8eadfe
--- /dev/null
+++ b/performance/room-join/fixture.js
@@ -0,0 +1,50 @@
+const database = db.getSiblingDB('bootcamp-chat');
+const fixtureId = process.env.FIXTURE_ID;
+const action = process.env.ACTION || 'create';
+const accountCount = Number(process.env.ACCOUNT_COUNT || 30);
+const accountPrefix = process.env.ACCOUNT_PREFIX || 'loadtest';
+const creatorEmail = process.env.CREATOR_EMAIL;
+const roomsPerAccount = Number(process.env.ROOMS_PER_ACCOUNT || 200);
+const prefix = `${fixtureId}:room:`;
+
+if (!fixtureId) throw new Error('FIXTURE_ID is required');
+
+function cleanup() {
+  const removed = database.rooms.deleteMany({ _id: { $regex: `^${prefix}` } }).deletedCount;
+  printjson({ action: 'cleanup', fixtureId, removedRooms: removed });
+}
+
+function create() {
+  cleanup();
+  if (!creatorEmail) throw new Error('CREATOR_EMAIL is required');
+  const creator = database.users.findOne({ email: creatorEmail });
+  if (!creator) throw new Error(`Missing room creator ${creatorEmail}`);
+  const users = [];
+  for (let index = 0; index < accountCount; index += 1) {
+    const email = `${accountPrefix}-${index}@test.com`;
+    const user = database.users.findOne({ email });
+    if (!user) throw new Error(`Missing load-test account ${email}`);
+    users.push(user);
+  }
+  const creatorId = creator._id.toString();
+  const now = new Date();
+  for (let vu = 1; vu <= accountCount; vu += 1) {
+    const rooms = [];
+    for (let iteration = 0; iteration < roomsPerAccount; iteration += 1) {
+      rooms.push({
+        _id: `${prefix}${vu}:${iteration}`,
+        name: `Room join mutation ${vu}:${iteration}`,
+        creator: creatorId,
+        hasPassword: false,
+        participantIds: [creatorId],
+        createdAt: now,
+      });
+    }
+    database.rooms.insertMany(rooms, { ordered: false });
+  }
+  printjson({ action: 'create', fixtureId, accountCount, roomsPerAccount });
+}
+
+if (action === 'create') create();
+else if (action === 'cleanup') cleanup();
+else throw new Error(`Unknown ACTION: ${action}`);
diff --git a/performance/room-join/results/join-final-mutation-vu10-1.json b/performance/room-join/results/join-final-mutation-vu10-1.json
new file mode 100644
index 00000000..562f39d6
--- /dev/null
+++ b/performance/room-join/results/join-final-mutation-vu10-1.json
@@ -0,0 +1,19 @@
+{
+  "testName": "join-final-mutation-vu10-1",
+  "fixtureId": "room-join-exp2-20260811",
+  "apiBaseUrl": "http://localhost:15001",
+  "model": "HTTP-only room join, one new participant per request",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "vus": 10,
+  "duration": "10s",
+  "thinkTimeMs": 100,
+  "startedAt": "2026-08-11T09:09:47.361Z",
+  "endedAt": "2026-08-11T09:09:57.529Z",
+  "p95Ms": 25.91114999999999,
+  "p99Ms": 41.08063,
+  "averageMs": 15.689969767441873,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 860
+}
diff --git a/performance/room-join/results/join-final-mutation-vu10-2.json b/performance/room-join/results/join-final-mutation-vu10-2.json
new file mode 100644
index 00000000..5b10febb
--- /dev/null
+++ b/performance/room-join/results/join-final-mutation-vu10-2.json
@@ -0,0 +1,19 @@
+{
+  "testName": "join-final-mutation-vu10-2",
+  "fixtureId": "room-join-exp2-20260811",
+  "apiBaseUrl": "http://localhost:15001",
+  "model": "HTTP-only room join, one new participant per request",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "vus": 10,
+  "duration": "10s",
+  "thinkTimeMs": 100,
+  "startedAt": "2026-08-11T09:09:58.631Z",
+  "endedAt": "2026-08-11T09:10:08.798Z",
+  "p95Ms": 24.340749999999996,
+  "p99Ms": 49.80357,
+  "averageMs": 15.457447674418631,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 860
+}
diff --git a/performance/room-join/results/join-final-mutation-vu10-3.json b/performance/room-join/results/join-final-mutation-vu10-3.json
new file mode 100644
index 00000000..3ff40726
--- /dev/null
+++ b/performance/room-join/results/join-final-mutation-vu10-3.json
@@ -0,0 +1,19 @@
+{
+  "testName": "join-final-mutation-vu10-3",
+  "fixtureId": "room-join-exp2-20260811",
+  "apiBaseUrl": "http://localhost:15001",
+  "model": "HTTP-only room join, one new participant per request",
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "vus": 10,
+  "duration": "10s",
+  "thinkTimeMs": 100,
+  "startedAt": "2026-08-11T09:10:09.861Z",
+  "endedAt": "2026-08-11T09:10:20.047Z",
+  "p95Ms": 27.385399999999997,
+  "p99Ms": 63.7213,
+  "averageMs": 15.927647674418596,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 860
+}
diff --git a/performance/room-join/results/join-final-repeated-vu10-1.json b/performance/room-join/results/join-final-repeated-vu10-1.json
new file mode 100644
index 00000000..d2cfff56
--- /dev/null
+++ b/performance/room-join/results/join-final-repeated-vu10-1.json
@@ -0,0 +1,19 @@
+{
+  "testName": "join-final-repeated-vu10-1",
+  "fixtureId": "room-join-exp2-20260811",
+  "apiBaseUrl": "http://localhost:15001",
+  "model": "HTTP-only room join",
+  "vus": 10,
+  "duration": "10s",
+  "thinkTimeMs": 100,
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "startedAt": "2026-08-11T09:00:27.133Z",
+  "endedAt": "2026-08-11T09:00:37.411Z",
+  "p95Ms": 24.095999999999997,
+  "p99Ms": 43.75119999999989,
+  "averageMs": 14.266164179104479,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 871
+}
diff --git a/performance/room-join/results/join-final-repeated-vu10-2.json b/performance/room-join/results/join-final-repeated-vu10-2.json
new file mode 100644
index 00000000..b18f8d78
--- /dev/null
+++ b/performance/room-join/results/join-final-repeated-vu10-2.json
@@ -0,0 +1,19 @@
+{
+  "testName": "join-final-repeated-vu10-2",
+  "fixtureId": "room-join-exp2-20260811",
+  "apiBaseUrl": "http://localhost:15001",
+  "model": "HTTP-only room join",
+  "vus": 10,
+  "duration": "10s",
+  "thinkTimeMs": 100,
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "startedAt": "2026-08-11T09:00:37.734Z",
+  "endedAt": "2026-08-11T09:00:47.986Z",
+  "p95Ms": 28.7301,
+  "p99Ms": 32.512539999999994,
+  "averageMs": 14.941232183908037,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 870
+}
diff --git a/performance/room-join/results/join-final-repeated-vu10-3.json b/performance/room-join/results/join-final-repeated-vu10-3.json
new file mode 100644
index 00000000..49a130f2
--- /dev/null
+++ b/performance/room-join/results/join-final-repeated-vu10-3.json
@@ -0,0 +1,19 @@
+{
+  "testName": "join-final-repeated-vu10-3",
+  "fixtureId": "room-join-exp2-20260811",
+  "apiBaseUrl": "http://localhost:15001",
+  "model": "HTTP-only room join",
+  "vus": 10,
+  "duration": "10s",
+  "thinkTimeMs": 100,
+  "commit": "4bdede9",
+  "mongoIndexes": "users.email_unique_messages.room_timestamp_idx",
+  "startedAt": "2026-08-11T09:00:48.292Z",
+  "endedAt": "2026-08-11T09:00:58.515Z",
+  "p95Ms": 24.02685,
+  "p99Ms": 28.17489,
+  "averageMs": 16.213308139534902,
+  "errorRate": 0,
+  "timeoutCount": 0,
+  "completedRequests": 860
+}
diff --git a/performance/room-join/room-join-mutation.js b/performance/room-join/room-join-mutation.js
new file mode 100644
index 00000000..eb225df0
--- /dev/null
+++ b/performance/room-join/room-join-mutation.js
@@ -0,0 +1,100 @@
+import http from 'k6/http';
+import { check, sleep } from 'k6';
+import { Counter, Rate, Trend } from 'k6/metrics';
+
+const API_BASE_URL = __ENV.API_BASE_URL || 'http://localhost:5001';
+const TEST_NAME = __ENV.TEST_NAME || 'room-join-mutation-local';
+const FIXTURE_ID = __ENV.FIXTURE_ID;
+const VUS = Number(__ENV.VUS || 10);
+const DURATION = __ENV.DURATION || '30s';
+const THINK_TIME_MS = Number(__ENV.THINK_TIME_MS || 1000);
+const ACCOUNT_COUNT = Number(__ENV.ACCOUNT_COUNT || 30);
+const ACCOUNT_PREFIX = __ENV.ACCOUNT_PREFIX || 'loadtest';
+const PASSWORD = __ENV.LOAD_TEST_PASSWORD || 'Test1234!';
+
+const duration = new Trend('room_join_mutation_duration', true);
+const errors = new Rate('room_join_mutation_errors');
+const completed = new Counter('room_join_mutation_completed');
+const timeouts = new Counter('room_join_mutation_timeouts');
+
+export const options = {
+  vus: VUS,
+  duration: DURATION,
+  summaryTrendStats: ['avg', 'min', 'med', 'max', 'p(90)', 'p(95)', 'p(99)'],
+  thresholds: {
+    room_join_mutation_errors: ['rate<=0.01'],
+    room_join_mutation_timeouts: ['count==0'],
+  },
+};
+
+function account(index) {
+  return { email: `${ACCOUNT_PREFIX}-${index}@test.com`, password: PASSWORD };
+}
+
+export function setup() {
+  if (!FIXTURE_ID) throw new Error('FIXTURE_ID is required');
+  if (VUS > ACCOUNT_COUNT) throw new Error(`VUS (${VUS}) exceeds ACCOUNT_COUNT (${ACCOUNT_COUNT})`);
+  const tokens = [];
+  for (let index = 0; index < ACCOUNT_COUNT; index += 1) {
+    const response = http.post(`${API_BASE_URL}/api/auth/login`, JSON.stringify(account(index)), {
+      headers: { 'Content-Type': 'application/json' }, timeout: '10s', tags: { phase: 'setup' },
+    });
+    if (response.status !== 200 || !response.json('token')) {
+      throw new Error(`Fixed account ${account(index).email} unavailable (HTTP ${response.status})`);
+    }
+    tokens.push(response.json('token'));
+  }
+  return { tokens };
+}
+
+export default function (data) {
+  const roomId = `${FIXTURE_ID}:room:${__VU}:${__ITER}`;
+  const token = data.tokens[(__VU - 1) % data.tokens.length];
+  const response = http.post(`${API_BASE_URL}/api/rooms/${roomId}/join`, '{}', {
+    headers: { Authorization: `Bearer ${token}`, 'Content-Type': 'application/json' },
+    timeout: '30s', tags: { phase: 'measurement', endpoint: 'room-join-mutation' },
+  });
+  const success = check(response, {
+    'mutation join returned HTTP 200': (res) => res.status === 200,
+    'mutation join preserved success contract': (res) => res.json('success') === true,
+    'mutation join returned expected room': (res) => res.json('data._id') === roomId,
+    'mutation join added participant': (res) => res.json('data.participantsCount') >= 2,
+  });
+  duration.add(response.timings.duration);
+  errors.add(!success);
+  completed.add(1);
+  if (response.error_code === 1050 || response.timings.duration >= 30000) timeouts.add(1);
+  sleep(THINK_TIME_MS / 1000);
+}
+
+function value(data, metric, field, fallback = 0) {
+  return data.metrics[metric]?.values[field] ?? fallback;
+}
+
+export function handleSummary(data) {
+  const endedAt = new Date();
+  const startedAt = new Date(endedAt.getTime() - data.state.testRunDurationMs);
+  const summary = {
+    testName: TEST_NAME,
+    fixtureId: FIXTURE_ID,
+    apiBaseUrl: API_BASE_URL,
+    model: 'HTTP-only room join, one new participant per request',
+    commit: __ENV.COMMIT || 'unknown',
+    mongoIndexes: __ENV.MONGO_INDEXES || 'unknown',
+    vus: VUS,
+    duration: DURATION,
+    thinkTimeMs: THINK_TIME_MS,
+    startedAt: startedAt.toISOString(),
+    endedAt: endedAt.toISOString(),
+    p95Ms: value(data, 'room_join_mutation_duration', 'p(95)'),
+    p99Ms: value(data, 'room_join_mutation_duration', 'p(99)'),
+    averageMs: value(data, 'room_join_mutation_duration', 'avg'),
+    errorRate: value(data, 'room_join_mutation_errors', 'rate'),
+    timeoutCount: value(data, 'room_join_mutation_timeouts', 'count'),
+    completedRequests: value(data, 'room_join_mutation_completed', 'count'),
+  };
+  return {
+    stdout: `${JSON.stringify(summary, null, 2)}\n`,
+    [`performance/room-join/results/${TEST_NAME}.json`]: `${JSON.stringify(summary, null, 2)}\n`,
+  };
+}
diff --git a/performance/room-join/room-join-optimization-report.md b/performance/room-join/room-join-optimization-report.md
new file mode 100644
index 00000000..4e9256bf
--- /dev/null
+++ b/performance/room-join/room-join-optimization-report.md
@@ -0,0 +1,56 @@
+# REST Room Join sequential optimization report
+
+Date: 2026-08-11 (Asia/Seoul)  
+Base: `origin/master` (`01b725d`)  
+Backend: isolated `http://localhost:15001`  
+Fixture: `room-join-exp2-20260811`
+
+## Implemented path
+
+1. Creator and participants are loaded by one `findAllById` call.
+2. The controller passes the session-validated JWT `userId`; Join no longer calls `findByEmail`.
+3. New participants use Mongo `findAndModify` with `$addToSet` and `returnNew=true`; existing
+   participants reuse the initial Room read.
+4. RoomService builds one final `RoomResponse` and shares the same object with `RoomUpdatedEvent`
+   and the HTTP response.
+
+The repeated scenario uses one password-free room. The mutation scenario recreates 6,000 rooms
+before every run and maps each VU/iteration to a unique room, so every measured request is a new
+participant mutation.
+
+## Results
+
+| Scenario | Runs | Requests | Median p95 (ms) | Median p99 (ms) | Max error | Timeouts |
+|---|---:|---:|---:|---:|---:|---:|
+| Existing checked-in baseline | 1 | 1,647 | 137.10 | 245.30 | 0 | 0 |
+| Final repeated Join | 3 | 2,601 | 24.10 | 32.51 | 0 | 0 |
+| Final new-participant mutation | 3 | 2,580 | 25.91 | 49.80 | 0 | 0 |
+
+The final repeated path improves p95 by 82.4% and p99 by 86.7% against the available baseline.
+The new-participant path remains within 7.5% of the repeated-path p95 while performing an actual
+atomic write on every request.
+
+After restarting the backend to reset Micrometer counters, one existing-participant request and one
+new-participant request produced: Room find 2, User find 4 (two logins plus two Join mappings),
+messages aggregate 2, and Room findAndModify 1. Thus each Join performs one user batch lookup and one
+recent-message aggregate, while only a new participant performs the atomic mutation.
+
+Unit tests verify no `findByEmail` or whole-document `save`, ID-based `isCreator`, participant order,
+and that the event and HTTP layers share the service response. Mongo integration tests verify the
+returned updated Room and 20 concurrent joins without lost or duplicate participants.
+
+## Recent-message counter decision
+
+For the two isolated Join requests, related Mongo command time was 16.319 ms and the two messages
+aggregates used 2.919 ms, a 17.89% share. This is below the 20% threshold and the final p95 target is
+met, so the exact 30-minute query and `{room: 1, timestamp: 1}` index are retained. No counter code
+is introduced. A minute-bucket design remains a follow-up only if a future representative run
+crosses the 20% threshold.
+
+## Scope and limitations
+
+The after runs are 10 seconds each rather than the README's 3-minute production-profile run, and
+the isolated port was not scraped by Prometheus for CPU/heap peaks. The final combined path is
+adopted based on contract, command counts, integration tests, zero errors/timeouts, and latency;
+the precise contribution of each intermediate Join change was not independently benchmarked.
+Socket.IO Join remains a separate follow-up that can reuse `addParticipantAndReturn`.
diff --git a/performance/room-join/room-join.js b/performance/room-join/room-join.js
index d77f3069..23c757b3 100644
--- a/performance/room-join/room-join.js
+++ b/performance/room-join/room-join.js
@@ -9,6 +9,8 @@ const VUS = Number(__ENV.VUS || 10);
 const DURATION = __ENV.DURATION || '3m';
 const THINK_TIME_MS = Number(__ENV.THINK_TIME_MS || 1000);
 const ACCOUNT_COUNT = Number(__ENV.ACCOUNT_COUNT || 30);
+const ACCOUNT_PREFIX = __ENV.ACCOUNT_PREFIX || 'loadtest';
+const TARGET_ROOM_ID = __ENV.ROOM_ID;
 const PASSWORD = __ENV.LOAD_TEST_PASSWORD || 'Test1234!';
 
 const duration = new Trend('room_join_duration', true);
@@ -27,7 +29,7 @@ export const options = {
 };
 
 function account(index) {
-  return { email: `loadtest-${index}@test.com`, password: PASSWORD };
+  return { email: `${ACCOUNT_PREFIX}-${index}@test.com`, password: PASSWORD };
 }
 
 export function setup() {
@@ -42,11 +44,14 @@ export function setup() {
     }
     tokens.push(response.json('token'));
   }
-  const rooms = http.get(`${API_BASE_URL}/api/rooms`, {
-    headers: { Authorization: `Bearer ${tokens[0]}` }, timeout: '30s', tags: { phase: 'setup' },
-  });
-  const roomId = rooms.json('data.0._id') || rooms.json('data.0.id');
-  if (rooms.status !== 200 || !roomId) throw new Error('No room is available for room-join measurement');
+  let roomId = TARGET_ROOM_ID;
+  if (!roomId) {
+    const rooms = http.get(`${API_BASE_URL}/api/rooms`, {
+      headers: { Authorization: `Bearer ${tokens[0]}` }, timeout: '30s', tags: { phase: 'setup' },
+    });
+    roomId = rooms.json('data.0._id') || rooms.json('data.0.id');
+    if (rooms.status !== 200 || !roomId) throw new Error('No room is available for room-join measurement');
+  }
   return { tokens, roomId };
 }
 
@@ -73,10 +78,13 @@ function value(data, metric, field, fallback = 0) {
 }
 
 export function handleSummary(data) {
+  const endedAt = new Date();
+  const startedAt = new Date(endedAt.getTime() - data.state.testRunDurationMs);
   const summary = {
     testName: TEST_NAME, fixtureId: FIXTURE_ID, apiBaseUrl: API_BASE_URL,
     model: 'HTTP-only room join', vus: VUS, duration: DURATION, thinkTimeMs: THINK_TIME_MS,
-    endedAt: new Date().toISOString(),
+    commit: __ENV.COMMIT || 'unknown', mongoIndexes: __ENV.MONGO_INDEXES || 'unknown',
+    startedAt: startedAt.toISOString(), endedAt: endedAt.toISOString(),
     p95Ms: value(data, 'room_join_duration', 'p(95)'),
     p99Ms: value(data, 'room_join_duration', 'p(99)'),
     averageMs: value(data, 'room_join_duration', 'avg'),

From ece990fac1eca4749ff87f09f274fabf594a3cf9 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 21:26:48 +0900
Subject: [PATCH 22/32] =?UTF-8?q?fix(frontend):=20=EB=A1=9C=EA=B7=B8?=
 =?UTF-8?q?=EC=9D=B8=EA=B3=BC=20=EC=B1=84=ED=8C=85=20=ED=99=94=EB=A9=B4=20?=
 =?UTF-8?q?=EC=95=88=EC=A0=95=ED=99=94?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/frontend/components/ChatHeader.js        |  1 +
 apps/frontend/components/ChatInput.js         | 33 +++++--
 .../components/ConnectionErrorBanner.js       |  2 +
 apps/frontend/components/FileMessage.js       |  6 +-
 apps/frontend/components/FilePreview.js       |  6 +-
 .../components/__tests__/ChatInput.test.js    | 28 +++++-
 .../features/chat/room/ChatRoomView.js        | 91 +++++-------------
 .../chat/room/__tests__/ChatRoomView.test.js  |  2 +
 .../room/__tests__/useMessageHandling.test.js |  4 +-
 .../features/chat/room/useMessageHandling.js  |  7 +-
 .../features/chat/rooms/ChatRoomsView.js      |  6 +-
 .../rooms/__tests__/ChatRoomsView.test.js     |  6 ++
 apps/frontend/next.config.js                  | 11 ++-
 apps/frontend/pages/chat/new.js               | 17 +++-
 apps/frontend/pages/index.js                  | 47 +++-------
 apps/frontend/pages/register.js               |  2 +-
 apps/frontend/test/pages/index.test.js        | 30 ++++++
 e2e/actions/auth.actions.js                   | 15 ++-
 e2e/actions/chat.actions.js                   |  5 +-
 e2e/artillery/aggregate-observations.js       | 19 +++-
 e2e/artillery/observation.js                  | 94 ++++++++++++++++++-
 e2e/artillery/observation.test.js             |  8 +-
 e2e/artillery/report-observations.js          |  9 +-
 e2e/artillery/scenarios/auth.scenario.js      |  8 +-
 e2e/artillery/scenarios/chat.scenario.js      |  9 +-
 25 files changed, 326 insertions(+), 140 deletions(-)
 create mode 100644 apps/frontend/test/pages/index.test.js

diff --git a/apps/frontend/components/ChatHeader.js b/apps/frontend/components/ChatHeader.js
index 8dc55672..f6c0a6e0 100644
--- a/apps/frontend/components/ChatHeader.js
+++ b/apps/frontend/components/ChatHeader.js
@@ -31,6 +31,7 @@ const ChatHeader = () => {
         <img
           src="/images/logo.png"
           alt="Chat App Logo"
+          width={120}
           height={15}
           className="logo"
         />
diff --git a/apps/frontend/components/ChatInput.js b/apps/frontend/components/ChatInput.js
index 63603ac1..0911b1db 100644
--- a/apps/frontend/components/ChatInput.js
+++ b/apps/frontend/components/ChatInput.js
@@ -46,6 +46,7 @@ const ChatInput = forwardRef(({
   const [uploading, setUploading] = useState(false);
   const [uploadProgress, setUploadProgress] = useState(0);
   const [uploadError, setUploadError] = useState(null);
+  const [submissionStatus, setSubmissionStatus] = useState('idle');
   const [isDragging, setIsDragging] = useState(false);
   const [mentionPosition, setMentionPosition] = useState({ top: 0, left: 0 });
 
@@ -109,7 +110,10 @@ const ChatInput = forwardRef(({
           throw new Error('파일이 선택되지 않았습니다.');
         }
 
-        onSubmit({
+        setUploading(true);
+        setSubmissionStatus('uploading-and-waiting-for-message');
+        setUploadError(null);
+        await onSubmit({
           type: 'file',
           content: message.trim(),
           fileData: file
@@ -119,19 +123,29 @@ const ChatInput = forwardRef(({
         setShowEmojiPicker(false);
         setShowMentionList(false);
         setFiles([]);
+        setSubmissionStatus('complete');
 
       } catch (error) {
         console.error('File submit error:', error);
         setUploadError(error.message);
+        setSubmissionStatus('failed');
+      } finally {
+        setUploading(false);
       }
     } else if (message.trim()) {
-      onSubmit({
-        type: 'text',
-        content: message.trim()
-      });
-      setMessage('');
-      setShowEmojiPicker(false);
-      setShowMentionList(false);
+      try {
+        setSubmissionStatus('waiting-for-message');
+        await onSubmit({
+          type: 'text',
+          content: message.trim()
+        });
+        setMessage('');
+        setShowEmojiPicker(false);
+        setShowMentionList(false);
+        setSubmissionStatus('complete');
+      } catch (error) {
+        setSubmissionStatus('failed');
+      }
     }
   }, [files, message, onSubmit, setMessage, setShowEmojiPicker, setShowMentionList]);
 
@@ -403,6 +417,9 @@ const ChatInput = forwardRef(({
         )}
 
         <VStack $css={{ gap: '$100', width: '100%' }}>
+          <span className="sr-only" role="status" data-testid="message-submission-status">
+            {submissionStatus}
+          </span>
           <VStack className="relative" $css={{ gap: '$025' }}>
             <HStack $css={{ gap: '$200' }}>
                 <Textarea
diff --git a/apps/frontend/components/ConnectionErrorBanner.js b/apps/frontend/components/ConnectionErrorBanner.js
index 17ce8fee..7e3c3574 100644
--- a/apps/frontend/components/ConnectionErrorBanner.js
+++ b/apps/frontend/components/ConnectionErrorBanner.js
@@ -13,6 +13,8 @@ const ConnectionErrorBanner = ({ message, title = '연결에 문제가 생겼어
     <img
       src="/error.svg"
       alt=""
+      width="220"
+      height="165"
       style={{ width: '220px', maxWidth: '70%', height: 'auto' }}
     />
     <Text typography="heading4" foreground="normal-100">
diff --git a/apps/frontend/components/FileMessage.js b/apps/frontend/components/FileMessage.js
index 9b75b5a2..87815233 100644
--- a/apps/frontend/components/FileMessage.js
+++ b/apps/frontend/components/FileMessage.js
@@ -172,11 +172,13 @@ const FileMessage = ({
       const previewUrl = fileService.getPreviewUrl(msg.file, user?.token, true);
 
       return (
-        <div className="bg-transparent-pattern">
+        <div className="bg-transparent-pattern" style={{ width: 'min(400px, 100%)', aspectRatio: '4 / 3' }}>
           <img
             src={previewUrl}
             alt={originalname}
-            className="max-w-[400px] max-h-[400px] object-cover object-center rounded-md"
+            width="400"
+            height="300"
+            className="w-full h-full object-cover object-center rounded-md"
             onLoad={() => {
               console.debug('Image loaded successfully:', originalname);
             }}
diff --git a/apps/frontend/components/FilePreview.js b/apps/frontend/components/FilePreview.js
index 29220c62..ab250520 100644
--- a/apps/frontend/components/FilePreview.js
+++ b/apps/frontend/components/FilePreview.js
@@ -208,11 +208,13 @@ const FilePreview = ({
 
     if (file.type.startsWith('image/')) {
       return (
-        <div className={`${previewContainer} ${previewBackground}`}>
+        <div className={`${previewContainer} ${previewBackground}`} style={{ aspectRatio: '4 / 3' }}>
           <img
             src={previewUrl || file.url}
             alt={`${file.name} 미리보기`}
             className="w-full h-full object-cover"
+            width="320"
+            height="240"
             onError={(e) => {
               e.target.onerror = null;
               e.target.src = '/placeholder-image.png';
@@ -226,7 +228,7 @@ const FilePreview = ({
 
     if (file.type.startsWith('video/')) {
       return (
-        <div className={`${previewContainer}`}>
+        <div className={`${previewContainer}`} style={{ aspectRatio: '4 / 3' }}>
           <video
             src={previewUrl || file.url}
             className="w-full h-full object-cover"
diff --git a/apps/frontend/components/__tests__/ChatInput.test.js b/apps/frontend/components/__tests__/ChatInput.test.js
index 1c1848ea..a222933d 100644
--- a/apps/frontend/components/__tests__/ChatInput.test.js
+++ b/apps/frontend/components/__tests__/ChatInput.test.js
@@ -1,8 +1,12 @@
 import React from 'react';
 import { render, waitFor, fireEvent } from '@testing-library/react';
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, vi } from 'vitest';
 import ChatInput from '../ChatInput';
 
+vi.mock('../EmojiPicker', () => ({
+  default: () => React.createElement('em-emoji-picker'),
+}));
+
 describe('ChatInput', () => {
   it('renders the lazy emoji picker under React 19', async () => {
     const { container, getByLabelText } = render(
@@ -18,4 +22,26 @@ describe('ChatInput', () => {
       expect(container.querySelector('em-emoji-picker')).toBeInTheDocument();
     });
   });
+
+  it('waits for the socket-confirmed submit before clearing the message', async () => {
+    let confirmSubmit;
+    const onSubmit = vi.fn(() => new Promise((resolve) => { confirmSubmit = resolve; }));
+    const { getByTestId } = render(
+      <ChatInput
+        fileInputRef={{ current: null }}
+        room={{ participants: [] }}
+        onSubmit={onSubmit}
+      />
+    );
+
+    fireEvent.change(getByTestId('chat-message-input'), { target: { value: 'hello' } });
+    fireEvent.click(getByTestId('chat-send-button'));
+
+    await waitFor(() => expect(getByTestId('message-submission-status')).toHaveTextContent('waiting-for-message'));
+    expect(getByTestId('chat-message-input')).toHaveValue('hello');
+
+    confirmSubmit();
+    await waitFor(() => expect(getByTestId('message-submission-status')).toHaveTextContent('complete'));
+    expect(getByTestId('chat-message-input')).toHaveValue('');
+  });
 });
diff --git a/apps/frontend/features/chat/room/ChatRoomView.js b/apps/frontend/features/chat/room/ChatRoomView.js
index 75401bf9..4ab892c8 100644
--- a/apps/frontend/features/chat/room/ChatRoomView.js
+++ b/apps/frontend/features/chat/room/ChatRoomView.js
@@ -4,9 +4,9 @@ import {
   Button,
   Text,
   Callout,
-  Card,
   Spinner,
   Flex,
+  Box,
 } from '@vapor-ui/core';
 import { ErrorCircleOutlineIcon } from '@vapor-ui/icons';
 import { useChatRoom } from './useChatRoom';
@@ -35,52 +35,6 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
     handleLoadMore,
   } = useChatRoom({ roomId, onNavigate, onReplace, asPath });
 
-  const renderLoadingState = () => (
-    <div className="chat-container">
-      <Card.Root className="chat-room-card">
-        <Card.Body
-          style={{
-            display: 'flex',
-            alignItems: 'center',
-            justifyContent: 'center',
-          }}
-        >
-          <Flex
-            style={{ textAlign: 'center', marginTop: 'var(--vapor-space-500)' }}
-            $css={{ gap: '$100', alignItems: 'center' }}
-          >
-            <Spinner
-              size="lg"
-              colorPalette="primary"
-              aria-label="채팅방 연결 중"
-            />
-            <Text typography="heading5">채팅방 연결 중...</Text>
-          </Flex>
-        </Card.Body>
-      </Card.Root>
-    </div>
-  );
-
-  const renderErrorState = () => (
-    <div className="chat-container">
-      <Card.Root className="chat-room-card">
-        <Card.Body
-          style={{
-            display: 'flex',
-            flexDirection: 'column',
-            alignItems: 'center',
-            justifyContent: 'center',
-          }}
-        >
-          <ConnectionErrorBanner
-            message={error || '채팅방을 불러오는데 실패했습니다.'}
-          />
-          <Button onClick={() => window.location.reload()}>다시 시도</Button>
-        </Card.Body>
-      </Card.Root>
-    </div>
-  );
-
   const renderContent = () => {
     if (loading) {
       return (
@@ -145,16 +99,9 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
     );
   };
 
-  if (error) {
-    return renderErrorState();
-  }
-
-  if (loading || !room) {
-    return renderLoadingState();
-  }
-
   return (
     <VStack
+      data-testid="chat-room-layout"
       $css={{
         gap: '$0',
         height: 'calc(100vh - 80px)',
@@ -162,8 +109,9 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
         backgroundColor: 'var(--vapor-color-surface-normal)',
       }}
     >
-      {/* 채팅방 정보 (참여자 목록 및 연결 상태) */}
-      <ChatRoomInfo room={room} connectionStatus={connectionStatus} />
+      <Box data-testid="chat-room-header-slot" $css={{ minHeight: '64px', width: '100%' }}>
+        {room ? <ChatRoomInfo room={room} connectionStatus={connectionStatus} /> : null}
+      </Box>
 
       {/* 메시지 영역 */}
       <VStack
@@ -173,16 +121,29 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
           minHeight: '0',
         }}
       >
-        {renderContent()}
+        {error ? (
+          <Flex $css={{ height: '100%', alignItems: 'center', justifyContent: 'center', flexDirection: 'column', gap: '$200' }}>
+            <ConnectionErrorBanner message={error || '채팅방을 불러오는데 실패했습니다.'} />
+            <Button onClick={() => window.location.reload()}>다시 시도</Button>
+          </Flex>
+        ) : loading || !room ? (
+          <Flex $css={{ height: '100%', alignItems: 'center', justifyContent: 'center', gap: '$100' }}>
+            <Spinner size="lg" colorPalette="primary" aria-label="채팅방 연결 중" />
+            <Text typography="heading5">채팅방 연결 중...</Text>
+          </Flex>
+        ) : renderContent()}
       </VStack>
 
-      {/* 입력 영역 */}
-      <ChatInput
-        onSubmit={handleMessageSubmit}
-        fileInputRef={fileInputRef}
-        disabled={connectionStatus !== 'connected'}
-        room={room}
-      />
+      <Box data-testid="chat-room-input-slot" $css={{ minHeight: '72px', width: '100%' }}>
+        {room ? (
+          <ChatInput
+            onSubmit={handleMessageSubmit}
+            fileInputRef={fileInputRef}
+            disabled={connectionStatus !== 'connected'}
+            room={room}
+          />
+        ) : null}
+      </Box>
     </VStack>
   );
 };
diff --git a/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js b/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js
index 2b40e292..01a87bf2 100644
--- a/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js
+++ b/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js
@@ -64,5 +64,7 @@ describe('ChatRoomView', () => {
     expect(screen.getByText('chat messages')).toBeInTheDocument();
     expect(screen.getByText('room info: disconnected')).toBeInTheDocument();
     expect(screen.queryByText(/연결이 끊어졌습니다/)).not.toBeInTheDocument();
+    expect(screen.getByTestId('chat-room-header-slot')).toBeInTheDocument();
+    expect(screen.getByTestId('chat-room-input-slot')).toBeInTheDocument();
   });
 });
diff --git a/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js b/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
index ab6434b0..9876e7d3 100644
--- a/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
+++ b/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
@@ -65,9 +65,9 @@ describe('useMessageHandling', () => {
       useMessageHandling(currentUser, roomId, vi.fn())
     );
 
-    await act(async () => {
+    await expect(act(async () => {
       await result.current.handleMessageSubmit({ content: 'hello' });
-    });
+    })).rejects.toThrow('채팅 서버와 연결이 끊어졌습니다.');
 
     expect(socketClient.sendChatMessageAndWait).not.toHaveBeenCalled();
     expect(Toast.error).toHaveBeenCalledWith('채팅 서버와 연결이 끊어졌습니다.');
diff --git a/apps/frontend/features/chat/room/useMessageHandling.js b/apps/frontend/features/chat/room/useMessageHandling.js
index d9d8769d..a4ec6654 100644
--- a/apps/frontend/features/chat/room/useMessageHandling.js
+++ b/apps/frontend/features/chat/room/useMessageHandling.js
@@ -69,12 +69,12 @@ export const useMessageHandling = (
    const roomSocket = getRoomSocket();
    if (!canSendOnRoomSocket() || !currentUser) {
      Toast.error('채팅 서버와 연결이 끊어졌습니다.');
-     return;
+     throw new Error('채팅 서버와 연결이 끊어졌습니다.');
    }
 
    if (!roomId) {
      Toast.error('채팅방 정보를 찾을 수 없습니다.');
-     return;
+     throw new Error('채팅방 정보를 찾을 수 없습니다.');
    }
 
    try {
@@ -112,7 +112,7 @@ export const useMessageHandling = (
          error.message?.includes('인증') ||
          error.message?.includes('토큰')) {
        await handleSessionError();
-       return;
+       throw error;
      }
 
      // 서버가 거부한 메시지는 onError 핸들러가 이미 토스트로 알렸다.
@@ -124,6 +124,7 @@ export const useMessageHandling = (
        setUploadError(error.message);
        setUploading(false);
      }
+     throw error;
    }
  }, [currentUser, roomId, handleSessionError, uploadChatFile, resetFileUpload, setUploadError, setUploading, canSendOnRoomSocket, getRoomSocket]);
 
diff --git a/apps/frontend/features/chat/rooms/ChatRoomsView.js b/apps/frontend/features/chat/rooms/ChatRoomsView.js
index 77c624d6..c7f9416a 100644
--- a/apps/frontend/features/chat/rooms/ChatRoomsView.js
+++ b/apps/frontend/features/chat/rooms/ChatRoomsView.js
@@ -222,10 +222,11 @@ export default function ChatRoomsView({ router }) {
           </Callout.Root>
         )}
 
+        <Box data-testid="rooms-content-slot" $css={{ minHeight: '430px', width: '100%' }}>
         {connectionStatus === CONNECTION_STATUS.ERROR ? (
           <ConnectionErrorBanner message="채팅 서버와 연결할 수 없습니다. 잠시 후 다시 시도해주세요." />
         ) : loading ? (
-          <Box $css={{ padding: '$400' }}>
+          <Box $css={{ minHeight: '430px', display: 'flex', alignItems: 'center', justifyContent: 'center' }}>
             <LoadingIndicator text="채팅방 목록을 불러오는 중..." />
           </Box>
         ) : rooms.length > 0 ? (
@@ -236,7 +237,7 @@ export default function ChatRoomsView({ router }) {
           />
         ) : !error && (
           <VStack
-            $css={{ gap: '$300', alignItems: 'center', padding: '$400' }}
+            $css={{ gap: '$300', alignItems: 'center', justifyContent: 'center', minHeight: '430px', padding: '$400' }}
             data-testid="rooms-empty"
           >
             <Text typography="body1">생성된 채팅방이 없습니다.</Text>
@@ -249,6 +250,7 @@ export default function ChatRoomsView({ router }) {
             </Button>
           </VStack>
         )}
+        </Box>
       </VStack>
     </Box>
   );
diff --git a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
index 204be521..0d1d71ef 100644
--- a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
@@ -143,4 +143,10 @@ describe('ChatRoomsView', () => {
 
     expect(screen.queryByTestId('refresh-rooms-button')).toBeNull();
   });
+
+  it('always reserves a stable slot for room-list states', () => {
+    render(<ChatRoomsView router={{ push: vi.fn() }} />);
+
+    expect(screen.getByTestId('rooms-content-slot')).toBeInTheDocument();
+  });
 });
diff --git a/apps/frontend/next.config.js b/apps/frontend/next.config.js
index 994749dd..66f2b0c8 100644
--- a/apps/frontend/next.config.js
+++ b/apps/frontend/next.config.js
@@ -24,7 +24,16 @@ const nextConfig = {
   // Docker 빌드를 위한 standalone 출력 모드 (개발 환경에는 영향 없음)
   output: 'standalone',
   // monorepo에서 standalone 빌드 시 중첩 경로 방지
-  outputFileTracingRoot: workspaceRoot
+  outputFileTracingRoot: workspaceRoot,
+  async redirects() {
+    return [
+      {
+        source: '/login',
+        destination: '/',
+        permanent: false,
+      },
+    ];
+  }
 };
 
 module.exports = nextConfig;
diff --git a/apps/frontend/pages/chat/new.js b/apps/frontend/pages/chat/new.js
index 9cbbdda9..c9914289 100644
--- a/apps/frontend/pages/chat/new.js
+++ b/apps/frontend/pages/chat/new.js
@@ -25,13 +25,16 @@ function NewChatRoom() {
     password: ''
   });
   const [loading, setLoading] = useState(false);
+  const [submissionStage, setSubmissionStage] = useState('idle');
   const [error, setError] = useState('');
 
   const joinRoom = async (roomId, password) => {
     try {
+      setSubmissionStage('joining');
       await api.post(`/api/rooms/${roomId}/join`, { password });
 
-      router.push(`/chat/${roomId}`);
+      setSubmissionStage('navigating');
+      await router.push(`/chat/${roomId}`);
     } catch (error) {
       console.error('Room join error:', error);
       throw error;
@@ -58,6 +61,7 @@ function NewChatRoom() {
 
     try {
       setLoading(true);
+      setSubmissionStage('creating');
       setError('');
 
       const response = await api.post('/api/rooms', {
@@ -71,6 +75,7 @@ function NewChatRoom() {
     } catch (error) {
       console.error('Room creation/join error:', error);
       setError(error.message);
+      setSubmissionStage('failed');
     } finally {
       setLoading(false);
     }
@@ -99,6 +104,16 @@ function NewChatRoom() {
       >
         <Text typography="heading4">새 채팅방</Text>
 
+        <Text typography="body2" role="status" data-testid="room-creation-status">
+          {{
+            idle: '',
+            creating: '채팅방을 생성하고 있습니다.',
+            joining: '생성된 채팅방에 입장하고 있습니다.',
+            navigating: '채팅방 화면을 여는 중입니다.',
+            failed: '채팅방 생성 또는 입장에 실패했습니다.',
+          }[submissionStage]}
+        </Text>
+
         {error && (
           <Callout.Root colorPalette="danger">
             <HStack $css={{ gap: '$200', alignItems: 'center' }}>
diff --git a/apps/frontend/pages/index.js b/apps/frontend/pages/index.js
index bd1f05aa..76a2cf06 100644
--- a/apps/frontend/pages/index.js
+++ b/apps/frontend/pages/index.js
@@ -81,12 +81,6 @@ const Login = () => {
   const handleSubmit = async (e) => {
     e.preventDefault();
 
-    // 서버 연결 상태 확인 — 로그인을 시도조차 못 한 것이므로 서버 안내로 알린다.
-    if (!serverStatus.connected) {
-      setServerNotice('서버와 연결할 수 없습니다. 인터넷 연결을 확인하고 잠시 후 다시 시도해주세요.');
-      return;
-    }
-
     // 폼 유효성 검사
     if (!validateForm()) {
       return;
@@ -116,29 +110,6 @@ const Login = () => {
     }
   };
 
-  if (serverStatus.checking) {
-    return (
-      <div className="min-h-screen flex items-center justify-center p-[var(--vapor-space-300)] bg-[var(--vapor-color-background)]">
-        <VStack
-          $css={{
-            gap: '$250',
-            width: '400px',
-            padding: '$300',
-            borderRadius: '$300',
-            border: '1px solid var(--vapor-color-border-normal)',
-          }}
-        >
-          <div className="text-center mb-[2rem]">
-            <img src="images/logo-h.png" className="w-1/2 mx-auto" alt="KTB Chat 로고" />
-          </div>
-          <div className="text-center">
-            <Text typography="body1">서버 연결 확인 중...</Text>
-          </div>
-        </VStack>
-      </div>
-    );
-  }
-
   return (
     <div className="min-h-screen flex items-center justify-center p-(--vapor-space-300) bg-(--vapor-color-background)">
       <VStack
@@ -152,9 +123,21 @@ const Login = () => {
         render={<Form onSubmit={handleSubmit} />}
       >
         <div className="text-center mb-4">
-          <img src="images/logo-h.png" className="w-1/2 mx-auto" alt="KTB Chat 로고" />
+          <img
+            src="/images/logo-h.png"
+            width="240"
+            height="80"
+            className="w-1/2 h-auto mx-auto"
+            alt="KTB Chat 로고"
+          />
         </div>
 
+        {serverStatus.checking && (
+          <Text typography="body2" foreground="hint-100" role="status" data-testid="server-status-checking">
+            서버 연결을 백그라운드에서 확인하고 있습니다.
+          </Text>
+        )}
+
         {serverNotice && (
           <Callout.Root colorPalette="warning" data-testid="server-status-message">
             <Callout.Icon>
@@ -224,7 +207,7 @@ const Login = () => {
           <Button
             type="submit"
             size="lg"
-            disabled={loading || !serverStatus.connected}
+            disabled={loading}
             data-testid="login-submit-button"
           >
             {loading ? '로그인 중...' : '로그인'}
@@ -238,7 +221,7 @@ const Login = () => {
             size="sm"
             variant="ghost"
             onClick={() => router.push('/register')}
-            disabled={loading || !serverStatus.connected}
+            disabled={loading}
           >
             회원가입
           </Button>
diff --git a/apps/frontend/pages/register.js b/apps/frontend/pages/register.js
index 57f36a2c..00e33a5a 100644
--- a/apps/frontend/pages/register.js
+++ b/apps/frontend/pages/register.js
@@ -77,7 +77,7 @@ const Register = () => {
         render={<Form onSubmit={handleSubmit} />}
       >
         <div className="text-center mb-4">
-          <img src="images/logo-h.png" className="w-1/2 mx-auto" alt="KTB Chat 로고" />
+          <img src="/images/logo-h.png" width="240" height="80" className="w-1/2 h-auto mx-auto" alt="KTB Chat 로고" />
         </div>
 
         {error && (
diff --git a/apps/frontend/test/pages/index.test.js b/apps/frontend/test/pages/index.test.js
new file mode 100644
index 00000000..6af33ed1
--- /dev/null
+++ b/apps/frontend/test/pages/index.test.js
@@ -0,0 +1,30 @@
+import React from 'react';
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+import LoginPage from '../../pages/index';
+
+const mocks = vi.hoisted(() => ({
+  login: vi.fn(),
+  checkServerConnection: vi.fn(() => new Promise(() => {})),
+  router: { query: {}, push: vi.fn(), replace: vi.fn(), isReady: true },
+}));
+
+vi.mock('next/router', () => ({ useRouter: () => mocks.router }));
+vi.mock('@/contexts/AuthContext', () => ({
+  useAuth: () => ({ login: mocks.login }),
+  withoutAuth: (Component) => Component,
+}));
+vi.mock('@/services/authService', () => ({
+  default: { checkServerConnection: mocks.checkServerConnection },
+}));
+
+describe('Login page', () => {
+  it('keeps the login form usable while the health check is pending', () => {
+    render(<LoginPage />);
+
+    expect(screen.getByTestId('server-status-checking')).toBeInTheDocument();
+    expect(screen.getByTestId('login-email-input')).toBeEnabled();
+    expect(screen.getByTestId('login-password-input')).toBeEnabled();
+    expect(screen.getByTestId('login-submit-button')).toBeEnabled();
+  });
+});
diff --git a/e2e/actions/auth.actions.js b/e2e/actions/auth.actions.js
index 074754c2..3b05742b 100644
--- a/e2e/actions/auth.actions.js
+++ b/e2e/actions/auth.actions.js
@@ -6,12 +6,20 @@ const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
  * @param {Object} credentials - { email: string, password: string }
  * @param waitForRedirect
  */
-async function loginAction(page, credentials, waitForRedirect = true) {
+async function openLoginAction(page) {
   await page.goto(`${BASE_URL}/login`);
+  const emailInput = page.getByTestId('login-email-input');
+  await emailInput.waitFor({ state: 'visible' });
+  if (!await emailInput.isEnabled()) {
+    throw new Error('로그인 입력 필드가 활성화되지 않았습니다.');
+  }
+}
+
+async function loginAction(page, credentials, waitForRedirect = true, navigate = true) {
+  if (navigate) await openLoginAction(page);
   await page.getByTestId('login-email-input').fill(credentials.email);
   await page.getByTestId('login-password-input').fill(credentials.password);
   await page.getByTestId('login-submit-button').click();
-  await page.waitForTimeout(1000); // 잠시 대기
   if (waitForRedirect) {
     await page.waitForURL(`${BASE_URL}/chat`);
   }
@@ -32,7 +40,7 @@ async function registerAction(page, userData) {
   // 성공 시 앱이 1000ms 뒤 router.push('/login')로 이동하므로, 그 내비게이션이 끝난 뒤에
   // 다음 goto가 실행되어야 경합(net::ERR_ABORTED)이 없다. 실패 시엔 에러 메시지 표시까지 대기.
   await Promise.race([
-    page.waitForURL(`${BASE_URL}/login`, { timeout: 10000 }).catch(() => {}),
+    page.waitForURL(`${BASE_URL}/`, { timeout: 10000 }).catch(() => {}),
     page.getByTestId('register-error-message').waitFor({ state: 'visible', timeout: 10000 }).catch(() => {}),
   ]);
 }
@@ -47,6 +55,7 @@ async function logoutAction(page) {
 
 module.exports = {
   loginAction,
+  openLoginAction,
   registerAction,
   logoutAction,
 };
diff --git a/e2e/actions/chat.actions.js b/e2e/actions/chat.actions.js
index 7ff1d470..52411350 100644
--- a/e2e/actions/chat.actions.js
+++ b/e2e/actions/chat.actions.js
@@ -54,9 +54,12 @@ async function createChatRoomAction(page, roomName) {
  * @param {import('@playwright/test').Page} page
  * @param {string} message - 전송할 메시지 내용
  */
-async function sendMessageAction(page, message) {
+async function sendMessageAction(page, message, { expectFailure = false } = {}) {
   await page.getByTestId('chat-message-input').fill(message);
   await page.getByTestId('chat-send-button').click();
+  await page.getByTestId('message-submission-status').filter({
+    hasText: expectFailure ? 'failed' : 'complete',
+  }).waitFor({ state: 'visible', timeout: 15000 });
 }
 
 /**
diff --git a/e2e/artillery/aggregate-observations.js b/e2e/artillery/aggregate-observations.js
index 3188f00a..3304c7f4 100644
--- a/e2e/artillery/aggregate-observations.js
+++ b/e2e/artillery/aggregate-observations.js
@@ -12,11 +12,13 @@ const reports = runDirectories.flatMap((runDirectory) => fs.readdirSync(runDirec
     .filter((file) => file.startsWith('vu-') && file.endsWith('.json'))
     .map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8'))));
 
-const samples = { actions: [], http: [], socket: [] };
+const samples = { actions: [], documents: [], http: [], socket: [], layoutShifts: [] };
 for (const report of reports) {
     samples.actions.push(...report.samples.actions);
+    samples.documents.push(...(report.samples.documents || []));
     samples.http.push(...report.samples.http);
     samples.socket.push(...report.samples.socket);
+    samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
 
 const timestamps = Object.values(samples).flat()
@@ -26,6 +28,10 @@ const timestamps = Object.values(samples).flat()
 const rankedHttp = summarizeSamples(samples.http.filter((sample) =>
     sample.name !== 'GET /api/health' &&
     !(sample.name === 'POST /api/auth/login' && sample.status === 401)));
+const expectedPerRun = Number(process.env.EXPECTED_VUS || process.env.PHASE1_ARRIVAL_COUNT || 0);
+const expectedVirtualUsers = expectedPerRun > 0 ? expectedPerRun * runDirectories.length : null;
+const completedVirtualUsers = reports.filter((report) => report.samples.actions.length > 0 &&
+    report.samples.actions.every((sample) => sample.success)).length;
 
 const aggregate = {
     schemaVersion: 1,
@@ -39,13 +45,22 @@ const aggregate = {
             report.samples.actions.every((sample) => sample.success)).length,
         failed: reports.filter((report) => report.samples.actions.some((sample) => !sample.success)).length,
         allSamplesHaveWindowsAndNames: Object.values(samples).flat().every((sample) =>
-            Boolean(sample.name && sample.startedAt && sample.endedAt)),
+            sample.value !== undefined || Boolean(sample.name && sample.startedAt && sample.endedAt)),
+        expectedVirtualUsers,
+        targetReached: expectedVirtualUsers === null ? null : reports.length === expectedVirtualUsers,
+        adoptable: expectedVirtualUsers === null ? false :
+            reports.length === expectedVirtualUsers && completedVirtualUsers / expectedVirtualUsers >= 0.99,
     },
     summary: {
         actions: summarizeSamples(samples.actions),
+        documents: summarizeSamples(samples.documents),
         httpDiagnostic: summarizeSamples(samples.http),
         httpRanked: rankedHttp,
         socket: summarizeSamples(samples.socket),
+        cls: {
+            total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
+            entries: samples.layoutShifts.length,
+        },
     },
     samples,
 };
diff --git a/e2e/artillery/observation.js b/e2e/artillery/observation.js
index de15b9a3..e3d19464 100644
--- a/e2e/artillery/observation.js
+++ b/e2e/artillery/observation.js
@@ -38,6 +38,13 @@ function normalizeApiPath(rawUrl) {
     }).join('/')}`;
 }
 
+function normalizeDocumentPath(rawUrl) {
+    const url = new URL(rawUrl, 'http://localhost');
+    if (url.pathname === '/' || url.pathname === '/login') return url.pathname;
+    if (/^\/chat\/[a-f\d]{24}$/i.test(url.pathname)) return '/chat/{roomId}';
+    return url.pathname;
+}
+
 function parseSocketEvent(payload) {
     const text = Buffer.isBuffer(payload) ? payload.toString('utf8') : String(payload);
     const arrayStart = text.indexOf('[');
@@ -80,7 +87,7 @@ function summarizeSamples(samples) {
 function createObservation(page, vuContext) {
     const startedRequests = new Map();
     const pendingSocketEvents = new Map();
-    const samples = { http: [], socket: [], actions: [] };
+    const samples = { documents: [], http: [], socket: [], actions: [], layoutShifts: [] };
     let currentAction = null;
     let socketSequence = 0;
 
@@ -108,6 +115,21 @@ function createObservation(page, vuContext) {
     };
     const onRequestFinished = (request) => { void recordHttp(request, true); };
     const onRequestFailed = (request) => { void recordHttp(request, false); };
+    const onResponse = (response) => {
+        const request = response.request();
+        if (request.resourceType() !== 'document') return;
+        const timing = request.timing();
+        const ttfbMs = timing?.responseStart;
+        samples.documents.push({
+            name: `GET ${normalizeDocumentPath(response.url())}`,
+            durationMs: Number.isFinite(ttfbMs) && ttfbMs >= 0 ? ttfbMs : 0,
+            startedAt: new Date(Date.now() - Math.max(ttfbMs || 0, 0)).toISOString(),
+            endedAt: new Date().toISOString(),
+            success: response.status() < 400,
+            status: response.status(),
+            redirectedFrom: request.redirectedFrom()?.url() || null,
+        });
+    };
     const onWebSocket = (webSocket) => {
         const connectionKey = `connection:${socketSequence += 1}`;
         pendingSocketEvents.set(connectionKey, {
@@ -151,8 +173,66 @@ function createObservation(page, vuContext) {
     page.on('request', onRequest);
     page.on('requestfinished', onRequestFinished);
     page.on('requestfailed', onRequestFailed);
+    page.on('response', onResponse);
     page.on('websocket', onWebSocket);
 
+    void page.addInitScript(() => {
+        window.__e2eLayoutShifts = [];
+        const selectorFor = (node) => {
+            if (!node || node.nodeType !== Node.ELEMENT_NODE) return null;
+            if (node.id) return `#${node.id}`;
+            const testId = node.getAttribute('data-testid');
+            if (testId) return `[data-testid="${testId}"]`;
+            return node.tagName.toLowerCase() + (node.classList.length ? `.${[...node.classList].slice(0, 2).join('.')}` : '');
+        };
+        new PerformanceObserver((list) => {
+            for (const entry of list.getEntries()) {
+                if (entry.hadRecentInput) continue;
+                window.__e2eLayoutShifts.push({
+                    value: entry.value,
+                    atMs: entry.startTime,
+                    sources: (entry.sources || []).map((source) => ({
+                        selector: selectorFor(source.node),
+                        previousRect: source.previousRect,
+                        currentRect: source.currentRect,
+                        image: source.node?.tagName === 'IMG' ? {
+                            src: source.node.currentSrc || source.node.src,
+                            naturalWidth: source.node.naturalWidth,
+                            naturalHeight: source.node.naturalHeight,
+                            renderedWidth: source.node.clientWidth,
+                            renderedHeight: source.node.clientHeight,
+                        } : null,
+                    })),
+                });
+            }
+        }).observe({ type: 'layout-shift', buffered: true });
+    });
+
+    async function collectPageDiagnostics() {
+        const selectors = [
+            '[data-testid="login-email-input"]', '[data-testid="login-submit-button"]',
+            '[data-testid="chat-room-name-input"]', '[data-testid="chat-message-input"]',
+            '[data-testid="message-submission-status"]', '[data-testid="file-message-container"]',
+        ];
+        const ui = {};
+        for (const selector of selectors) {
+            const locator = page.locator(selector).first();
+            ui[selector] = {
+                count: await locator.count().catch(() => 0),
+                visible: await locator.isVisible().catch(() => false),
+                enabled: await locator.isEnabled().catch(() => false),
+                text: await locator.textContent().catch(() => null),
+            };
+        }
+        return {
+            url: page.url(),
+            ui,
+            recentHttp: samples.http.slice(-10).map(({ name, status, success, action, startedAt, endedAt }) =>
+                ({ name, status, success, action, startedAt, endedAt })),
+            pendingSocketEvents: [...pendingSocketEvents.keys()].map(String),
+        };
+    }
+
     async function action(name, callback) {
         const previousAction = currentAction;
         currentAction = name;
@@ -169,6 +249,8 @@ function createObservation(page, vuContext) {
             samples.actions.push({
                 name, durationMs: performance.now() - startedAt, startedAt: wallStartedAt,
                 endedAt: new Date().toISOString(), success: false,
+                error: { name: error.name, message: error.message },
+                diagnostics: await collectPageDiagnostics(),
             });
             throw error;
         } finally {
@@ -181,8 +263,11 @@ function createObservation(page, vuContext) {
         page.off('request', onRequest);
         page.off('requestfinished', onRequestFinished);
         page.off('requestfailed', onRequestFailed);
+        page.off('response', onResponse);
         page.off('websocket', onWebSocket);
 
+        samples.layoutShifts = await page.evaluate(() => window.__e2eLayoutShifts || []).catch(() => []);
+
         const runId = process.env.OBSERVATION_RUN_ID || new Date().toISOString().replace(/[:.]/g, '-');
         const outputRoot = process.env.OBSERVATION_OUTPUT_DIR || path.resolve(__dirname, 'results');
         const runDirectory = path.join(outputRoot, runId);
@@ -195,8 +280,13 @@ function createObservation(page, vuContext) {
             samples,
             summary: {
                 actions: summarizeSamples(samples.actions),
+                documents: summarizeSamples(samples.documents),
                 http: summarizeSamples(samples.http),
                 socket: summarizeSamples(samples.socket),
+                cls: {
+                    total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
+                    entries: samples.layoutShifts.length,
+                },
             },
         };
         const filename = `vu-${process.pid}-${randomUUID()}.json`;
@@ -217,6 +307,7 @@ function printTable(title, rows) {
 
 function printSummary(summary) {
     printTable('User actions', summary.actions);
+    printTable('Next.js documents (TTFB)', summary.documents);
     printTable('HTTP API (ranked by cumulative response time)', summary.http);
     printTable('Socket.IO actions', summary.socket);
 }
@@ -224,6 +315,7 @@ function printSummary(summary) {
 module.exports = {
     createObservation,
     normalizeApiPath,
+    normalizeDocumentPath,
     parseSocketEvent,
     summarizeSamples,
 };
diff --git a/e2e/artillery/observation.test.js b/e2e/artillery/observation.test.js
index 320ecdd9..9d880f9a 100644
--- a/e2e/artillery/observation.test.js
+++ b/e2e/artillery/observation.test.js
@@ -1,6 +1,6 @@
 const test = require('node:test');
 const assert = require('node:assert/strict');
-const { normalizeApiPath, parseSocketEvent, summarizeSamples } = require('./observation');
+const { normalizeApiPath, normalizeDocumentPath, parseSocketEvent, summarizeSamples } = require('./observation');
 
 test('normalizes dynamic API identifiers without changing static routes', () => {
     assert.equal(normalizeApiPath('http://localhost:5001/api/rooms'), '/api/rooms');
@@ -10,6 +10,12 @@ test('normalizes dynamic API identifiers without changing static routes', () =>
     assert.equal(normalizeApiPath('http://localhost:3000/_next/static/a.js'), null);
 });
 
+test('keeps Next document paths separate from API paths', () => {
+    assert.equal(normalizeDocumentPath('http://localhost:3000/'), '/');
+    assert.equal(normalizeDocumentPath('http://localhost:3000/login?redirect=/chat'), '/login');
+    assert.equal(normalizeDocumentPath('http://localhost:3000/chat/507f1f77bcf86cd799439011'), '/chat/{roomId}');
+});
+
 test('extracts Socket.IO event names from websocket frames', () => {
     assert.equal(parseSocketEvent('42["joinRoom","room-1"]'), 'joinRoom');
     assert.equal(parseSocketEvent('451-["message",{"content":"hello"}]'), 'message');
diff --git a/e2e/artillery/report-observations.js b/e2e/artillery/report-observations.js
index f3c1b652..e754f3a5 100644
--- a/e2e/artillery/report-observations.js
+++ b/e2e/artillery/report-observations.js
@@ -8,11 +8,13 @@ const files = fs.readdirSync(runDirectory).filter((file) => file.startsWith('vu-
 if (files.length === 0) throw new Error(`No observation JSON files found in ${runDirectory}`);
 
 const reports = files.map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8')));
-const samples = { actions: [], http: [], socket: [] };
+const samples = { actions: [], documents: [], http: [], socket: [], layoutShifts: [] };
 for (const report of reports) {
     samples.actions.push(...report.samples.actions);
+    samples.documents.push(...(report.samples.documents || []));
     samples.http.push(...report.samples.http);
     samples.socket.push(...report.samples.socket);
+    samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
 
 const aggregate = {
@@ -22,8 +24,13 @@ const aggregate = {
     virtualUsers: reports.length,
     summary: {
         actions: summarizeSamples(samples.actions),
+        documents: summarizeSamples(samples.documents),
         http: summarizeSamples(samples.http),
         socket: summarizeSamples(samples.socket),
+        cls: {
+            total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
+            entries: samples.layoutShifts.length,
+        },
     },
 };
 fs.writeFileSync(path.join(runDirectory, 'summary.json'), `${JSON.stringify(aggregate, null, 2)}\n`);
diff --git a/e2e/artillery/scenarios/auth.scenario.js b/e2e/artillery/scenarios/auth.scenario.js
index 577eb006..e7a70bfd 100644
--- a/e2e/artillery/scenarios/auth.scenario.js
+++ b/e2e/artillery/scenarios/auth.scenario.js
@@ -1,4 +1,4 @@
-const { loginAction, registerAction, logoutAction } = require('../../actions/auth.actions');
+const { loginAction, openLoginAction, registerAction, logoutAction } = require('../../actions/auth.actions');
 const { expect } = require('@playwright/test');
 
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
@@ -21,7 +21,8 @@ async function loginScenario(page, vuContext) {
         await page.waitForTimeout(ACTION_TIMEOUT);
 
         // 2. 로그인
-        await observe('login', () => loginAction(page, testUser));
+        await observe('login_form_ready', () => openLoginAction(page));
+        await observe('login', () => loginAction(page, testUser, true, false));
         await expect(page).toHaveURL(`${BASE_URL}/chat`);
 
         // URL 만으로는 완주를 판정할 수 없다. 방 목록이 실제로 그려져야
@@ -46,10 +47,11 @@ async function loginScenario(page, vuContext) {
 async function failedLoginScenario(page, vuContext) {
     const observe = vuContext.vars.observation.action;
     try {
+        await observe('login_form_ready_for_failed_login', () => openLoginAction(page));
         await observe('failed_login', () => loginAction(page, {
             email: 'nonexistent@example.com',
             password: 'WrongPassword123!',
-        }, false));
+        }, false, false));
 
         await page.waitForTimeout(500);
         const errorElement = page.getByTestId('login-error-message');
diff --git a/e2e/artillery/scenarios/chat.scenario.js b/e2e/artillery/scenarios/chat.scenario.js
index f78a1664..d8021931 100644
--- a/e2e/artillery/scenarios/chat.scenario.js
+++ b/e2e/artillery/scenarios/chat.scenario.js
@@ -13,11 +13,6 @@ const { randomUUID } = require('crypto');
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
 const MASS_MESSAGE_COUNT = process.env.MASS_MESSAGE_COUNT || 10;
 
-// Action 간 timeout 설정 (환경변수로 조절 가능)
-const ACTION_TIMEOUT = parseInt(process.env.ACTION_TIMEOUT || '1000', 10);
-const ACTION_TIMEOUT_SHORT = parseInt(process.env.ACTION_TIMEOUT_SHORT || '500', 10);
-const ACTION_TIMEOUT_LONG = parseInt(process.env.ACTION_TIMEOUT_LONG || '2000', 10);
-
 async function gotoChatPage(page, vuContext) {
     await page.goto(`${BASE_URL}/chat`);
     await expect(page).toHaveURL(`${BASE_URL}/chat`);
@@ -46,7 +41,6 @@ async function chatRoomCreationScenario(page, vuContext) {
         const message = `테스트 메시지 ${bannedWordSafeText(Date.now())}`;
         await observe('message_send', async () => {
             await sendMessageAction(page, message);
-            await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
             const messageElement = page.getByTestId('message-content').filter({ hasText: message });
             await expect(messageElement).toBeVisible();
         });
@@ -108,7 +102,6 @@ async function fileUploadScenario(page, vuContext) {
         await observe('file_upload', async () => {
             await uploadFileAction(page, filePath, message);
             await uploadPromise;
-            await page.waitForTimeout(ACTION_TIMEOUT);
             const fileMessageContainer = page.getByTestId('file-message-container').filter({ hasText: message });
             await expect(fileMessageContainer).toBeVisible({ timeout: 10000 });
         });
@@ -143,7 +136,7 @@ async function forbiddenWordScenario(page, vuContext) {
         // 2. 금칙어 메시지 전송 시도
         const forbiddenWord = FORBIDDEN_WORDS[Math.floor(Math.random() * FORBIDDEN_WORDS.length)];
         await observe('forbidden_message_send', async () => {
-            await sendMessageAction(page, forbiddenWord);
+            await sendMessageAction(page, forbiddenWord, { expectFailure: true });
             const errorToast = page.getByTestId('toast-error');
             await expect(errorToast).toBeVisible({ timeout: 5000 });
             const sentMessage = page.getByTestId('message-content').filter({ hasText: forbiddenWord });

From c26ea4a0e6238bbffb7990e6906a824df53634cc Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 21:28:55 +0900
Subject: [PATCH 23/32] =?UTF-8?q?perf(mongo):=20=ED=8C=8C=EC=9D=BC=20?=
 =?UTF-8?q?=EC=B0=B8=EC=A1=B0=20=EC=A1=B0=ED=9A=8C=20=EC=9D=B8=EB=8D=B1?=
 =?UTF-8?q?=EC=8A=A4=20=EC=B6=94=EA=B0=80?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/backend/src/main/java/com/ktb/chatapp/model/File.java    | 4 +++-
 apps/backend/src/main/java/com/ktb/chatapp/model/Message.java | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/File.java b/apps/backend/src/main/java/com/ktb/chatapp/model/File.java
index e3780e9d..ed56a2bf 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/File.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/File.java
@@ -6,6 +6,7 @@
 import lombok.NoArgsConstructor;
 import org.springframework.data.annotation.CreatedDate;
 import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.index.Indexed;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
 
@@ -25,6 +26,7 @@ public class File {
     @Id
     private String id;
 
+    @Indexed(name = "filename_idx")
     private String filename;
 
     private String originalname;
@@ -78,4 +80,4 @@ public String getFileUrl(String type) {
             type,
             URLEncoder.encode(this.filename, StandardCharsets.UTF_8));
     }
-}
\ No newline at end of file
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java b/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
index dba4283c..a2fe71fb 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
@@ -10,6 +10,7 @@
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.index.CompoundIndex;
+import org.springframework.data.mongodb.core.index.Indexed;
 
 import java.time.LocalDateTime;
 import java.util.ArrayList;
@@ -50,6 +51,7 @@ public class Message {
     private MessageType type;
 
     // Mongo 문서 필드명 "file" 사용
+    @Indexed(name = "file_idx")
     @Field("file")
     private String fileId;
 

From b579e9745fdd4baa61d2e33867199cbdd1889d95 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 21:34:51 +0900
Subject: [PATCH 24/32] =?UTF-8?q?build(frontend):=20webpack=20=EB=B9=8C?=
 =?UTF-8?q?=EB=93=9C=EB=A1=9C=20=EA=B3=A0=EC=A0=95?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/frontend/package.json | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/apps/frontend/package.json b/apps/frontend/package.json
index 914e133f..4511f932 100644
--- a/apps/frontend/package.json
+++ b/apps/frontend/package.json
@@ -5,12 +5,12 @@
   "private": true,
   "scripts": {
     "dev": "next dev",
-    "build": "next build",
+    "build": "next build --webpack",
     "start": "next start",
     "lint": "eslint .",
     "test": "vitest run --passWithNoTests",
     "format": "prettier . --write --ignore-path .prettierignore",
-    "build:production": "test -f .env.production || { echo '.env.production 이 없습니다. cp .env.production.example .env.production 후 배포 주소로 값을 채우세요.' >&2; exit 1; } && env $(grep -vE '^[[:space:]]*(#|$)' .env.production | xargs) next build && pnpm run deploy:static",
+    "build:production": "test -f .env.production || { echo '.env.production 이 없습니다. cp .env.production.example .env.production 후 배포 주소로 값을 채우세요.' >&2; exit 1; } && env $(grep -vE '^[[:space:]]*(#|$)' .env.production | xargs) next build --webpack && pnpm run deploy:static",
     "deploy:static": "cp -r .next/static .next/standalone/apps/frontend/.next/ && cp -r public .next/standalone/apps/frontend/"
   },
   "dependencies": {

From a790293d9d3a41f094cf32ba9629dda3661d76f0 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 21:26:48 +0900
Subject: [PATCH 25/32] =?UTF-8?q?fix(frontend):=20=EB=A1=9C=EA=B7=B8?=
 =?UTF-8?q?=EC=9D=B8=EA=B3=BC=20=EC=B1=84=ED=8C=85=20=ED=99=94=EB=A9=B4=20?=
 =?UTF-8?q?=EC=95=88=EC=A0=95=ED=99=94?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/frontend/components/ChatHeader.js        |  1 +
 apps/frontend/components/ChatInput.js         | 33 +++++--
 .../components/ConnectionErrorBanner.js       |  2 +
 apps/frontend/components/FileMessage.js       |  6 +-
 apps/frontend/components/FilePreview.js       |  6 +-
 .../components/__tests__/ChatInput.test.js    | 28 +++++-
 .../features/chat/room/ChatRoomView.js        | 91 +++++-------------
 .../chat/room/__tests__/ChatRoomView.test.js  |  2 +
 .../room/__tests__/useMessageHandling.test.js |  4 +-
 .../features/chat/room/useMessageHandling.js  |  7 +-
 .../features/chat/rooms/ChatRoomsView.js      |  6 +-
 .../rooms/__tests__/ChatRoomsView.test.js     |  6 ++
 apps/frontend/next.config.js                  | 11 ++-
 apps/frontend/pages/chat/new.js               | 17 +++-
 apps/frontend/pages/index.js                  | 47 +++-------
 apps/frontend/pages/register.js               |  2 +-
 apps/frontend/test/pages/index.test.js        | 30 ++++++
 e2e/actions/auth.actions.js                   | 15 ++-
 e2e/actions/chat.actions.js                   |  5 +-
 e2e/artillery/aggregate-observations.js       | 19 +++-
 e2e/artillery/observation.js                  | 94 ++++++++++++++++++-
 e2e/artillery/observation.test.js             |  8 +-
 e2e/artillery/report-observations.js          |  9 +-
 e2e/artillery/scenarios/auth.scenario.js      |  8 +-
 e2e/artillery/scenarios/chat.scenario.js      |  9 +-
 25 files changed, 326 insertions(+), 140 deletions(-)
 create mode 100644 apps/frontend/test/pages/index.test.js

diff --git a/apps/frontend/components/ChatHeader.js b/apps/frontend/components/ChatHeader.js
index 8dc55672..f6c0a6e0 100644
--- a/apps/frontend/components/ChatHeader.js
+++ b/apps/frontend/components/ChatHeader.js
@@ -31,6 +31,7 @@ const ChatHeader = () => {
         <img
           src="/images/logo.png"
           alt="Chat App Logo"
+          width={120}
           height={15}
           className="logo"
         />
diff --git a/apps/frontend/components/ChatInput.js b/apps/frontend/components/ChatInput.js
index 63603ac1..0911b1db 100644
--- a/apps/frontend/components/ChatInput.js
+++ b/apps/frontend/components/ChatInput.js
@@ -46,6 +46,7 @@ const ChatInput = forwardRef(({
   const [uploading, setUploading] = useState(false);
   const [uploadProgress, setUploadProgress] = useState(0);
   const [uploadError, setUploadError] = useState(null);
+  const [submissionStatus, setSubmissionStatus] = useState('idle');
   const [isDragging, setIsDragging] = useState(false);
   const [mentionPosition, setMentionPosition] = useState({ top: 0, left: 0 });
 
@@ -109,7 +110,10 @@ const ChatInput = forwardRef(({
           throw new Error('파일이 선택되지 않았습니다.');
         }
 
-        onSubmit({
+        setUploading(true);
+        setSubmissionStatus('uploading-and-waiting-for-message');
+        setUploadError(null);
+        await onSubmit({
           type: 'file',
           content: message.trim(),
           fileData: file
@@ -119,19 +123,29 @@ const ChatInput = forwardRef(({
         setShowEmojiPicker(false);
         setShowMentionList(false);
         setFiles([]);
+        setSubmissionStatus('complete');
 
       } catch (error) {
         console.error('File submit error:', error);
         setUploadError(error.message);
+        setSubmissionStatus('failed');
+      } finally {
+        setUploading(false);
       }
     } else if (message.trim()) {
-      onSubmit({
-        type: 'text',
-        content: message.trim()
-      });
-      setMessage('');
-      setShowEmojiPicker(false);
-      setShowMentionList(false);
+      try {
+        setSubmissionStatus('waiting-for-message');
+        await onSubmit({
+          type: 'text',
+          content: message.trim()
+        });
+        setMessage('');
+        setShowEmojiPicker(false);
+        setShowMentionList(false);
+        setSubmissionStatus('complete');
+      } catch (error) {
+        setSubmissionStatus('failed');
+      }
     }
   }, [files, message, onSubmit, setMessage, setShowEmojiPicker, setShowMentionList]);
 
@@ -403,6 +417,9 @@ const ChatInput = forwardRef(({
         )}
 
         <VStack $css={{ gap: '$100', width: '100%' }}>
+          <span className="sr-only" role="status" data-testid="message-submission-status">
+            {submissionStatus}
+          </span>
           <VStack className="relative" $css={{ gap: '$025' }}>
             <HStack $css={{ gap: '$200' }}>
                 <Textarea
diff --git a/apps/frontend/components/ConnectionErrorBanner.js b/apps/frontend/components/ConnectionErrorBanner.js
index 17ce8fee..7e3c3574 100644
--- a/apps/frontend/components/ConnectionErrorBanner.js
+++ b/apps/frontend/components/ConnectionErrorBanner.js
@@ -13,6 +13,8 @@ const ConnectionErrorBanner = ({ message, title = '연결에 문제가 생겼어
     <img
       src="/error.svg"
       alt=""
+      width="220"
+      height="165"
       style={{ width: '220px', maxWidth: '70%', height: 'auto' }}
     />
     <Text typography="heading4" foreground="normal-100">
diff --git a/apps/frontend/components/FileMessage.js b/apps/frontend/components/FileMessage.js
index 9b75b5a2..87815233 100644
--- a/apps/frontend/components/FileMessage.js
+++ b/apps/frontend/components/FileMessage.js
@@ -172,11 +172,13 @@ const FileMessage = ({
       const previewUrl = fileService.getPreviewUrl(msg.file, user?.token, true);
 
       return (
-        <div className="bg-transparent-pattern">
+        <div className="bg-transparent-pattern" style={{ width: 'min(400px, 100%)', aspectRatio: '4 / 3' }}>
           <img
             src={previewUrl}
             alt={originalname}
-            className="max-w-[400px] max-h-[400px] object-cover object-center rounded-md"
+            width="400"
+            height="300"
+            className="w-full h-full object-cover object-center rounded-md"
             onLoad={() => {
               console.debug('Image loaded successfully:', originalname);
             }}
diff --git a/apps/frontend/components/FilePreview.js b/apps/frontend/components/FilePreview.js
index 29220c62..ab250520 100644
--- a/apps/frontend/components/FilePreview.js
+++ b/apps/frontend/components/FilePreview.js
@@ -208,11 +208,13 @@ const FilePreview = ({
 
     if (file.type.startsWith('image/')) {
       return (
-        <div className={`${previewContainer} ${previewBackground}`}>
+        <div className={`${previewContainer} ${previewBackground}`} style={{ aspectRatio: '4 / 3' }}>
           <img
             src={previewUrl || file.url}
             alt={`${file.name} 미리보기`}
             className="w-full h-full object-cover"
+            width="320"
+            height="240"
             onError={(e) => {
               e.target.onerror = null;
               e.target.src = '/placeholder-image.png';
@@ -226,7 +228,7 @@ const FilePreview = ({
 
     if (file.type.startsWith('video/')) {
       return (
-        <div className={`${previewContainer}`}>
+        <div className={`${previewContainer}`} style={{ aspectRatio: '4 / 3' }}>
           <video
             src={previewUrl || file.url}
             className="w-full h-full object-cover"
diff --git a/apps/frontend/components/__tests__/ChatInput.test.js b/apps/frontend/components/__tests__/ChatInput.test.js
index 1c1848ea..a222933d 100644
--- a/apps/frontend/components/__tests__/ChatInput.test.js
+++ b/apps/frontend/components/__tests__/ChatInput.test.js
@@ -1,8 +1,12 @@
 import React from 'react';
 import { render, waitFor, fireEvent } from '@testing-library/react';
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, vi } from 'vitest';
 import ChatInput from '../ChatInput';
 
+vi.mock('../EmojiPicker', () => ({
+  default: () => React.createElement('em-emoji-picker'),
+}));
+
 describe('ChatInput', () => {
   it('renders the lazy emoji picker under React 19', async () => {
     const { container, getByLabelText } = render(
@@ -18,4 +22,26 @@ describe('ChatInput', () => {
       expect(container.querySelector('em-emoji-picker')).toBeInTheDocument();
     });
   });
+
+  it('waits for the socket-confirmed submit before clearing the message', async () => {
+    let confirmSubmit;
+    const onSubmit = vi.fn(() => new Promise((resolve) => { confirmSubmit = resolve; }));
+    const { getByTestId } = render(
+      <ChatInput
+        fileInputRef={{ current: null }}
+        room={{ participants: [] }}
+        onSubmit={onSubmit}
+      />
+    );
+
+    fireEvent.change(getByTestId('chat-message-input'), { target: { value: 'hello' } });
+    fireEvent.click(getByTestId('chat-send-button'));
+
+    await waitFor(() => expect(getByTestId('message-submission-status')).toHaveTextContent('waiting-for-message'));
+    expect(getByTestId('chat-message-input')).toHaveValue('hello');
+
+    confirmSubmit();
+    await waitFor(() => expect(getByTestId('message-submission-status')).toHaveTextContent('complete'));
+    expect(getByTestId('chat-message-input')).toHaveValue('');
+  });
 });
diff --git a/apps/frontend/features/chat/room/ChatRoomView.js b/apps/frontend/features/chat/room/ChatRoomView.js
index 75401bf9..4ab892c8 100644
--- a/apps/frontend/features/chat/room/ChatRoomView.js
+++ b/apps/frontend/features/chat/room/ChatRoomView.js
@@ -4,9 +4,9 @@ import {
   Button,
   Text,
   Callout,
-  Card,
   Spinner,
   Flex,
+  Box,
 } from '@vapor-ui/core';
 import { ErrorCircleOutlineIcon } from '@vapor-ui/icons';
 import { useChatRoom } from './useChatRoom';
@@ -35,52 +35,6 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
     handleLoadMore,
   } = useChatRoom({ roomId, onNavigate, onReplace, asPath });
 
-  const renderLoadingState = () => (
-    <div className="chat-container">
-      <Card.Root className="chat-room-card">
-        <Card.Body
-          style={{
-            display: 'flex',
-            alignItems: 'center',
-            justifyContent: 'center',
-          }}
-        >
-          <Flex
-            style={{ textAlign: 'center', marginTop: 'var(--vapor-space-500)' }}
-            $css={{ gap: '$100', alignItems: 'center' }}
-          >
-            <Spinner
-              size="lg"
-              colorPalette="primary"
-              aria-label="채팅방 연결 중"
-            />
-            <Text typography="heading5">채팅방 연결 중...</Text>
-          </Flex>
-        </Card.Body>
-      </Card.Root>
-    </div>
-  );
-
-  const renderErrorState = () => (
-    <div className="chat-container">
-      <Card.Root className="chat-room-card">
-        <Card.Body
-          style={{
-            display: 'flex',
-            flexDirection: 'column',
-            alignItems: 'center',
-            justifyContent: 'center',
-          }}
-        >
-          <ConnectionErrorBanner
-            message={error || '채팅방을 불러오는데 실패했습니다.'}
-          />
-          <Button onClick={() => window.location.reload()}>다시 시도</Button>
-        </Card.Body>
-      </Card.Root>
-    </div>
-  );
-
   const renderContent = () => {
     if (loading) {
       return (
@@ -145,16 +99,9 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
     );
   };
 
-  if (error) {
-    return renderErrorState();
-  }
-
-  if (loading || !room) {
-    return renderLoadingState();
-  }
-
   return (
     <VStack
+      data-testid="chat-room-layout"
       $css={{
         gap: '$0',
         height: 'calc(100vh - 80px)',
@@ -162,8 +109,9 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
         backgroundColor: 'var(--vapor-color-surface-normal)',
       }}
     >
-      {/* 채팅방 정보 (참여자 목록 및 연결 상태) */}
-      <ChatRoomInfo room={room} connectionStatus={connectionStatus} />
+      <Box data-testid="chat-room-header-slot" $css={{ minHeight: '64px', width: '100%' }}>
+        {room ? <ChatRoomInfo room={room} connectionStatus={connectionStatus} /> : null}
+      </Box>
 
       {/* 메시지 영역 */}
       <VStack
@@ -173,16 +121,29 @@ const ChatRoomView = ({ roomId, onNavigate, onReplace, asPath }) => {
           minHeight: '0',
         }}
       >
-        {renderContent()}
+        {error ? (
+          <Flex $css={{ height: '100%', alignItems: 'center', justifyContent: 'center', flexDirection: 'column', gap: '$200' }}>
+            <ConnectionErrorBanner message={error || '채팅방을 불러오는데 실패했습니다.'} />
+            <Button onClick={() => window.location.reload()}>다시 시도</Button>
+          </Flex>
+        ) : loading || !room ? (
+          <Flex $css={{ height: '100%', alignItems: 'center', justifyContent: 'center', gap: '$100' }}>
+            <Spinner size="lg" colorPalette="primary" aria-label="채팅방 연결 중" />
+            <Text typography="heading5">채팅방 연결 중...</Text>
+          </Flex>
+        ) : renderContent()}
       </VStack>
 
-      {/* 입력 영역 */}
-      <ChatInput
-        onSubmit={handleMessageSubmit}
-        fileInputRef={fileInputRef}
-        disabled={connectionStatus !== 'connected'}
-        room={room}
-      />
+      <Box data-testid="chat-room-input-slot" $css={{ minHeight: '72px', width: '100%' }}>
+        {room ? (
+          <ChatInput
+            onSubmit={handleMessageSubmit}
+            fileInputRef={fileInputRef}
+            disabled={connectionStatus !== 'connected'}
+            room={room}
+          />
+        ) : null}
+      </Box>
     </VStack>
   );
 };
diff --git a/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js b/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js
index 2b40e292..01a87bf2 100644
--- a/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js
+++ b/apps/frontend/features/chat/room/__tests__/ChatRoomView.test.js
@@ -64,5 +64,7 @@ describe('ChatRoomView', () => {
     expect(screen.getByText('chat messages')).toBeInTheDocument();
     expect(screen.getByText('room info: disconnected')).toBeInTheDocument();
     expect(screen.queryByText(/연결이 끊어졌습니다/)).not.toBeInTheDocument();
+    expect(screen.getByTestId('chat-room-header-slot')).toBeInTheDocument();
+    expect(screen.getByTestId('chat-room-input-slot')).toBeInTheDocument();
   });
 });
diff --git a/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js b/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
index ab6434b0..9876e7d3 100644
--- a/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
+++ b/apps/frontend/features/chat/room/__tests__/useMessageHandling.test.js
@@ -65,9 +65,9 @@ describe('useMessageHandling', () => {
       useMessageHandling(currentUser, roomId, vi.fn())
     );
 
-    await act(async () => {
+    await expect(act(async () => {
       await result.current.handleMessageSubmit({ content: 'hello' });
-    });
+    })).rejects.toThrow('채팅 서버와 연결이 끊어졌습니다.');
 
     expect(socketClient.sendChatMessageAndWait).not.toHaveBeenCalled();
     expect(Toast.error).toHaveBeenCalledWith('채팅 서버와 연결이 끊어졌습니다.');
diff --git a/apps/frontend/features/chat/room/useMessageHandling.js b/apps/frontend/features/chat/room/useMessageHandling.js
index d9d8769d..a4ec6654 100644
--- a/apps/frontend/features/chat/room/useMessageHandling.js
+++ b/apps/frontend/features/chat/room/useMessageHandling.js
@@ -69,12 +69,12 @@ export const useMessageHandling = (
    const roomSocket = getRoomSocket();
    if (!canSendOnRoomSocket() || !currentUser) {
      Toast.error('채팅 서버와 연결이 끊어졌습니다.');
-     return;
+     throw new Error('채팅 서버와 연결이 끊어졌습니다.');
    }
 
    if (!roomId) {
      Toast.error('채팅방 정보를 찾을 수 없습니다.');
-     return;
+     throw new Error('채팅방 정보를 찾을 수 없습니다.');
    }
 
    try {
@@ -112,7 +112,7 @@ export const useMessageHandling = (
          error.message?.includes('인증') ||
          error.message?.includes('토큰')) {
        await handleSessionError();
-       return;
+       throw error;
      }
 
      // 서버가 거부한 메시지는 onError 핸들러가 이미 토스트로 알렸다.
@@ -124,6 +124,7 @@ export const useMessageHandling = (
        setUploadError(error.message);
        setUploading(false);
      }
+     throw error;
    }
  }, [currentUser, roomId, handleSessionError, uploadChatFile, resetFileUpload, setUploadError, setUploading, canSendOnRoomSocket, getRoomSocket]);
 
diff --git a/apps/frontend/features/chat/rooms/ChatRoomsView.js b/apps/frontend/features/chat/rooms/ChatRoomsView.js
index 77c624d6..c7f9416a 100644
--- a/apps/frontend/features/chat/rooms/ChatRoomsView.js
+++ b/apps/frontend/features/chat/rooms/ChatRoomsView.js
@@ -222,10 +222,11 @@ export default function ChatRoomsView({ router }) {
           </Callout.Root>
         )}
 
+        <Box data-testid="rooms-content-slot" $css={{ minHeight: '430px', width: '100%' }}>
         {connectionStatus === CONNECTION_STATUS.ERROR ? (
           <ConnectionErrorBanner message="채팅 서버와 연결할 수 없습니다. 잠시 후 다시 시도해주세요." />
         ) : loading ? (
-          <Box $css={{ padding: '$400' }}>
+          <Box $css={{ minHeight: '430px', display: 'flex', alignItems: 'center', justifyContent: 'center' }}>
             <LoadingIndicator text="채팅방 목록을 불러오는 중..." />
           </Box>
         ) : rooms.length > 0 ? (
@@ -236,7 +237,7 @@ export default function ChatRoomsView({ router }) {
           />
         ) : !error && (
           <VStack
-            $css={{ gap: '$300', alignItems: 'center', padding: '$400' }}
+            $css={{ gap: '$300', alignItems: 'center', justifyContent: 'center', minHeight: '430px', padding: '$400' }}
             data-testid="rooms-empty"
           >
             <Text typography="body1">생성된 채팅방이 없습니다.</Text>
@@ -249,6 +250,7 @@ export default function ChatRoomsView({ router }) {
             </Button>
           </VStack>
         )}
+        </Box>
       </VStack>
     </Box>
   );
diff --git a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
index 204be521..0d1d71ef 100644
--- a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
@@ -143,4 +143,10 @@ describe('ChatRoomsView', () => {
 
     expect(screen.queryByTestId('refresh-rooms-button')).toBeNull();
   });
+
+  it('always reserves a stable slot for room-list states', () => {
+    render(<ChatRoomsView router={{ push: vi.fn() }} />);
+
+    expect(screen.getByTestId('rooms-content-slot')).toBeInTheDocument();
+  });
 });
diff --git a/apps/frontend/next.config.js b/apps/frontend/next.config.js
index 994749dd..66f2b0c8 100644
--- a/apps/frontend/next.config.js
+++ b/apps/frontend/next.config.js
@@ -24,7 +24,16 @@ const nextConfig = {
   // Docker 빌드를 위한 standalone 출력 모드 (개발 환경에는 영향 없음)
   output: 'standalone',
   // monorepo에서 standalone 빌드 시 중첩 경로 방지
-  outputFileTracingRoot: workspaceRoot
+  outputFileTracingRoot: workspaceRoot,
+  async redirects() {
+    return [
+      {
+        source: '/login',
+        destination: '/',
+        permanent: false,
+      },
+    ];
+  }
 };
 
 module.exports = nextConfig;
diff --git a/apps/frontend/pages/chat/new.js b/apps/frontend/pages/chat/new.js
index 9cbbdda9..c9914289 100644
--- a/apps/frontend/pages/chat/new.js
+++ b/apps/frontend/pages/chat/new.js
@@ -25,13 +25,16 @@ function NewChatRoom() {
     password: ''
   });
   const [loading, setLoading] = useState(false);
+  const [submissionStage, setSubmissionStage] = useState('idle');
   const [error, setError] = useState('');
 
   const joinRoom = async (roomId, password) => {
     try {
+      setSubmissionStage('joining');
       await api.post(`/api/rooms/${roomId}/join`, { password });
 
-      router.push(`/chat/${roomId}`);
+      setSubmissionStage('navigating');
+      await router.push(`/chat/${roomId}`);
     } catch (error) {
       console.error('Room join error:', error);
       throw error;
@@ -58,6 +61,7 @@ function NewChatRoom() {
 
     try {
       setLoading(true);
+      setSubmissionStage('creating');
       setError('');
 
       const response = await api.post('/api/rooms', {
@@ -71,6 +75,7 @@ function NewChatRoom() {
     } catch (error) {
       console.error('Room creation/join error:', error);
       setError(error.message);
+      setSubmissionStage('failed');
     } finally {
       setLoading(false);
     }
@@ -99,6 +104,16 @@ function NewChatRoom() {
       >
         <Text typography="heading4">새 채팅방</Text>
 
+        <Text typography="body2" role="status" data-testid="room-creation-status">
+          {{
+            idle: '',
+            creating: '채팅방을 생성하고 있습니다.',
+            joining: '생성된 채팅방에 입장하고 있습니다.',
+            navigating: '채팅방 화면을 여는 중입니다.',
+            failed: '채팅방 생성 또는 입장에 실패했습니다.',
+          }[submissionStage]}
+        </Text>
+
         {error && (
           <Callout.Root colorPalette="danger">
             <HStack $css={{ gap: '$200', alignItems: 'center' }}>
diff --git a/apps/frontend/pages/index.js b/apps/frontend/pages/index.js
index bd1f05aa..76a2cf06 100644
--- a/apps/frontend/pages/index.js
+++ b/apps/frontend/pages/index.js
@@ -81,12 +81,6 @@ const Login = () => {
   const handleSubmit = async (e) => {
     e.preventDefault();
 
-    // 서버 연결 상태 확인 — 로그인을 시도조차 못 한 것이므로 서버 안내로 알린다.
-    if (!serverStatus.connected) {
-      setServerNotice('서버와 연결할 수 없습니다. 인터넷 연결을 확인하고 잠시 후 다시 시도해주세요.');
-      return;
-    }
-
     // 폼 유효성 검사
     if (!validateForm()) {
       return;
@@ -116,29 +110,6 @@ const Login = () => {
     }
   };
 
-  if (serverStatus.checking) {
-    return (
-      <div className="min-h-screen flex items-center justify-center p-[var(--vapor-space-300)] bg-[var(--vapor-color-background)]">
-        <VStack
-          $css={{
-            gap: '$250',
-            width: '400px',
-            padding: '$300',
-            borderRadius: '$300',
-            border: '1px solid var(--vapor-color-border-normal)',
-          }}
-        >
-          <div className="text-center mb-[2rem]">
-            <img src="images/logo-h.png" className="w-1/2 mx-auto" alt="KTB Chat 로고" />
-          </div>
-          <div className="text-center">
-            <Text typography="body1">서버 연결 확인 중...</Text>
-          </div>
-        </VStack>
-      </div>
-    );
-  }
-
   return (
     <div className="min-h-screen flex items-center justify-center p-(--vapor-space-300) bg-(--vapor-color-background)">
       <VStack
@@ -152,9 +123,21 @@ const Login = () => {
         render={<Form onSubmit={handleSubmit} />}
       >
         <div className="text-center mb-4">
-          <img src="images/logo-h.png" className="w-1/2 mx-auto" alt="KTB Chat 로고" />
+          <img
+            src="/images/logo-h.png"
+            width="240"
+            height="80"
+            className="w-1/2 h-auto mx-auto"
+            alt="KTB Chat 로고"
+          />
         </div>
 
+        {serverStatus.checking && (
+          <Text typography="body2" foreground="hint-100" role="status" data-testid="server-status-checking">
+            서버 연결을 백그라운드에서 확인하고 있습니다.
+          </Text>
+        )}
+
         {serverNotice && (
           <Callout.Root colorPalette="warning" data-testid="server-status-message">
             <Callout.Icon>
@@ -224,7 +207,7 @@ const Login = () => {
           <Button
             type="submit"
             size="lg"
-            disabled={loading || !serverStatus.connected}
+            disabled={loading}
             data-testid="login-submit-button"
           >
             {loading ? '로그인 중...' : '로그인'}
@@ -238,7 +221,7 @@ const Login = () => {
             size="sm"
             variant="ghost"
             onClick={() => router.push('/register')}
-            disabled={loading || !serverStatus.connected}
+            disabled={loading}
           >
             회원가입
           </Button>
diff --git a/apps/frontend/pages/register.js b/apps/frontend/pages/register.js
index 57f36a2c..00e33a5a 100644
--- a/apps/frontend/pages/register.js
+++ b/apps/frontend/pages/register.js
@@ -77,7 +77,7 @@ const Register = () => {
         render={<Form onSubmit={handleSubmit} />}
       >
         <div className="text-center mb-4">
-          <img src="images/logo-h.png" className="w-1/2 mx-auto" alt="KTB Chat 로고" />
+          <img src="/images/logo-h.png" width="240" height="80" className="w-1/2 h-auto mx-auto" alt="KTB Chat 로고" />
         </div>
 
         {error && (
diff --git a/apps/frontend/test/pages/index.test.js b/apps/frontend/test/pages/index.test.js
new file mode 100644
index 00000000..6af33ed1
--- /dev/null
+++ b/apps/frontend/test/pages/index.test.js
@@ -0,0 +1,30 @@
+import React from 'react';
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+import LoginPage from '../../pages/index';
+
+const mocks = vi.hoisted(() => ({
+  login: vi.fn(),
+  checkServerConnection: vi.fn(() => new Promise(() => {})),
+  router: { query: {}, push: vi.fn(), replace: vi.fn(), isReady: true },
+}));
+
+vi.mock('next/router', () => ({ useRouter: () => mocks.router }));
+vi.mock('@/contexts/AuthContext', () => ({
+  useAuth: () => ({ login: mocks.login }),
+  withoutAuth: (Component) => Component,
+}));
+vi.mock('@/services/authService', () => ({
+  default: { checkServerConnection: mocks.checkServerConnection },
+}));
+
+describe('Login page', () => {
+  it('keeps the login form usable while the health check is pending', () => {
+    render(<LoginPage />);
+
+    expect(screen.getByTestId('server-status-checking')).toBeInTheDocument();
+    expect(screen.getByTestId('login-email-input')).toBeEnabled();
+    expect(screen.getByTestId('login-password-input')).toBeEnabled();
+    expect(screen.getByTestId('login-submit-button')).toBeEnabled();
+  });
+});
diff --git a/e2e/actions/auth.actions.js b/e2e/actions/auth.actions.js
index 074754c2..3b05742b 100644
--- a/e2e/actions/auth.actions.js
+++ b/e2e/actions/auth.actions.js
@@ -6,12 +6,20 @@ const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
  * @param {Object} credentials - { email: string, password: string }
  * @param waitForRedirect
  */
-async function loginAction(page, credentials, waitForRedirect = true) {
+async function openLoginAction(page) {
   await page.goto(`${BASE_URL}/login`);
+  const emailInput = page.getByTestId('login-email-input');
+  await emailInput.waitFor({ state: 'visible' });
+  if (!await emailInput.isEnabled()) {
+    throw new Error('로그인 입력 필드가 활성화되지 않았습니다.');
+  }
+}
+
+async function loginAction(page, credentials, waitForRedirect = true, navigate = true) {
+  if (navigate) await openLoginAction(page);
   await page.getByTestId('login-email-input').fill(credentials.email);
   await page.getByTestId('login-password-input').fill(credentials.password);
   await page.getByTestId('login-submit-button').click();
-  await page.waitForTimeout(1000); // 잠시 대기
   if (waitForRedirect) {
     await page.waitForURL(`${BASE_URL}/chat`);
   }
@@ -32,7 +40,7 @@ async function registerAction(page, userData) {
   // 성공 시 앱이 1000ms 뒤 router.push('/login')로 이동하므로, 그 내비게이션이 끝난 뒤에
   // 다음 goto가 실행되어야 경합(net::ERR_ABORTED)이 없다. 실패 시엔 에러 메시지 표시까지 대기.
   await Promise.race([
-    page.waitForURL(`${BASE_URL}/login`, { timeout: 10000 }).catch(() => {}),
+    page.waitForURL(`${BASE_URL}/`, { timeout: 10000 }).catch(() => {}),
     page.getByTestId('register-error-message').waitFor({ state: 'visible', timeout: 10000 }).catch(() => {}),
   ]);
 }
@@ -47,6 +55,7 @@ async function logoutAction(page) {
 
 module.exports = {
   loginAction,
+  openLoginAction,
   registerAction,
   logoutAction,
 };
diff --git a/e2e/actions/chat.actions.js b/e2e/actions/chat.actions.js
index 7ff1d470..52411350 100644
--- a/e2e/actions/chat.actions.js
+++ b/e2e/actions/chat.actions.js
@@ -54,9 +54,12 @@ async function createChatRoomAction(page, roomName) {
  * @param {import('@playwright/test').Page} page
  * @param {string} message - 전송할 메시지 내용
  */
-async function sendMessageAction(page, message) {
+async function sendMessageAction(page, message, { expectFailure = false } = {}) {
   await page.getByTestId('chat-message-input').fill(message);
   await page.getByTestId('chat-send-button').click();
+  await page.getByTestId('message-submission-status').filter({
+    hasText: expectFailure ? 'failed' : 'complete',
+  }).waitFor({ state: 'visible', timeout: 15000 });
 }
 
 /**
diff --git a/e2e/artillery/aggregate-observations.js b/e2e/artillery/aggregate-observations.js
index 3188f00a..3304c7f4 100644
--- a/e2e/artillery/aggregate-observations.js
+++ b/e2e/artillery/aggregate-observations.js
@@ -12,11 +12,13 @@ const reports = runDirectories.flatMap((runDirectory) => fs.readdirSync(runDirec
     .filter((file) => file.startsWith('vu-') && file.endsWith('.json'))
     .map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8'))));
 
-const samples = { actions: [], http: [], socket: [] };
+const samples = { actions: [], documents: [], http: [], socket: [], layoutShifts: [] };
 for (const report of reports) {
     samples.actions.push(...report.samples.actions);
+    samples.documents.push(...(report.samples.documents || []));
     samples.http.push(...report.samples.http);
     samples.socket.push(...report.samples.socket);
+    samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
 
 const timestamps = Object.values(samples).flat()
@@ -26,6 +28,10 @@ const timestamps = Object.values(samples).flat()
 const rankedHttp = summarizeSamples(samples.http.filter((sample) =>
     sample.name !== 'GET /api/health' &&
     !(sample.name === 'POST /api/auth/login' && sample.status === 401)));
+const expectedPerRun = Number(process.env.EXPECTED_VUS || process.env.PHASE1_ARRIVAL_COUNT || 0);
+const expectedVirtualUsers = expectedPerRun > 0 ? expectedPerRun * runDirectories.length : null;
+const completedVirtualUsers = reports.filter((report) => report.samples.actions.length > 0 &&
+    report.samples.actions.every((sample) => sample.success)).length;
 
 const aggregate = {
     schemaVersion: 1,
@@ -39,13 +45,22 @@ const aggregate = {
             report.samples.actions.every((sample) => sample.success)).length,
         failed: reports.filter((report) => report.samples.actions.some((sample) => !sample.success)).length,
         allSamplesHaveWindowsAndNames: Object.values(samples).flat().every((sample) =>
-            Boolean(sample.name && sample.startedAt && sample.endedAt)),
+            sample.value !== undefined || Boolean(sample.name && sample.startedAt && sample.endedAt)),
+        expectedVirtualUsers,
+        targetReached: expectedVirtualUsers === null ? null : reports.length === expectedVirtualUsers,
+        adoptable: expectedVirtualUsers === null ? false :
+            reports.length === expectedVirtualUsers && completedVirtualUsers / expectedVirtualUsers >= 0.99,
     },
     summary: {
         actions: summarizeSamples(samples.actions),
+        documents: summarizeSamples(samples.documents),
         httpDiagnostic: summarizeSamples(samples.http),
         httpRanked: rankedHttp,
         socket: summarizeSamples(samples.socket),
+        cls: {
+            total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
+            entries: samples.layoutShifts.length,
+        },
     },
     samples,
 };
diff --git a/e2e/artillery/observation.js b/e2e/artillery/observation.js
index de15b9a3..e3d19464 100644
--- a/e2e/artillery/observation.js
+++ b/e2e/artillery/observation.js
@@ -38,6 +38,13 @@ function normalizeApiPath(rawUrl) {
     }).join('/')}`;
 }
 
+function normalizeDocumentPath(rawUrl) {
+    const url = new URL(rawUrl, 'http://localhost');
+    if (url.pathname === '/' || url.pathname === '/login') return url.pathname;
+    if (/^\/chat\/[a-f\d]{24}$/i.test(url.pathname)) return '/chat/{roomId}';
+    return url.pathname;
+}
+
 function parseSocketEvent(payload) {
     const text = Buffer.isBuffer(payload) ? payload.toString('utf8') : String(payload);
     const arrayStart = text.indexOf('[');
@@ -80,7 +87,7 @@ function summarizeSamples(samples) {
 function createObservation(page, vuContext) {
     const startedRequests = new Map();
     const pendingSocketEvents = new Map();
-    const samples = { http: [], socket: [], actions: [] };
+    const samples = { documents: [], http: [], socket: [], actions: [], layoutShifts: [] };
     let currentAction = null;
     let socketSequence = 0;
 
@@ -108,6 +115,21 @@ function createObservation(page, vuContext) {
     };
     const onRequestFinished = (request) => { void recordHttp(request, true); };
     const onRequestFailed = (request) => { void recordHttp(request, false); };
+    const onResponse = (response) => {
+        const request = response.request();
+        if (request.resourceType() !== 'document') return;
+        const timing = request.timing();
+        const ttfbMs = timing?.responseStart;
+        samples.documents.push({
+            name: `GET ${normalizeDocumentPath(response.url())}`,
+            durationMs: Number.isFinite(ttfbMs) && ttfbMs >= 0 ? ttfbMs : 0,
+            startedAt: new Date(Date.now() - Math.max(ttfbMs || 0, 0)).toISOString(),
+            endedAt: new Date().toISOString(),
+            success: response.status() < 400,
+            status: response.status(),
+            redirectedFrom: request.redirectedFrom()?.url() || null,
+        });
+    };
     const onWebSocket = (webSocket) => {
         const connectionKey = `connection:${socketSequence += 1}`;
         pendingSocketEvents.set(connectionKey, {
@@ -151,8 +173,66 @@ function createObservation(page, vuContext) {
     page.on('request', onRequest);
     page.on('requestfinished', onRequestFinished);
     page.on('requestfailed', onRequestFailed);
+    page.on('response', onResponse);
     page.on('websocket', onWebSocket);
 
+    void page.addInitScript(() => {
+        window.__e2eLayoutShifts = [];
+        const selectorFor = (node) => {
+            if (!node || node.nodeType !== Node.ELEMENT_NODE) return null;
+            if (node.id) return `#${node.id}`;
+            const testId = node.getAttribute('data-testid');
+            if (testId) return `[data-testid="${testId}"]`;
+            return node.tagName.toLowerCase() + (node.classList.length ? `.${[...node.classList].slice(0, 2).join('.')}` : '');
+        };
+        new PerformanceObserver((list) => {
+            for (const entry of list.getEntries()) {
+                if (entry.hadRecentInput) continue;
+                window.__e2eLayoutShifts.push({
+                    value: entry.value,
+                    atMs: entry.startTime,
+                    sources: (entry.sources || []).map((source) => ({
+                        selector: selectorFor(source.node),
+                        previousRect: source.previousRect,
+                        currentRect: source.currentRect,
+                        image: source.node?.tagName === 'IMG' ? {
+                            src: source.node.currentSrc || source.node.src,
+                            naturalWidth: source.node.naturalWidth,
+                            naturalHeight: source.node.naturalHeight,
+                            renderedWidth: source.node.clientWidth,
+                            renderedHeight: source.node.clientHeight,
+                        } : null,
+                    })),
+                });
+            }
+        }).observe({ type: 'layout-shift', buffered: true });
+    });
+
+    async function collectPageDiagnostics() {
+        const selectors = [
+            '[data-testid="login-email-input"]', '[data-testid="login-submit-button"]',
+            '[data-testid="chat-room-name-input"]', '[data-testid="chat-message-input"]',
+            '[data-testid="message-submission-status"]', '[data-testid="file-message-container"]',
+        ];
+        const ui = {};
+        for (const selector of selectors) {
+            const locator = page.locator(selector).first();
+            ui[selector] = {
+                count: await locator.count().catch(() => 0),
+                visible: await locator.isVisible().catch(() => false),
+                enabled: await locator.isEnabled().catch(() => false),
+                text: await locator.textContent().catch(() => null),
+            };
+        }
+        return {
+            url: page.url(),
+            ui,
+            recentHttp: samples.http.slice(-10).map(({ name, status, success, action, startedAt, endedAt }) =>
+                ({ name, status, success, action, startedAt, endedAt })),
+            pendingSocketEvents: [...pendingSocketEvents.keys()].map(String),
+        };
+    }
+
     async function action(name, callback) {
         const previousAction = currentAction;
         currentAction = name;
@@ -169,6 +249,8 @@ function createObservation(page, vuContext) {
             samples.actions.push({
                 name, durationMs: performance.now() - startedAt, startedAt: wallStartedAt,
                 endedAt: new Date().toISOString(), success: false,
+                error: { name: error.name, message: error.message },
+                diagnostics: await collectPageDiagnostics(),
             });
             throw error;
         } finally {
@@ -181,8 +263,11 @@ function createObservation(page, vuContext) {
         page.off('request', onRequest);
         page.off('requestfinished', onRequestFinished);
         page.off('requestfailed', onRequestFailed);
+        page.off('response', onResponse);
         page.off('websocket', onWebSocket);
 
+        samples.layoutShifts = await page.evaluate(() => window.__e2eLayoutShifts || []).catch(() => []);
+
         const runId = process.env.OBSERVATION_RUN_ID || new Date().toISOString().replace(/[:.]/g, '-');
         const outputRoot = process.env.OBSERVATION_OUTPUT_DIR || path.resolve(__dirname, 'results');
         const runDirectory = path.join(outputRoot, runId);
@@ -195,8 +280,13 @@ function createObservation(page, vuContext) {
             samples,
             summary: {
                 actions: summarizeSamples(samples.actions),
+                documents: summarizeSamples(samples.documents),
                 http: summarizeSamples(samples.http),
                 socket: summarizeSamples(samples.socket),
+                cls: {
+                    total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
+                    entries: samples.layoutShifts.length,
+                },
             },
         };
         const filename = `vu-${process.pid}-${randomUUID()}.json`;
@@ -217,6 +307,7 @@ function printTable(title, rows) {
 
 function printSummary(summary) {
     printTable('User actions', summary.actions);
+    printTable('Next.js documents (TTFB)', summary.documents);
     printTable('HTTP API (ranked by cumulative response time)', summary.http);
     printTable('Socket.IO actions', summary.socket);
 }
@@ -224,6 +315,7 @@ function printSummary(summary) {
 module.exports = {
     createObservation,
     normalizeApiPath,
+    normalizeDocumentPath,
     parseSocketEvent,
     summarizeSamples,
 };
diff --git a/e2e/artillery/observation.test.js b/e2e/artillery/observation.test.js
index 320ecdd9..9d880f9a 100644
--- a/e2e/artillery/observation.test.js
+++ b/e2e/artillery/observation.test.js
@@ -1,6 +1,6 @@
 const test = require('node:test');
 const assert = require('node:assert/strict');
-const { normalizeApiPath, parseSocketEvent, summarizeSamples } = require('./observation');
+const { normalizeApiPath, normalizeDocumentPath, parseSocketEvent, summarizeSamples } = require('./observation');
 
 test('normalizes dynamic API identifiers without changing static routes', () => {
     assert.equal(normalizeApiPath('http://localhost:5001/api/rooms'), '/api/rooms');
@@ -10,6 +10,12 @@ test('normalizes dynamic API identifiers without changing static routes', () =>
     assert.equal(normalizeApiPath('http://localhost:3000/_next/static/a.js'), null);
 });
 
+test('keeps Next document paths separate from API paths', () => {
+    assert.equal(normalizeDocumentPath('http://localhost:3000/'), '/');
+    assert.equal(normalizeDocumentPath('http://localhost:3000/login?redirect=/chat'), '/login');
+    assert.equal(normalizeDocumentPath('http://localhost:3000/chat/507f1f77bcf86cd799439011'), '/chat/{roomId}');
+});
+
 test('extracts Socket.IO event names from websocket frames', () => {
     assert.equal(parseSocketEvent('42["joinRoom","room-1"]'), 'joinRoom');
     assert.equal(parseSocketEvent('451-["message",{"content":"hello"}]'), 'message');
diff --git a/e2e/artillery/report-observations.js b/e2e/artillery/report-observations.js
index f3c1b652..e754f3a5 100644
--- a/e2e/artillery/report-observations.js
+++ b/e2e/artillery/report-observations.js
@@ -8,11 +8,13 @@ const files = fs.readdirSync(runDirectory).filter((file) => file.startsWith('vu-
 if (files.length === 0) throw new Error(`No observation JSON files found in ${runDirectory}`);
 
 const reports = files.map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8')));
-const samples = { actions: [], http: [], socket: [] };
+const samples = { actions: [], documents: [], http: [], socket: [], layoutShifts: [] };
 for (const report of reports) {
     samples.actions.push(...report.samples.actions);
+    samples.documents.push(...(report.samples.documents || []));
     samples.http.push(...report.samples.http);
     samples.socket.push(...report.samples.socket);
+    samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
 
 const aggregate = {
@@ -22,8 +24,13 @@ const aggregate = {
     virtualUsers: reports.length,
     summary: {
         actions: summarizeSamples(samples.actions),
+        documents: summarizeSamples(samples.documents),
         http: summarizeSamples(samples.http),
         socket: summarizeSamples(samples.socket),
+        cls: {
+            total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
+            entries: samples.layoutShifts.length,
+        },
     },
 };
 fs.writeFileSync(path.join(runDirectory, 'summary.json'), `${JSON.stringify(aggregate, null, 2)}\n`);
diff --git a/e2e/artillery/scenarios/auth.scenario.js b/e2e/artillery/scenarios/auth.scenario.js
index 577eb006..e7a70bfd 100644
--- a/e2e/artillery/scenarios/auth.scenario.js
+++ b/e2e/artillery/scenarios/auth.scenario.js
@@ -1,4 +1,4 @@
-const { loginAction, registerAction, logoutAction } = require('../../actions/auth.actions');
+const { loginAction, openLoginAction, registerAction, logoutAction } = require('../../actions/auth.actions');
 const { expect } = require('@playwright/test');
 
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
@@ -21,7 +21,8 @@ async function loginScenario(page, vuContext) {
         await page.waitForTimeout(ACTION_TIMEOUT);
 
         // 2. 로그인
-        await observe('login', () => loginAction(page, testUser));
+        await observe('login_form_ready', () => openLoginAction(page));
+        await observe('login', () => loginAction(page, testUser, true, false));
         await expect(page).toHaveURL(`${BASE_URL}/chat`);
 
         // URL 만으로는 완주를 판정할 수 없다. 방 목록이 실제로 그려져야
@@ -46,10 +47,11 @@ async function loginScenario(page, vuContext) {
 async function failedLoginScenario(page, vuContext) {
     const observe = vuContext.vars.observation.action;
     try {
+        await observe('login_form_ready_for_failed_login', () => openLoginAction(page));
         await observe('failed_login', () => loginAction(page, {
             email: 'nonexistent@example.com',
             password: 'WrongPassword123!',
-        }, false));
+        }, false, false));
 
         await page.waitForTimeout(500);
         const errorElement = page.getByTestId('login-error-message');
diff --git a/e2e/artillery/scenarios/chat.scenario.js b/e2e/artillery/scenarios/chat.scenario.js
index f78a1664..d8021931 100644
--- a/e2e/artillery/scenarios/chat.scenario.js
+++ b/e2e/artillery/scenarios/chat.scenario.js
@@ -13,11 +13,6 @@ const { randomUUID } = require('crypto');
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
 const MASS_MESSAGE_COUNT = process.env.MASS_MESSAGE_COUNT || 10;
 
-// Action 간 timeout 설정 (환경변수로 조절 가능)
-const ACTION_TIMEOUT = parseInt(process.env.ACTION_TIMEOUT || '1000', 10);
-const ACTION_TIMEOUT_SHORT = parseInt(process.env.ACTION_TIMEOUT_SHORT || '500', 10);
-const ACTION_TIMEOUT_LONG = parseInt(process.env.ACTION_TIMEOUT_LONG || '2000', 10);
-
 async function gotoChatPage(page, vuContext) {
     await page.goto(`${BASE_URL}/chat`);
     await expect(page).toHaveURL(`${BASE_URL}/chat`);
@@ -46,7 +41,6 @@ async function chatRoomCreationScenario(page, vuContext) {
         const message = `테스트 메시지 ${bannedWordSafeText(Date.now())}`;
         await observe('message_send', async () => {
             await sendMessageAction(page, message);
-            await page.waitForTimeout(ACTION_TIMEOUT_SHORT);
             const messageElement = page.getByTestId('message-content').filter({ hasText: message });
             await expect(messageElement).toBeVisible();
         });
@@ -108,7 +102,6 @@ async function fileUploadScenario(page, vuContext) {
         await observe('file_upload', async () => {
             await uploadFileAction(page, filePath, message);
             await uploadPromise;
-            await page.waitForTimeout(ACTION_TIMEOUT);
             const fileMessageContainer = page.getByTestId('file-message-container').filter({ hasText: message });
             await expect(fileMessageContainer).toBeVisible({ timeout: 10000 });
         });
@@ -143,7 +136,7 @@ async function forbiddenWordScenario(page, vuContext) {
         // 2. 금칙어 메시지 전송 시도
         const forbiddenWord = FORBIDDEN_WORDS[Math.floor(Math.random() * FORBIDDEN_WORDS.length)];
         await observe('forbidden_message_send', async () => {
-            await sendMessageAction(page, forbiddenWord);
+            await sendMessageAction(page, forbiddenWord, { expectFailure: true });
             const errorToast = page.getByTestId('toast-error');
             await expect(errorToast).toBeVisible({ timeout: 5000 });
             const sentMessage = page.getByTestId('message-content').filter({ hasText: forbiddenWord });

From 347b7e531a10a840da484e45a38d21fd5e0761fe Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 21:28:55 +0900
Subject: [PATCH 26/32] =?UTF-8?q?perf(mongo):=20=ED=8C=8C=EC=9D=BC=20?=
 =?UTF-8?q?=EC=B0=B8=EC=A1=B0=20=EC=A1=B0=ED=9A=8C=20=EC=9D=B8=EB=8D=B1?=
 =?UTF-8?q?=EC=8A=A4=20=EC=B6=94=EA=B0=80?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/backend/src/main/java/com/ktb/chatapp/model/File.java    | 4 +++-
 apps/backend/src/main/java/com/ktb/chatapp/model/Message.java | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/File.java b/apps/backend/src/main/java/com/ktb/chatapp/model/File.java
index e3780e9d..ed56a2bf 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/File.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/File.java
@@ -6,6 +6,7 @@
 import lombok.NoArgsConstructor;
 import org.springframework.data.annotation.CreatedDate;
 import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.index.Indexed;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
 
@@ -25,6 +26,7 @@ public class File {
     @Id
     private String id;
 
+    @Indexed(name = "filename_idx")
     private String filename;
 
     private String originalname;
@@ -78,4 +80,4 @@ public String getFileUrl(String type) {
             type,
             URLEncoder.encode(this.filename, StandardCharsets.UTF_8));
     }
-}
\ No newline at end of file
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java b/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
index dba4283c..a2fe71fb 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/Message.java
@@ -10,6 +10,7 @@
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.index.CompoundIndex;
+import org.springframework.data.mongodb.core.index.Indexed;
 
 import java.time.LocalDateTime;
 import java.util.ArrayList;
@@ -50,6 +51,7 @@ public class Message {
     private MessageType type;
 
     // Mongo 문서 필드명 "file" 사용
+    @Indexed(name = "file_idx")
     @Field("file")
     private String fileId;
 

From ed4b774a5e43bf0c4d4f28f61e1a23e4db7aad46 Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 21:34:51 +0900
Subject: [PATCH 27/32] =?UTF-8?q?build(frontend):=20webpack=20=EB=B9=8C?=
 =?UTF-8?q?=EB=93=9C=EB=A1=9C=20=EA=B3=A0=EC=A0=95?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/frontend/package.json | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/apps/frontend/package.json b/apps/frontend/package.json
index 914e133f..4511f932 100644
--- a/apps/frontend/package.json
+++ b/apps/frontend/package.json
@@ -5,12 +5,12 @@
   "private": true,
   "scripts": {
     "dev": "next dev",
-    "build": "next build",
+    "build": "next build --webpack",
     "start": "next start",
     "lint": "eslint .",
     "test": "vitest run --passWithNoTests",
     "format": "prettier . --write --ignore-path .prettierignore",
-    "build:production": "test -f .env.production || { echo '.env.production 이 없습니다. cp .env.production.example .env.production 후 배포 주소로 값을 채우세요.' >&2; exit 1; } && env $(grep -vE '^[[:space:]]*(#|$)' .env.production | xargs) next build && pnpm run deploy:static",
+    "build:production": "test -f .env.production || { echo '.env.production 이 없습니다. cp .env.production.example .env.production 후 배포 주소로 값을 채우세요.' >&2; exit 1; } && env $(grep -vE '^[[:space:]]*(#|$)' .env.production | xargs) next build --webpack && pnpm run deploy:static",
     "deploy:static": "cp -r .next/static .next/standalone/apps/frontend/.next/ && cp -r public .next/standalone/apps/frontend/"
   },
   "dependencies": {

From 869cdb20af245e3cd550d7f4b647940b3567c0e9 Mon Sep 17 00:00:00 2001
From: MINJU JEONG <108784922+jung-min-ju@users.noreply.github.com>
Date: Tue, 11 Aug 2026 23:26:26 +0900
Subject: [PATCH 28/32] perf(socket): batch room join critical path

---
 .../chatapp/repository/MessageRepository.java |   4 +-
 .../service/MessageReadStatusService.java     |  36 ++-
 .../socketio/handler/JoinAckDispatcher.java   | 144 +++++++++++
 .../handler/MessageFetchDispatcher.java       | 147 +++++++++++
 .../socketio/handler/MessageFetchHandler.java |  20 +-
 .../socketio/handler/MessageLoader.java       |  41 ++-
 .../handler/ReadStatusDispatcher.java         |  82 ++++++
 .../socketio/handler/RoomJoinHandler.java     | 102 ++------
 .../socketio/handler/RoomLeaveHandler.java    |  64 +----
 .../handler/RoomPostJoinDispatcher.java       | 237 ++++++++++++++++++
 .../src/main/resources/application.properties |  13 +
 .../service/MessageReadStatusServiceTest.java |  40 +++
 .../handler/JoinAckDispatcherTest.java        |  70 ++++++
 .../handler/MessageFetchDispatcherTest.java   |  60 +++++
 .../handler/MessageFetchHandlerTest.java      |  24 +-
 .../handler/MessageLoaderIntegrationTest.java |   3 +-
 .../socketio/handler/MessageLoaderTest.java   |  12 +-
 .../handler/ReadStatusDispatcherTest.java     |  38 +++
 .../socketio/handler/RoomJoinHandlerTest.java |  96 +++----
 .../handler/RoomLeaveHandlerTest.java         |  62 +----
 .../handler/RoomPostJoinDispatcherTest.java   |  72 ++++++
 loadtest/load-test.js                         |  97 ++++++-
 loadtest/ramp-up-test.js                      |  40 +--
 performance/room-join/fixture.js              |   2 +-
 24 files changed, 1130 insertions(+), 376 deletions(-)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcher.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcher.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcher.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcher.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/service/MessageReadStatusServiceTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcherTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcherTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcherTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcherTest.java

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/MessageRepository.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/MessageRepository.java
index 932ce8e8..300b1c82 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/repository/MessageRepository.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/MessageRepository.java
@@ -3,15 +3,15 @@
 import com.ktb.chatapp.model.Message;
 import java.time.LocalDateTime;
 import java.util.Optional;
-import org.springframework.data.domain.Page;
 import org.springframework.data.domain.Pageable;
+import org.springframework.data.domain.Slice;
 import org.springframework.data.mongodb.repository.MongoRepository;
 import org.springframework.data.mongodb.repository.Query;
 import org.springframework.stereotype.Repository;
 
 @Repository
 public interface MessageRepository extends MongoRepository<Message, String> {
-    Page<Message> findByRoomIdAndTimestampBefore(String roomId, LocalDateTime timestamp, Pageable pageable);
+    Slice<Message> findByRoomIdAndTimestampBefore(String roomId, LocalDateTime timestamp, Pageable pageable);
     /**
      * 특정 시간 이후의 메시지 수 카운트
      * 최근 N분간 메시지 수를 조회할 때 사용
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/MessageReadStatusService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/MessageReadStatusService.java
index 4dacf9cc..75d0408a 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/MessageReadStatusService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/MessageReadStatusService.java
@@ -1,13 +1,15 @@
 package com.ktb.chatapp.service;
 
 import com.ktb.chatapp.model.Message;
-import com.ktb.chatapp.repository.MessageRepository;
 import java.time.LocalDateTime;
-import java.util.ArrayList;
 import java.util.List;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.stereotype.Service;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 
 /**
  * 메시지 읽음 상태 관리 서비스
@@ -17,7 +19,7 @@
 @RequiredArgsConstructor
 public class MessageReadStatusService {
 
-    private final MessageRepository messageRepository;
+    private final MongoTemplate mongoTemplate;
 
     /**
      * 메시지 읽음 상태 업데이트
@@ -26,7 +28,7 @@ public class MessageReadStatusService {
      * @param userId 읽은 사용자 ID
      */
     public void updateReadStatus(List<String> messageIds, String userId) {
-        if (messageIds.isEmpty()) {
+        if (messageIds == null || messageIds.isEmpty() || userId == null) {
             return;
         }
 
@@ -36,23 +38,19 @@ public void updateReadStatus(List<String> messageIds, String userId) {
                 .build();
 
         try {
-            for (String messageId : messageIds) {
-                var messageOptional = messageRepository.findById(messageId);
-                if (messageOptional.isPresent()) {
-                    var message = messageOptional.get();
-                    if (message.getReaders() == null) {
-                        message.setReaders(new ArrayList<>());
-                    }
-                    boolean alreadyRead = message.getReaders().stream()
-                            .anyMatch(r -> r.getUserId().equals(userId));
-                    if (!alreadyRead) {
-                        message.getReaders().add(readerInfo);
-                    }
-                    messageRepository.save(message);
-                }
+            List<String> distinctMessageIds = messageIds.stream()
+                    .filter(java.util.Objects::nonNull)
+                    .distinct()
+                    .toList();
+            if (distinctMessageIds.isEmpty()) {
+                return;
             }
+
+            Query query = Query.query(Criteria.where("_id").in(distinctMessageIds)
+                    .and("readers.userId").ne(userId));
+            mongoTemplate.updateMulti(query, new Update().push("readers", readerInfo), Message.class);
             log.debug("Read status updated for {} messages by user {}",
-                    messageIds.size(), userId);
+                    distinctMessageIds.size(), userId);
         } catch (Exception e) {
             log.error("Read status update error for user {}", userId, e);
         }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcher.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcher.java
new file mode 100644
index 00000000..007aa13e
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcher.java
@@ -0,0 +1,144 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.JOIN_ROOM_SUCCESS;
+
+import com.corundumstudio.socketio.SocketIOClient;
+import com.ktb.chatapp.dto.FetchMessagesRequest;
+import com.ktb.chatapp.dto.FetchMessagesResponse;
+import com.ktb.chatapp.dto.JoinRoomSuccessResponse;
+import io.micrometer.core.instrument.Counter;
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+import jakarta.annotation.PreDestroy;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.stereotype.Component;
+
+@Slf4j
+@Component
+@ConditionalOnProperty(name = "socketio.enabled", havingValue = "true", matchIfMissing = true)
+public class JoinAckDispatcher {
+
+    private final MessageLoader messageLoader;
+    private final ReadStatusDispatcher readStatusDispatcher;
+    private final RoomPostJoinDispatcher postJoinDispatcher;
+    private final ScheduledExecutorService scheduler;
+    private final ThreadPoolExecutor worker;
+    private final Map<String, JoinBatch> pending = new ConcurrentHashMap<>();
+    private final long debounceMs;
+    private final Counter requestCounter;
+    private final Counter queryCounter;
+
+    public JoinAckDispatcher(
+            MessageLoader messageLoader,
+            ReadStatusDispatcher readStatusDispatcher,
+            RoomPostJoinDispatcher postJoinDispatcher,
+            MeterRegistry meterRegistry,
+            @Value("${socketio.join-ack.workers:8}") int workers,
+            @Value("${socketio.join-ack.queue-capacity:1000}") int queueCapacity,
+            @Value("${socketio.join-ack.debounce-ms:100}") long debounceMs) {
+        this.messageLoader = messageLoader;
+        this.readStatusDispatcher = readStatusDispatcher;
+        this.postJoinDispatcher = postJoinDispatcher;
+        this.debounceMs = debounceMs;
+        this.scheduler = Executors.newSingleThreadScheduledExecutor(namedThreadFactory("join-ack-debounce-"));
+        this.worker = new ThreadPoolExecutor(
+                Math.max(1, workers),
+                Math.max(1, workers),
+                0L,
+                TimeUnit.MILLISECONDS,
+                new ArrayBlockingQueue<>(Math.max(1, queueCapacity)),
+                namedThreadFactory("join-ack-worker-"),
+                new ThreadPoolExecutor.CallerRunsPolicy());
+        this.requestCounter = meterRegistry.counter("socketio.join_ack.requests");
+        this.queryCounter = meterRegistry.counter("socketio.join_ack.history_queries");
+        Gauge.builder("socketio.join_ack.pending", pending, Map::size).register(meterRegistry);
+        Gauge.builder("socketio.join_ack.queue.size", worker.getQueue(), java.util.Collection::size)
+                .register(meterRegistry);
+    }
+
+    public void request(SocketIOClient client, String roomId, String userId, String userName) {
+        requestCounter.increment();
+        pending.compute(roomId, (key, existing) -> {
+            JoinBatch batch = existing != null ? existing : new JoinBatch();
+            batch.add(new WaitingJoin(client, userId, userName));
+            if (existing == null) {
+                scheduler.schedule(() -> flush(roomId, batch), debounceMs, TimeUnit.MILLISECONDS);
+            }
+            return batch;
+        });
+    }
+
+    private void flush(String roomId, JoinBatch batch) {
+        if (!pending.remove(roomId, batch)) {
+            return;
+        }
+        worker.execute(() -> {
+            queryCounter.increment();
+            FetchMessagesResponse history = messageLoader.loadMessages(
+                    new FetchMessagesRequest(roomId, 30, null), null);
+            JoinRoomSuccessResponse response = JoinRoomSuccessResponse.builder()
+                    .roomId(roomId)
+                    .messages(history.getMessages())
+                    .hasMore(history.isHasMore())
+                    .activeStreams(Collections.emptyList())
+                    .build();
+            List<String> messageIds = history.getMessages().stream()
+                    .map(com.ktb.chatapp.dto.MessageResponse::getId)
+                    .toList();
+            List<WaitingJoin> clients = batch.clients();
+
+            // 모든 참가자에게 화면 준비 ACK를 먼저 큐잉한 뒤 입장 fan-out을 시작한다.
+            for (WaitingJoin waiting : clients) {
+                waiting.client().sendEvent(JOIN_ROOM_SUCCESS, response);
+            }
+            for (WaitingJoin waiting : clients) {
+                readStatusDispatcher.schedule(messageIds, waiting.userId());
+                postJoinDispatcher.scheduleSystemMessage(
+                        roomId, waiting.userName() + "님이 입장하였습니다.");
+            }
+        });
+    }
+
+    private static ThreadFactory namedThreadFactory(String prefix) {
+        AtomicInteger sequence = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, prefix + sequence.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+
+    @PreDestroy
+    void shutdown() {
+        scheduler.shutdown();
+        worker.shutdown();
+    }
+
+    private record WaitingJoin(SocketIOClient client, String userId, String userName) {}
+
+    private static final class JoinBatch {
+        private final List<WaitingJoin> clients = new ArrayList<>();
+
+        private synchronized void add(WaitingJoin client) {
+            clients.add(client);
+        }
+
+        private synchronized List<WaitingJoin> clients() {
+            return List.copyOf(clients);
+        }
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcher.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcher.java
new file mode 100644
index 00000000..df82c5c4
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcher.java
@@ -0,0 +1,147 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.ERROR;
+import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.PREVIOUS_MESSAGES_LOADED;
+
+import com.corundumstudio.socketio.SocketIOClient;
+import com.ktb.chatapp.dto.FetchMessagesRequest;
+import com.ktb.chatapp.dto.FetchMessagesResponse;
+import io.micrometer.core.instrument.Counter;
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+import jakarta.annotation.PreDestroy;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.stereotype.Component;
+
+@Slf4j
+@Component
+@ConditionalOnProperty(name = "socketio.enabled", havingValue = "true", matchIfMissing = true)
+public class MessageFetchDispatcher {
+
+    private final MessageLoader messageLoader;
+    private final ReadStatusDispatcher readStatusDispatcher;
+    private final ScheduledExecutorService scheduler;
+    private final ThreadPoolExecutor worker;
+    private final Map<FetchKey, FetchBatch> pending = new ConcurrentHashMap<>();
+    private final long debounceMs;
+    private final Counter requestCounter;
+    private final Counter queryCounter;
+
+    public MessageFetchDispatcher(
+            MessageLoader messageLoader,
+            ReadStatusDispatcher readStatusDispatcher,
+            MeterRegistry meterRegistry,
+            @Value("${socketio.message-fetch.workers:8}") int workers,
+            @Value("${socketio.message-fetch.queue-capacity:1000}") int queueCapacity,
+            @Value("${socketio.message-fetch.debounce-ms:100}") long debounceMs) {
+        this.messageLoader = messageLoader;
+        this.readStatusDispatcher = readStatusDispatcher;
+        this.debounceMs = debounceMs;
+        this.scheduler = Executors.newSingleThreadScheduledExecutor(namedThreadFactory("message-fetch-debounce-"));
+        this.worker = new ThreadPoolExecutor(
+                Math.max(1, workers),
+                Math.max(1, workers),
+                0L,
+                TimeUnit.MILLISECONDS,
+                new ArrayBlockingQueue<>(Math.max(1, queueCapacity)),
+                namedThreadFactory("message-fetch-worker-"),
+                new ThreadPoolExecutor.CallerRunsPolicy());
+        this.requestCounter = meterRegistry.counter("socketio.message_fetch.requests");
+        this.queryCounter = meterRegistry.counter("socketio.message_fetch.queries");
+        Gauge.builder("socketio.message_fetch.pending", pending, Map::size).register(meterRegistry);
+        Gauge.builder("socketio.message_fetch.queue.size", worker.getQueue(), java.util.Collection::size)
+                .register(meterRegistry);
+    }
+
+    public void request(SocketIOClient client, FetchMessagesRequest request, String userId) {
+        requestCounter.increment();
+        FetchKey key = new FetchKey(request.roomId(), request.limit(), request.before());
+        pending.compute(key, (ignored, existing) -> {
+            FetchBatch batch = existing != null ? existing : new FetchBatch(request);
+            batch.add(new WaitingClient(client, userId));
+            if (existing == null) {
+                scheduler.schedule(() -> flush(key, batch), debounceMs, TimeUnit.MILLISECONDS);
+            }
+            return batch;
+        });
+    }
+
+    private void flush(FetchKey key, FetchBatch batch) {
+        if (!pending.remove(key, batch)) {
+            return;
+        }
+        worker.execute(() -> {
+            queryCounter.increment();
+            try {
+                FetchMessagesResponse result = messageLoader.loadMessages(batch.request(), null);
+                List<String> messageIds = result.getMessages().stream()
+                        .map(com.ktb.chatapp.dto.MessageResponse::getId)
+                        .toList();
+                for (WaitingClient waiting : batch.clients()) {
+                    waiting.client().sendEvent(PREVIOUS_MESSAGES_LOADED, result);
+                    readStatusDispatcher.schedule(messageIds, waiting.userId());
+                }
+            } catch (Exception e) {
+                log.error("Failed to load messages for room {}", key.roomId(), e);
+                for (WaitingClient waiting : batch.clients()) {
+                    waiting.client().sendEvent(ERROR, Map.of(
+                            "code", "LOAD_ERROR",
+                            "message", "이전 메시지를 불러오는 중 오류가 발생했습니다."));
+                }
+            }
+        });
+    }
+
+    private static ThreadFactory namedThreadFactory(String prefix) {
+        AtomicInteger sequence = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, prefix + sequence.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+
+    @PreDestroy
+    void shutdown() {
+        scheduler.shutdown();
+        worker.shutdown();
+    }
+
+    private record FetchKey(String roomId, Integer limit, Long before) {}
+
+    private record WaitingClient(SocketIOClient client, String userId) {}
+
+    private static final class FetchBatch {
+        private final FetchMessagesRequest request;
+        private final List<WaitingClient> clients = new ArrayList<>();
+
+        private FetchBatch(FetchMessagesRequest request) {
+            this.request = request;
+        }
+
+        private synchronized void add(WaitingClient client) {
+            clients.add(client);
+        }
+
+        private FetchMessagesRequest request() {
+            return request;
+        }
+
+        private synchronized List<WaitingClient> clients() {
+            return List.copyOf(clients);
+        }
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandler.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandler.java
index 8be4f9f8..92ff0cb9 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandler.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandler.java
@@ -3,10 +3,8 @@
 import com.corundumstudio.socketio.SocketIOClient;
 import com.corundumstudio.socketio.annotation.OnEvent;
 import com.ktb.chatapp.dto.FetchMessagesRequest;
-import com.ktb.chatapp.dto.FetchMessagesResponse;
-import com.ktb.chatapp.model.Room;
-import com.ktb.chatapp.repository.RoomRepository;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
+import com.ktb.chatapp.websocket.socketio.UserRooms;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
@@ -26,8 +24,8 @@
 @RequiredArgsConstructor
 public class MessageFetchHandler {
 
-    private final RoomRepository roomRepository;
-    private final MessageLoader messageLoader;
+    private final UserRooms userRooms;
+    private final MessageFetchDispatcher messageFetchDispatcher;
 
     @OnEvent(FETCH_PREVIOUS_MESSAGES)
     public void handleFetchMessages(SocketIOClient client, FetchMessagesRequest data) {
@@ -42,8 +40,7 @@ public void handleFetchMessages(SocketIOClient client, FetchMessagesRequest data
         
         try {
             // 권한 체크
-            Room room = roomRepository.findById(data.roomId()).orElse(null);
-            if (room == null || !room.getParticipantIds().contains(userId)) {
+            if (!userRooms.isInRoom(userId, data.roomId())) {
                 client.sendEvent(ERROR, Map.of(
                         "code", "LOAD_ERROR",
                         "message", "채팅방 접근 권한이 없습니다."
@@ -54,14 +51,7 @@ public void handleFetchMessages(SocketIOClient client, FetchMessagesRequest data
             log.debug("Starting message load for user {} in room {}, limit: {}, before: {}",
                     userId, data.roomId(), data.limit(), data.before());
 
-            log.debug("Loading messages for room {}", data.roomId());
-            FetchMessagesResponse result = messageLoader.loadMessages(data, userId);
-            
-            log.debug("Previous messages loaded - room: {}, count: {}, hasMore: {}",
-                    data.roomId(), result.getMessages().size(),
-                    result.isHasMore());
-            
-            client.sendEvent(PREVIOUS_MESSAGES_LOADED, result);
+            messageFetchDispatcher.request(client, data, userId);
 
         } catch (Exception e) {
             log.error("Error handling fetchPreviousMessages", e);
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoader.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoader.java
index ec320cc5..78ca88e5 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoader.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoader.java
@@ -7,16 +7,18 @@
 import com.ktb.chatapp.model.User;
 import com.ktb.chatapp.repository.MessageRepository;
 import com.ktb.chatapp.repository.UserRepository;
-import com.ktb.chatapp.service.MessageReadStatusService;
-import jakarta.annotation.Nullable;
 import java.time.LocalDateTime;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
 import java.util.List;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
-import org.springframework.data.domain.Page;
 import org.springframework.data.domain.PageRequest;
 import org.springframework.data.domain.Pageable;
+import org.springframework.data.domain.Slice;
 import org.springframework.data.domain.Sort;
 import org.springframework.stereotype.Component;
 
@@ -30,7 +32,6 @@ public class MessageLoader {
     private final MessageRepository messageRepository;
     private final UserRepository userRepository;
     private final MessageResponseMapper messageResponseMapper;
-    private final MessageReadStatusService messageReadStatusService;
 
     private static final int BATCH_SIZE = 30;
 
@@ -56,7 +57,7 @@ private FetchMessagesResponse loadMessagesInternal(
             String userId) {
         Pageable pageable = PageRequest.of(0, limit, Sort.by("timestamp").descending());
 
-        Page<Message> messagePage = messageRepository
+        Slice<Message> messagePage = messageRepository
                 .findByRoomIdAndTimestampBefore(roomId, before, pageable);
 
         List<Message> messages = messagePage.getContent();
@@ -64,15 +65,20 @@ private FetchMessagesResponse loadMessagesInternal(
         // DESC로 조회했으므로 ASC로 재정렬 (채팅 UI 표시 순서)
         List<Message> sortedMessages = messages.reversed();
         
-        var messageIds = sortedMessages.stream().map(Message::getId).toList();
-        messageReadStatusService.updateReadStatus(messageIds, userId);
-        
+        Set<String> senderIds = sortedMessages.stream()
+                .map(Message::getSenderId)
+                .filter(Objects::nonNull)
+                .collect(Collectors.toSet());
+        Map<String, User> sendersById = senderIds.isEmpty()
+                ? Map.of()
+                : userRepository.findAllById(senderIds).stream()
+                        .filter(sender -> sender != null && sender.getId() != null)
+                        .collect(Collectors.toMap(User::getId, Function.identity()));
+
         // 메시지 응답 생성
         List<MessageResponse> messageResponses = sortedMessages.stream()
-                .map(message -> {
-                    var user = findUserById(message.getSenderId());
-                    return messageResponseMapper.mapToMessageResponse(message, user);
-                })
+                .map(message -> messageResponseMapper.mapToMessageResponse(
+                        message, sendersById.get(message.getSenderId())))
                 .collect(Collectors.toList());
 
         boolean hasMore = messagePage.hasNext();
@@ -86,15 +92,4 @@ private FetchMessagesResponse loadMessagesInternal(
                 .build();
     }
 
-    /**
-     * AI 경우 null 반환 가능
-     */
-    @Nullable
-    private User findUserById(String id) {
-        if (id == null) {
-            return null;
-        }
-        return userRepository.findById(id)
-                .orElse(null);
-    }
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcher.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcher.java
new file mode 100644
index 00000000..657c0dc9
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcher.java
@@ -0,0 +1,82 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import com.ktb.chatapp.service.MessageReadStatusService;
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+import jakarta.annotation.PreDestroy;
+import java.util.List;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.stereotype.Component;
+
+@Slf4j
+@Component
+@ConditionalOnProperty(name = "socketio.enabled", havingValue = "true", matchIfMissing = true)
+public class ReadStatusDispatcher {
+
+    private final MessageReadStatusService readStatusService;
+    private final ScheduledExecutorService scheduler;
+    private final ThreadPoolExecutor worker;
+    private final long delayMs;
+
+    public ReadStatusDispatcher(
+            MessageReadStatusService readStatusService,
+            MeterRegistry meterRegistry,
+            @Value("${socketio.read-status.workers:8}") int workers,
+            @Value("${socketio.read-status.queue-capacity:1000}") int queueCapacity,
+            @Value("${socketio.read-status.delay-ms:2000}") long delayMs) {
+        this.readStatusService = readStatusService;
+        this.delayMs = delayMs;
+        this.scheduler = Executors.newSingleThreadScheduledExecutor(namedThreadFactory("read-status-delay-"));
+        this.worker = new ThreadPoolExecutor(
+                Math.max(1, workers),
+                Math.max(1, workers),
+                0L,
+                TimeUnit.MILLISECONDS,
+                new ArrayBlockingQueue<>(Math.max(1, queueCapacity)),
+                namedThreadFactory("read-status-worker-"),
+                new ThreadPoolExecutor.CallerRunsPolicy());
+        Gauge.builder("socketio.read_status.queue.size", worker.getQueue(), java.util.Collection::size)
+                .register(meterRegistry);
+    }
+
+    public void schedule(List<String> messageIds, String userId) {
+        if (messageIds == null || messageIds.isEmpty() || userId == null) {
+            return;
+        }
+        List<String> snapshot = List.copyOf(messageIds);
+        scheduler.schedule(
+                () -> worker.execute(() -> {
+                    try {
+                        readStatusService.updateReadStatus(snapshot, userId);
+                    } catch (Exception e) {
+                        log.error("Deferred read-status update failed for user {}", userId, e);
+                    }
+                }),
+                delayMs,
+                TimeUnit.MILLISECONDS);
+    }
+
+    private static ThreadFactory namedThreadFactory(String prefix) {
+        AtomicInteger sequence = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, prefix + sequence.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+
+    @PreDestroy
+    void shutdown() {
+        scheduler.shutdown();
+        worker.shutdown();
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandler.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandler.java
index fe0e4f7b..7f49c123 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandler.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandler.java
@@ -1,25 +1,17 @@
 package com.ktb.chatapp.websocket.socketio.handler;
 
 import com.corundumstudio.socketio.SocketIOClient;
-import com.corundumstudio.socketio.SocketIOServer;
 import com.corundumstudio.socketio.annotation.OnEvent;
-import com.ktb.chatapp.dto.FetchMessagesRequest;
-import com.ktb.chatapp.dto.FetchMessagesResponse;
-import com.ktb.chatapp.dto.JoinRoomSuccessResponse;
-import com.ktb.chatapp.dto.UserResponse;
-import com.ktb.chatapp.model.Message;
-import com.ktb.chatapp.model.MessageType;
-import com.ktb.chatapp.model.Room;
-import com.ktb.chatapp.repository.MessageRepository;
-import com.ktb.chatapp.repository.RoomRepository;
-import com.ktb.chatapp.repository.UserRepository;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
 import com.ktb.chatapp.websocket.socketio.UserRooms;
-import java.time.LocalDateTime;
 import java.util.*;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.stereotype.Component;
 
 import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.*;
@@ -34,14 +26,10 @@
 @RequiredArgsConstructor
 public class RoomJoinHandler {
 
-    private final SocketIOServer socketIOServer;
-    private final MessageRepository messageRepository;
-    private final RoomRepository roomRepository;
-    private final UserRepository userRepository;
+    private final MongoTemplate mongoTemplate;
     private final UserRooms userRooms;
-    private final MessageLoader messageLoader;
-    private final MessageResponseMapper messageResponseMapper;
-    private final RoomLeaveHandler roomLeaveHandler;
+    private final RoomPostJoinDispatcher postJoinDispatcher;
+    private final JoinAckDispatcher joinAckDispatcher;
     
     @OnEvent(JOIN_ROOM)
     public void handleJoinRoom(SocketIOClient client, String roomId) {
@@ -54,16 +42,6 @@ public void handleJoinRoom(SocketIOClient client, String roomId) {
                 return;
             }
             
-            if (userRepository.findById(userId).isEmpty()) {
-                client.sendEvent(JOIN_ROOM_ERROR, Map.of("message", "User not found"));
-                return;
-            }
-            
-            if (roomRepository.findById(roomId).isEmpty()) {
-                client.sendEvent(JOIN_ROOM_ERROR, Map.of("message", "채팅방을 찾을 수 없습니다."));
-                return;
-            }
-            
             // 이미 해당 방에 참여 중인지 확인
             if (userRooms.isInRoom(userId, roomId)) {
                 log.debug("User {} already in room {}", userId, roomId);
@@ -72,65 +50,19 @@ public void handleJoinRoom(SocketIOClient client, String roomId) {
                 return;
             }
 
-            roomRepository.addParticipant(roomId, userId);
-
-            // Join socket room and add to user's room set
-            client.joinRoom(roomId);
-            userRooms.add(userId, roomId);
-
-            Message joinMessage = Message.builder()
-                .roomId(roomId)
-                .content(userName + "님이 입장하였습니다.")
-                .type(MessageType.system)
-                .timestamp(LocalDateTime.now())
-                .mentions(new ArrayList<>())
-                .reactions(new HashMap<>())
-                .readers(new ArrayList<>())
-                .metadata(new HashMap<>())
-                .build();
-
-            joinMessage = messageRepository.save(joinMessage);
-
-            // 초기 메시지 로드
-            FetchMessagesRequest req = new FetchMessagesRequest(roomId, 30, null);
-            FetchMessagesResponse messageLoadResult = messageLoader.loadMessages(req, userId);
-
-            // 업데이트된 room 다시 조회하여 최신 participantIds 가져오기
-            Optional<Room> roomOpt = roomRepository.findById(roomId);
-            if (roomOpt.isEmpty()) {
+            if (!addParticipant(roomId, userId)) {
                 client.sendEvent(JOIN_ROOM_ERROR, Map.of("message", "채팅방을 찾을 수 없습니다."));
                 return;
             }
 
-            // 참가자 정보 조회
-            List<UserResponse> participants = roomOpt.get().getParticipantIds()
-                    .stream()
-                    .map(userRepository::findById)
-                    .filter(Optional::isPresent)
-                    .map(Optional::get)
-                    .map(UserResponse::from)
-                    .toList();
-            
-            JoinRoomSuccessResponse response = JoinRoomSuccessResponse.builder()
-                .roomId(roomId)
-                .participants(participants)
-                .messages(messageLoadResult.getMessages())
-                .hasMore(messageLoadResult.isHasMore())
-                .activeStreams(Collections.emptyList())
-                .build();
-
-            client.sendEvent(JOIN_ROOM_SUCCESS, response);
-
-            // 입장 메시지 브로드캐스트
-            socketIOServer.getRoomOperations(roomId)
-                .sendEvent(MESSAGE, messageResponseMapper.mapToMessageResponse(joinMessage, null));
+            // Join socket room and add to user's room set
+            client.joinRoom(roomId);
+            userRooms.add(userId, roomId);
 
-            // 참가자 목록 업데이트 브로드캐스트
-            socketIOServer.getRoomOperations(roomId)
-                .sendEvent(PARTICIPANTS_UPDATE, participants);
+            joinAckDispatcher.request(client, roomId, userId, userName);
+            postJoinDispatcher.scheduleParticipantsUpdate(roomId);
 
-            log.info("User {} joined room {} successfully. Message count: {}, hasMore: {}",
-                userName, roomId, messageLoadResult.getMessages().size(), messageLoadResult.isHasMore());
+            log.info("User {} joined room {} successfully", userName, roomId);
 
         } catch (Exception e) {
             log.error("Error handling joinRoom", e);
@@ -139,6 +71,12 @@ public void handleJoinRoom(SocketIOClient client, String roomId) {
             ));
         }
     }
+
+    private boolean addParticipant(String roomId, String userId) {
+        Query query = Query.query(Criteria.where("_id").is(roomId));
+        return mongoTemplate.updateFirst(
+                query, new Update().addToSet("participantIds", userId), "rooms").getMatchedCount() > 0;
+    }
     
     private SocketUser getUser(SocketIOClient client) {
         return client.get("user");
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandler.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandler.java
index 0d9bab6c..02863a2a 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandler.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandler.java
@@ -1,24 +1,14 @@
 package com.ktb.chatapp.websocket.socketio.handler;
 
 import com.corundumstudio.socketio.SocketIOClient;
-import com.corundumstudio.socketio.SocketIOServer;
 import com.corundumstudio.socketio.annotation.OnEvent;
-import com.ktb.chatapp.dto.MessageResponse;
-import com.ktb.chatapp.dto.UserResponse;
-import com.ktb.chatapp.model.Message;
-import com.ktb.chatapp.model.MessageType;
 import com.ktb.chatapp.model.Room;
 import com.ktb.chatapp.model.User;
-import com.ktb.chatapp.repository.MessageRepository;
 import com.ktb.chatapp.repository.RoomRepository;
 import com.ktb.chatapp.repository.UserRepository;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
 import com.ktb.chatapp.websocket.socketio.UserRooms;
-import java.time.LocalDateTime;
-import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.Map;
-import java.util.Optional;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
@@ -36,12 +26,10 @@
 @RequiredArgsConstructor
 public class RoomLeaveHandler {
 
-    private final SocketIOServer socketIOServer;
-    private final MessageRepository messageRepository;
     private final RoomRepository roomRepository;
     private final UserRepository userRepository;
     private final UserRooms userRooms;
-    private final MessageResponseMapper messageResponseMapper;
+    private final RoomPostJoinDispatcher postJoinDispatcher;
     
     @OnEvent(LEAVE_ROOM)
     public void handleLeaveRoom(SocketIOClient client, String roomId) {
@@ -76,8 +64,8 @@ public void handleLeaveRoom(SocketIOClient client, String roomId) {
             
             log.debug("Leave room cleanup - roomId: {}, userId: {}", roomId, userId);
             
-            sendSystemMessage(roomId, userName + "님이 퇴장하였습니다.");
-            broadcastParticipantList(roomId);
+            postJoinDispatcher.scheduleSystemMessage(roomId, userName + "님이 퇴장하였습니다.");
+            postJoinDispatcher.scheduleParticipantsUpdate(roomId);
 
         } catch (Exception e) {
             log.error("Error handling leaveRoom", e);
@@ -85,52 +73,6 @@ public void handleLeaveRoom(SocketIOClient client, String roomId) {
         }
     }
     
-    private void sendSystemMessage(String roomId, String content) {
-        try {
-            Message systemMessage = new Message();
-            systemMessage.setRoomId(roomId);
-            systemMessage.setContent(content);
-            systemMessage.setType(MessageType.system);
-            systemMessage.setTimestamp(LocalDateTime.now());
-            systemMessage.setMentions(new ArrayList<>());
-            systemMessage.setReactions(new HashMap<>());
-            systemMessage.setReaders(new ArrayList<>());
-            systemMessage.setMetadata(new HashMap<>());
-
-            Message savedMessage = messageRepository.save(systemMessage);
-            MessageResponse response = messageResponseMapper.mapToMessageResponse(savedMessage, null);
-
-            socketIOServer.getRoomOperations(roomId)
-                    .sendEvent(MESSAGE, response);
-
-        } catch (Exception e) {
-            log.error("Error sending system message", e);
-        }
-    }
-    
-    private void broadcastParticipantList(String roomId) {
-        Optional<Room> roomOpt = roomRepository.findById(roomId);
-        if (roomOpt.isEmpty()) {
-            return;
-        }
-        
-        var participantList = roomOpt.get()
-                .getParticipantIds()
-                .stream()
-                .map(userRepository::findById)
-                .filter(Optional::isPresent)
-                .map(Optional::get)
-                .map(UserResponse::from)
-                .toList();
-        
-        if (participantList.isEmpty()) {
-            return;
-        }
-        
-        socketIOServer.getRoomOperations(roomId)
-                .sendEvent(PARTICIPANTS_UPDATE, participantList);
-    }
-
     private SocketUser getUserDto(SocketIOClient client) {
         return client.get("user");
     }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcher.java b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcher.java
new file mode 100644
index 00000000..bac613d8
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcher.java
@@ -0,0 +1,237 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.MESSAGE;
+import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.PARTICIPANTS_UPDATE;
+
+import com.corundumstudio.socketio.SocketIOServer;
+import com.ktb.chatapp.dto.UserResponse;
+import com.ktb.chatapp.model.Message;
+import com.ktb.chatapp.model.MessageType;
+import com.ktb.chatapp.model.Room;
+import com.ktb.chatapp.model.User;
+import com.ktb.chatapp.repository.MessageRepository;
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.Timer;
+import jakarta.annotation.PreDestroy;
+import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Collectors;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.stereotype.Component;
+
+@Slf4j
+@Component
+@ConditionalOnProperty(name = "socketio.enabled", havingValue = "true", matchIfMissing = true)
+public class RoomPostJoinDispatcher {
+
+    private final SocketIOServer socketIOServer;
+    private final MessageRepository messageRepository;
+    private final MongoTemplate mongoTemplate;
+    private final MessageResponseMapper messageResponseMapper;
+    private final ThreadPoolExecutor[] stripes;
+    private final ScheduledExecutorService scheduler;
+    private final Map<String, ScheduledFuture<?>> pendingParticipantUpdates = new ConcurrentHashMap<>();
+    private final Map<String, SystemMessageBatch> pendingSystemMessages = new ConcurrentHashMap<>();
+    private final long participantDebounceMs;
+    private final long systemMessageDebounceMs;
+    private final Timer queueDelayTimer;
+    private final Timer participantSnapshotTimer;
+
+    public RoomPostJoinDispatcher(
+            SocketIOServer socketIOServer,
+            MessageRepository messageRepository,
+            MongoTemplate mongoTemplate,
+            MessageResponseMapper messageResponseMapper,
+            MeterRegistry meterRegistry,
+            @Value("${socketio.post-join.stripes:16}") int stripeCount,
+            @Value("${socketio.post-join.queue-capacity:1000}") int queueCapacity,
+            @Value("${socketio.participants.debounce-ms:250}") long participantDebounceMs,
+            @Value("${socketio.system-messages.debounce-ms:1000}") long systemMessageDebounceMs) {
+        this.socketIOServer = socketIOServer;
+        this.messageRepository = messageRepository;
+        this.mongoTemplate = mongoTemplate;
+        this.messageResponseMapper = messageResponseMapper;
+        this.participantDebounceMs = participantDebounceMs;
+        this.systemMessageDebounceMs = systemMessageDebounceMs;
+        this.queueDelayTimer = meterRegistry.timer("socketio.room.post_join.queue.delay");
+        this.participantSnapshotTimer = meterRegistry.timer("socketio.room.participants.snapshot.time");
+        this.stripes = new ThreadPoolExecutor[Math.max(1, stripeCount)];
+        for (int i = 0; i < stripes.length; i++) {
+            stripes[i] = new ThreadPoolExecutor(
+                    1,
+                    1,
+                    0L,
+                    TimeUnit.MILLISECONDS,
+                    new ArrayBlockingQueue<>(Math.max(1, queueCapacity)),
+                    namedThreadFactory("room-post-join-" + i + "-"),
+                    new ThreadPoolExecutor.CallerRunsPolicy());
+        }
+        this.scheduler = Executors.newSingleThreadScheduledExecutor(
+                namedThreadFactory("room-participants-debounce-"));
+        Gauge.builder("socketio.room.post_join.queue.size", this, RoomPostJoinDispatcher::queuedTasks)
+                .register(meterRegistry);
+        Gauge.builder("socketio.room.participants.pending", pendingParticipantUpdates, Map::size)
+                .register(meterRegistry);
+    }
+
+    public void publishSystemMessage(String roomId, String content) {
+        execute(roomId, () -> {
+            try {
+                Message message = Message.builder()
+                        .roomId(roomId)
+                        .content(content)
+                        .type(MessageType.system)
+                        .timestamp(LocalDateTime.now())
+                        .mentions(new ArrayList<>())
+                        .reactions(new HashMap<>())
+                        .readers(new ArrayList<>())
+                        .metadata(new HashMap<>())
+                        .build();
+                Message saved = messageRepository.save(message);
+                socketIOServer.getRoomOperations(roomId)
+                        .sendEvent(MESSAGE, messageResponseMapper.mapToMessageResponse(saved, null));
+            } catch (Exception e) {
+                log.error("Failed to publish system message for room {}", roomId, e);
+            }
+        });
+    }
+
+    public void scheduleSystemMessage(String roomId, String content) {
+        pendingSystemMessages.compute(roomId, (key, existing) -> {
+            SystemMessageBatch batch = existing != null ? existing : new SystemMessageBatch();
+            batch.add(content);
+            batch.reschedule(scheduler.schedule(
+                    () -> flushSystemMessages(roomId, batch),
+                    systemMessageDebounceMs,
+                    TimeUnit.MILLISECONDS));
+            return batch;
+        });
+    }
+
+    private void flushSystemMessages(String roomId, SystemMessageBatch batch) {
+        if (!pendingSystemMessages.remove(roomId, batch)) {
+            return;
+        }
+        for (String content : batch.contents()) {
+            publishSystemMessage(roomId, content);
+        }
+    }
+
+    public void scheduleParticipantsUpdate(String roomId) {
+        pendingParticipantUpdates.compute(roomId, (key, previous) -> {
+            if (previous != null) {
+                previous.cancel(false);
+            }
+            return scheduler.schedule(
+                    () -> {
+                        pendingParticipantUpdates.remove(roomId);
+                        execute(roomId, () -> broadcastParticipantSnapshot(roomId));
+                    },
+                    participantDebounceMs,
+                    TimeUnit.MILLISECONDS);
+        });
+    }
+
+    private void broadcastParticipantSnapshot(String roomId) {
+        participantSnapshotTimer.record(() -> {
+            try {
+                Query roomQuery = Query.query(Criteria.where("_id").is(roomId));
+                roomQuery.fields().include("participantIds");
+                Room room = mongoTemplate.findOne(roomQuery, Room.class);
+                if (room == null || room.getParticipantIds() == null) {
+                    return;
+                }
+
+                Query usersQuery = Query.query(Criteria.where("_id").in(room.getParticipantIds()));
+                usersQuery.fields().include("_id").include("name").include("email").include("profileImage");
+                Map<String, User> usersById = mongoTemplate.find(usersQuery, User.class).stream()
+                        .filter(user -> user.getId() != null)
+                        .collect(Collectors.toMap(User::getId, user -> user));
+                List<UserResponse> participants = room.getParticipantIds().stream()
+                        .map(usersById::get)
+                        .filter(Objects::nonNull)
+                        .map(UserResponse::from)
+                        .toList();
+                socketIOServer.getRoomOperations(roomId).sendEvent(PARTICIPANTS_UPDATE, participants);
+            } catch (Exception e) {
+                log.error("Failed to broadcast participant snapshot for room {}", roomId, e);
+            }
+        });
+    }
+
+    private void execute(String roomId, Runnable task) {
+        long queuedAt = System.nanoTime();
+        stripe(roomId).execute(() -> {
+            queueDelayTimer.record(System.nanoTime() - queuedAt, TimeUnit.NANOSECONDS);
+            task.run();
+        });
+    }
+
+    private ThreadPoolExecutor stripe(String roomId) {
+        return stripes[Math.floorMod(roomId.hashCode(), stripes.length)];
+    }
+
+    private double queuedTasks() {
+        int queued = 0;
+        for (ThreadPoolExecutor stripe : stripes) {
+            queued += stripe.getQueue().size();
+        }
+        return queued;
+    }
+
+    private static ThreadFactory namedThreadFactory(String prefix) {
+        AtomicInteger sequence = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, prefix + sequence.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+
+    @PreDestroy
+    void shutdown() {
+        scheduler.shutdown();
+        for (ThreadPoolExecutor stripe : stripes) {
+            stripe.shutdown();
+        }
+    }
+
+    private static final class SystemMessageBatch {
+        private final List<String> contents = new ArrayList<>();
+        private ScheduledFuture<?> future;
+
+        private synchronized void add(String content) {
+            contents.add(content);
+        }
+
+        private synchronized void reschedule(ScheduledFuture<?> next) {
+            if (future != null) {
+                future.cancel(false);
+            }
+            future = next;
+        }
+
+        private synchronized List<String> contents() {
+            return List.copyOf(contents);
+        }
+    }
+}
diff --git a/apps/backend/src/main/resources/application.properties b/apps/backend/src/main/resources/application.properties
index c6b493d0..3418acb4 100644
--- a/apps/backend/src/main/resources/application.properties
+++ b/apps/backend/src/main/resources/application.properties
@@ -45,6 +45,19 @@ spring.ai.openai.chat.options.temperature=${OPENAI_TEMPERATURE:0.7}
 socketio.server.host=0.0.0.0
 socketio.server.port=${WS_PORT:5002}
 socketio.server.origin=${SOCKETIO_SERVER_ORIGIN:*}
+socketio.post-join.stripes=${SOCKETIO_POST_JOIN_STRIPES:16}
+socketio.post-join.queue-capacity=${SOCKETIO_POST_JOIN_QUEUE_CAPACITY:1000}
+socketio.participants.debounce-ms=${SOCKETIO_PARTICIPANTS_DEBOUNCE_MS:250}
+socketio.system-messages.debounce-ms=${SOCKETIO_SYSTEM_MESSAGES_DEBOUNCE_MS:1000}
+socketio.read-status.workers=${SOCKETIO_READ_STATUS_WORKERS:8}
+socketio.read-status.queue-capacity=${SOCKETIO_READ_STATUS_QUEUE_CAPACITY:1000}
+socketio.read-status.delay-ms=${SOCKETIO_READ_STATUS_DELAY_MS:2000}
+socketio.message-fetch.workers=${SOCKETIO_MESSAGE_FETCH_WORKERS:8}
+socketio.message-fetch.queue-capacity=${SOCKETIO_MESSAGE_FETCH_QUEUE_CAPACITY:1000}
+socketio.message-fetch.debounce-ms=${SOCKETIO_MESSAGE_FETCH_DEBOUNCE_MS:100}
+socketio.join-ack.workers=${SOCKETIO_JOIN_ACK_WORKERS:8}
+socketio.join-ack.queue-capacity=${SOCKETIO_JOIN_ACK_QUEUE_CAPACITY:1000}
+socketio.join-ack.debounce-ms=${SOCKETIO_JOIN_ACK_DEBOUNCE_MS:100}
 
 # CORS Configuration
 app.cors.allowed-origins=${CORS_ALLOWED_ORIGINS:*}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/MessageReadStatusServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/MessageReadStatusServiceTest.java
new file mode 100644
index 00000000..2b791014
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/MessageReadStatusServiceTest.java
@@ -0,0 +1,40 @@
+package com.ktb.chatapp.service;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+
+import com.ktb.chatapp.model.Message;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
+
+@ExtendWith(MockitoExtension.class)
+class MessageReadStatusServiceTest {
+
+    @Mock private MongoTemplate mongoTemplate;
+
+    @Test
+    void updateReadStatusUsesOneBulkUpdateForDistinctMessages() {
+        MessageReadStatusService service = new MessageReadStatusService(mongoTemplate);
+
+        service.updateReadStatus(List.of("message-1", "message-1", "message-2"), "user-1");
+
+        verify(mongoTemplate).updateMulti(any(Query.class), any(Update.class), eq(Message.class));
+    }
+
+    @Test
+    void updateReadStatusSkipsEmptyInput() {
+        MessageReadStatusService service = new MessageReadStatusService(mongoTemplate);
+
+        service.updateReadStatus(List.of(), "user-1");
+
+        verify(mongoTemplate, never()).updateMulti(any(Query.class), any(Update.class), eq(Message.class));
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcherTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcherTest.java
new file mode 100644
index 00000000..056c8bcd
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/JoinAckDispatcherTest.java
@@ -0,0 +1,70 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.corundumstudio.socketio.SocketIOClient;
+import com.ktb.chatapp.dto.FetchMessagesRequest;
+import com.ktb.chatapp.dto.FetchMessagesResponse;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import java.util.List;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class JoinAckDispatcherTest {
+
+    @Mock private MessageLoader messageLoader;
+    @Mock private ReadStatusDispatcher readStatusDispatcher;
+    @Mock private RoomPostJoinDispatcher postJoinDispatcher;
+    @Mock private SocketIOClient firstClient;
+    @Mock private SocketIOClient secondClient;
+
+    private JoinAckDispatcher dispatcher;
+
+    @BeforeEach
+    void setUp() {
+        dispatcher = new JoinAckDispatcher(
+                messageLoader,
+                readStatusDispatcher,
+                postJoinDispatcher,
+                new SimpleMeterRegistry(),
+                1,
+                10,
+                25);
+    }
+
+    @AfterEach
+    void tearDown() {
+        dispatcher.shutdown();
+    }
+
+    @Test
+    void coalescesHistoryAndAcknowledgesEveryWaitingClient() {
+        FetchMessagesResponse history = FetchMessagesResponse.builder()
+                .messages(List.of())
+                .hasMore(false)
+                .build();
+        when(messageLoader.loadMessages(any(FetchMessagesRequest.class), eq(null))).thenReturn(history);
+
+        dispatcher.request(firstClient, "room-1", "user-1", "first");
+        dispatcher.request(secondClient, "room-1", "user-2", "second");
+
+        verify(firstClient, org.mockito.Mockito.timeout(1_000))
+                .sendEvent(eq("joinRoomSuccess"), any());
+        verify(secondClient, org.mockito.Mockito.timeout(1_000))
+                .sendEvent(eq("joinRoomSuccess"), any());
+        verify(messageLoader, times(1)).loadMessages(any(FetchMessagesRequest.class), eq(null));
+        verify(postJoinDispatcher, org.mockito.Mockito.timeout(1_000))
+                .scheduleSystemMessage("room-1", "first님이 입장하였습니다.");
+        verify(postJoinDispatcher, org.mockito.Mockito.timeout(1_000))
+                .scheduleSystemMessage("room-1", "second님이 입장하였습니다.");
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcherTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcherTest.java
new file mode 100644
index 00000000..33940697
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchDispatcherTest.java
@@ -0,0 +1,60 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.corundumstudio.socketio.SocketIOClient;
+import com.ktb.chatapp.dto.FetchMessagesRequest;
+import com.ktb.chatapp.dto.FetchMessagesResponse;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import java.util.List;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class MessageFetchDispatcherTest {
+
+    @Mock private MessageLoader messageLoader;
+    @Mock private ReadStatusDispatcher readStatusDispatcher;
+    @Mock private SocketIOClient firstClient;
+    @Mock private SocketIOClient secondClient;
+
+    private MessageFetchDispatcher dispatcher;
+
+    @BeforeEach
+    void setUp() {
+        dispatcher = new MessageFetchDispatcher(
+                messageLoader, readStatusDispatcher, new SimpleMeterRegistry(), 1, 10, 25);
+    }
+
+    @AfterEach
+    void tearDown() {
+        dispatcher.shutdown();
+    }
+
+    @Test
+    void coalescesEquivalentConcurrentFetchesIntoOneQuery() {
+        FetchMessagesRequest request = new FetchMessagesRequest("room-1", 30, null);
+        FetchMessagesResponse response = FetchMessagesResponse.builder()
+                .messages(List.of())
+                .hasMore(false)
+                .build();
+        when(messageLoader.loadMessages(request, null)).thenReturn(response);
+
+        dispatcher.request(firstClient, request, "user-1");
+        dispatcher.request(secondClient, request, "user-2");
+
+        verify(firstClient, org.mockito.Mockito.timeout(1_000))
+                .sendEvent(eq("previousMessagesLoaded"), eq(response));
+        verify(secondClient, org.mockito.Mockito.timeout(1_000))
+                .sendEvent(eq("previousMessagesLoaded"), eq(response));
+        verify(messageLoader, times(1)).loadMessages(request, null);
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandlerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandlerTest.java
index d7faf98c..2cdb310f 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandlerTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageFetchHandlerTest.java
@@ -3,12 +3,9 @@
 import com.corundumstudio.socketio.SocketIOClient;
 import com.ktb.chatapp.dto.FetchMessagesRequest;
 import com.ktb.chatapp.dto.FetchMessagesResponse;
-import com.ktb.chatapp.model.Room;
-import com.ktb.chatapp.repository.RoomRepository;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
+import com.ktb.chatapp.websocket.socketio.UserRooms;
 import java.util.List;
-import java.util.Optional;
-import java.util.Set;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -26,15 +23,15 @@
 @ExtendWith(MockitoExtension.class)
 class MessageFetchHandlerTest {
 
-    @Mock private RoomRepository roomRepository;
-    @Mock private MessageLoader messageLoader;
+    @Mock private UserRooms userRooms;
+    @Mock private MessageFetchDispatcher messageFetchDispatcher;
     @Mock private SocketIOClient client;
 
     private MessageFetchHandler handler;
 
     @BeforeEach
     void setUp() {
-        handler = new MessageFetchHandler(roomRepository, messageLoader);
+        handler = new MessageFetchHandler(userRooms, messageFetchDispatcher);
     }
 
     @Test
@@ -45,26 +42,19 @@ void handleFetchMessages_rejectsUnauthorizedClient() {
         handler.handleFetchMessages(client, request);
 
         verify(client).sendEvent(eq(ERROR), any());
-        verify(messageLoader, never()).loadMessages(any(), any());
+        verify(messageFetchDispatcher, never()).request(any(), any(), any());
     }
 
     @Test
     void handleFetchMessages_loadsMessagesForParticipant() {
         FetchMessagesRequest request = new FetchMessagesRequest("room-1", 30, null);
-        FetchMessagesResponse response = FetchMessagesResponse.builder()
-                .messages(List.of())
-                .hasMore(false)
-                .build();
-        Room room = Room.builder().id("room-1").participantIds(Set.of("user-1")).build();
-
         when(client.get("user"))
                 .thenReturn(new SocketUser("user-1", "tester", "session-1", "socket-1"));
-        when(roomRepository.findById("room-1")).thenReturn(Optional.of(room));
-        when(messageLoader.loadMessages(request, "user-1")).thenReturn(response);
+        when(userRooms.isInRoom("user-1", "room-1")).thenReturn(true);
 
         handler.handleFetchMessages(client, request);
 
         verify(client, never()).sendEvent(eq("messageLoadStart"));
-        verify(client).sendEvent(PREVIOUS_MESSAGES_LOADED, response);
+        verify(messageFetchDispatcher).request(client, request, "user-1");
     }
 }
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderIntegrationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderIntegrationTest.java
index d3d96d86..e5ad79b2 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderIntegrationTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderIntegrationTest.java
@@ -67,8 +67,7 @@ void setUp() {
         messageLoader = new MessageLoader(
                 messageRepository,
                 userRepository,
-                new MessageResponseMapper(fileRepository),
-                messageReadStatusService
+                new MessageResponseMapper(fileRepository)
         );
 
         // 테스트 사용자 생성 및 저장
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderTest.java
index abd8348e..07744df2 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/MessageLoaderTest.java
@@ -60,8 +60,7 @@ void setUp() {
         messageLoader = new MessageLoader(
                 messageRepository,
                 userRepository,
-                new MessageResponseMapper(fileRepository),
-                messageReadStatusService
+                new MessageResponseMapper(fileRepository)
         );
         
         var testUser = User.builder()
@@ -81,7 +80,6 @@ void setUp() {
         
         lenient().when(userRepository.findAllById(anySet()))
                 .thenReturn(List.of(testUser));
-        lenient().doNothing().when(messageReadStatusService).updateReadStatus(anyList(), anyString());
     }
     
     private Message createMessage(String id, LocalDateTime timestamp) {
@@ -115,17 +113,19 @@ void loadMessages_shouldReturnAscendingOrderAfterReversing() {
         // Then: 결과는 오름차순으로 정렬되어야 함
         assertThat(result.getMessages()).hasSize(30);
         assertThat(result.isHasMore()).isTrue();
+        verify(userRepository).findAllById(anySet());
+        verify(userRepository, never()).findById(anyString());
         
         // 시간순 정렬 확인 (오름차순: 오래된 것 → 최신 것)
         // [50시간 전, 49시간 전, ..., 21시간 전]
         verifyAscending(result);
     }
     
-    private static @NotNull Page<Message> getMessagePage(List<Message> first30Messages) {
+    private static @NotNull Slice<Message> getMessagePage(List<Message> first30Messages) {
         List<Message> messages = new ArrayList<>(first30Messages.reversed());
         
         Pageable pageable = PageRequest.of(0, 30, Sort.by("timestamp").descending());
-        Page<Message> messagePage = new PageImpl<>(messages, pageable, 50);
+        Slice<Message> messagePage = new SliceImpl<>(messages, pageable, true);
         return messagePage;
     }
     
@@ -137,7 +137,7 @@ void loadInitialMessages_shouldReturnAscendingOrderAfterReversing() {
         
         // DB는 DESC 정렬로 반환 (최신 것부터)
         // [1시간 전, 2시간 전, ..., 30시간 전]
-        Page<Message> messagePage = getMessagePage(last30Messages);
+        Slice<Message> messagePage = getMessagePage(last30Messages);
         
         when(messageRepository.findByRoomIdAndTimestampBefore(
                 eq(roomId), any(LocalDateTime.class), any(Pageable.class)))
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcherTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcherTest.java
new file mode 100644
index 00000000..29d5d75e
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/ReadStatusDispatcherTest.java
@@ -0,0 +1,38 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static org.mockito.Mockito.verify;
+
+import com.ktb.chatapp.service.MessageReadStatusService;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import java.util.List;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class ReadStatusDispatcherTest {
+
+    @Mock private MessageReadStatusService readStatusService;
+    private ReadStatusDispatcher dispatcher;
+
+    @BeforeEach
+    void setUp() {
+        dispatcher = new ReadStatusDispatcher(readStatusService, new SimpleMeterRegistry(), 1, 10, 10);
+    }
+
+    @AfterEach
+    void tearDown() {
+        dispatcher.shutdown();
+    }
+
+    @Test
+    void schedulesReadStatusOutsideTheRequestThread() {
+        dispatcher.schedule(List.of("message-1"), "user-1");
+
+        verify(readStatusService, org.mockito.Mockito.timeout(1_000))
+                .updateReadStatus(List.of("message-1"), "user-1");
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandlerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandlerTest.java
index 372e2925..003d5e90 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandlerTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomJoinHandlerTest.java
@@ -1,66 +1,40 @@
 package com.ktb.chatapp.websocket.socketio.handler;
 
-import com.corundumstudio.socketio.BroadcastOperations;
 import com.corundumstudio.socketio.SocketIOClient;
-import com.corundumstudio.socketio.SocketIOServer;
-import com.ktb.chatapp.dto.FetchMessagesRequest;
-import com.ktb.chatapp.dto.FetchMessagesResponse;
-import com.ktb.chatapp.dto.MessageResponse;
-import com.ktb.chatapp.model.Message;
-import com.ktb.chatapp.model.MessageType;
-import com.ktb.chatapp.model.Room;
-import com.ktb.chatapp.model.User;
-import com.ktb.chatapp.repository.MessageRepository;
-import com.ktb.chatapp.repository.RoomRepository;
-import com.ktb.chatapp.repository.UserRepository;
+import com.mongodb.client.result.UpdateResult;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
 import com.ktb.chatapp.websocket.socketio.UserRooms;
-import java.time.LocalDateTime;
-import java.util.List;
-import java.util.Optional;
-import java.util.Set;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 
 import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.JOIN_ROOM_ERROR;
 import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.JOIN_ROOM_SUCCESS;
-import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.MESSAGE;
-import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.PARTICIPANTS_UPDATE;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.when;
 
 @ExtendWith(MockitoExtension.class)
 class RoomJoinHandlerTest {
 
-    @Mock private SocketIOServer socketIOServer;
-    @Mock private MessageRepository messageRepository;
-    @Mock private RoomRepository roomRepository;
-    @Mock private UserRepository userRepository;
+    @Mock private MongoTemplate mongoTemplate;
     @Mock private UserRooms userRooms;
-    @Mock private MessageLoader messageLoader;
-    @Mock private MessageResponseMapper messageResponseMapper;
-    @Mock private RoomLeaveHandler roomLeaveHandler;
+    @Mock private RoomPostJoinDispatcher postJoinDispatcher;
+    @Mock private JoinAckDispatcher joinAckDispatcher;
     @Mock private SocketIOClient client;
-    @Mock private BroadcastOperations roomOperations;
 
     private RoomJoinHandler handler;
 
     @BeforeEach
     void setUp() {
-        handler = new RoomJoinHandler(
-                socketIOServer,
-                messageRepository,
-                roomRepository,
-                userRepository,
-                userRooms,
-                messageLoader,
-                messageResponseMapper,
-                roomLeaveHandler);
+        handler = new RoomJoinHandler(mongoTemplate, userRooms, postJoinDispatcher, joinAckDispatcher);
     }
 
     @Test
@@ -73,45 +47,35 @@ void handleJoinRoom_rejectsUnauthorizedClient() {
     }
 
     @Test
-    void handleJoinRoom_addsParticipantLoadsMessagesAndBroadcasts() {
+    void handleJoinRoomAcknowledgesBeforeDispatchingPostJoinWork() {
         SocketUser socketUser = new SocketUser("user-1", "tester", "session-1", "socket-1");
-        User user = User.builder().id("user-1").name("tester").email("tester@example.com").build();
-        Room room = Room.builder().id("room-1").name("room").participantIds(Set.of("user-1")).build();
-        MessageResponse joinMessageResponse = MessageResponse.builder()
-                .id("message-1")
-                .roomId("room-1")
-                .content("tester님이 입장하였습니다.")
-                .type(MessageType.system)
-                .timestamp(1L)
-                .build();
-        FetchMessagesResponse loadResponse = FetchMessagesResponse.builder()
-                .messages(List.of())
-                .hasMore(false)
-                .build();
 
         when(client.get("user")).thenReturn(socketUser);
-        when(userRepository.findById("user-1")).thenReturn(Optional.of(user));
-        when(roomRepository.findById("room-1")).thenReturn(Optional.of(room));
         when(userRooms.isInRoom("user-1", "room-1")).thenReturn(false);
-        when(messageRepository.save(any(Message.class))).thenAnswer(invocation -> {
-            Message message = invocation.getArgument(0);
-            message.setId("message-1");
-            message.setTimestamp(LocalDateTime.now());
-            return message;
-        });
-        when(messageLoader.loadMessages(any(FetchMessagesRequest.class), eq("user-1")))
-                .thenReturn(loadResponse);
-        when(messageResponseMapper.mapToMessageResponse(any(Message.class), eq(null)))
-                .thenReturn(joinMessageResponse);
-        when(socketIOServer.getRoomOperations("room-1")).thenReturn(roomOperations);
+        when(mongoTemplate.updateFirst(any(Query.class), any(Update.class), eq("rooms")))
+                .thenReturn(UpdateResult.acknowledged(1, 1L, null));
 
         handler.handleJoinRoom(client, "room-1");
 
-        verify(roomRepository).addParticipant("room-1", "user-1");
+        verify(mongoTemplate).updateFirst(any(Query.class), any(Update.class), eq("rooms"));
         verify(client).joinRoom("room-1");
         verify(userRooms).add("user-1", "room-1");
-        verify(client).sendEvent(eq(JOIN_ROOM_SUCCESS), any());
-        verify(roomOperations).sendEvent(MESSAGE, joinMessageResponse);
-        verify(roomOperations).sendEvent(eq(PARTICIPANTS_UPDATE), any());
+        verify(joinAckDispatcher).request(client, "room-1", "user-1", "tester");
+        verify(postJoinDispatcher).scheduleParticipantsUpdate("room-1");
+    }
+
+    @Test
+    void handleJoinRoom_missingRoomStopsBeforeJoiningSocket() {
+        SocketUser socketUser = new SocketUser("user-1", "tester", "session-1", "socket-1");
+        when(client.get("user")).thenReturn(socketUser);
+        when(userRooms.isInRoom("user-1", "room-1")).thenReturn(false);
+        when(mongoTemplate.updateFirst(any(Query.class), any(Update.class), eq("rooms")))
+                .thenReturn(UpdateResult.acknowledged(0, 0L, null));
+
+        handler.handleJoinRoom(client, "room-1");
+
+        verify(client).sendEvent(eq(JOIN_ROOM_ERROR), any());
+        verify(client, never()).joinRoom(any());
+        verify(joinAckDispatcher, never()).request(any(), any(), any(), any());
     }
 }
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandlerTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandlerTest.java
index 76e6b007..f64d3ef1 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandlerTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomLeaveHandlerTest.java
@@ -1,34 +1,21 @@
 package com.ktb.chatapp.websocket.socketio.handler;
 
-import com.corundumstudio.socketio.BroadcastOperations;
 import com.corundumstudio.socketio.SocketIOClient;
-import com.corundumstudio.socketio.SocketIOServer;
-import com.ktb.chatapp.dto.MessageResponse;
-import com.ktb.chatapp.dto.UserResponse;
-import com.ktb.chatapp.model.Message;
-import com.ktb.chatapp.model.MessageType;
 import com.ktb.chatapp.model.Room;
 import com.ktb.chatapp.model.User;
-import com.ktb.chatapp.repository.MessageRepository;
 import com.ktb.chatapp.repository.RoomRepository;
 import com.ktb.chatapp.repository.UserRepository;
 import com.ktb.chatapp.websocket.socketio.SocketUser;
 import com.ktb.chatapp.websocket.socketio.UserRooms;
-import java.time.LocalDateTime;
-import java.util.List;
 import java.util.Optional;
 import java.util.Set;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
-import org.mockito.ArgumentCaptor;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 
 import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.ERROR;
-import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.MESSAGE;
-import static com.ktb.chatapp.websocket.socketio.SocketIOEvents.PARTICIPANTS_UPDATE;
-import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.never;
@@ -38,26 +25,17 @@
 @ExtendWith(MockitoExtension.class)
 class RoomLeaveHandlerTest {
 
-    @Mock private SocketIOServer socketIOServer;
-    @Mock private MessageRepository messageRepository;
     @Mock private RoomRepository roomRepository;
     @Mock private UserRepository userRepository;
     @Mock private UserRooms userRooms;
-    @Mock private MessageResponseMapper messageResponseMapper;
+    @Mock private RoomPostJoinDispatcher postJoinDispatcher;
     @Mock private SocketIOClient client;
-    @Mock private BroadcastOperations roomOperations;
 
     private RoomLeaveHandler handler;
 
     @BeforeEach
     void setUp() {
-        handler = new RoomLeaveHandler(
-                socketIOServer,
-                messageRepository,
-                roomRepository,
-                userRepository,
-                userRooms,
-                messageResponseMapper);
+        handler = new RoomLeaveHandler(roomRepository, userRepository, userRooms, postJoinDispatcher);
     }
 
     @Test
@@ -74,52 +52,22 @@ void handleLeaveRoom_rejectsUnauthorizedClient() {
     void handleLeaveRoom_removesParticipantAndBroadcasts() {
         SocketUser socketUser = new SocketUser("user-1", "tester", "session-1", "socket-1");
         User user = User.builder().id("user-1").name("tester").email("tester@example.com").build();
-        User remainingUser = User.builder().id("user-2").name("remaining").email("remaining@example.com").build();
         Room roomBeforeLeave = Room.builder()
                 .id("room-1")
                 .name("room")
                 .participantIds(Set.of("user-1", "user-2"))
                 .build();
-        Room roomAfterLeave = Room.builder()
-                .id("room-1")
-                .name("room")
-                .participantIds(Set.of("user-2"))
-                .build();
-        MessageResponse leaveMessageResponse = MessageResponse.builder()
-                .id("message-1")
-                .roomId("room-1")
-                .content("tester님이 퇴장하였습니다.")
-                .type(MessageType.system)
-                .timestamp(1L)
-                .build();
-
         when(client.get("user")).thenReturn(socketUser);
         when(userRooms.isInRoom("user-1", "room-1")).thenReturn(true);
         when(userRepository.findById("user-1")).thenReturn(Optional.of(user));
-        when(userRepository.findById("user-2")).thenReturn(Optional.of(remainingUser));
-        when(roomRepository.findById("room-1"))
-                .thenReturn(Optional.of(roomBeforeLeave), Optional.of(roomAfterLeave));
-        when(messageRepository.save(any(Message.class))).thenAnswer(invocation -> {
-            Message message = invocation.getArgument(0);
-            message.setId("message-1");
-            message.setTimestamp(LocalDateTime.now());
-            return message;
-        });
-        when(messageResponseMapper.mapToMessageResponse(any(Message.class), eq(null)))
-                .thenReturn(leaveMessageResponse);
-        when(socketIOServer.getRoomOperations("room-1")).thenReturn(roomOperations);
+        when(roomRepository.findById("room-1")).thenReturn(Optional.of(roomBeforeLeave));
 
         handler.handleLeaveRoom(client, "room-1");
 
         verify(roomRepository).removeParticipant("room-1", "user-1");
         verify(client).leaveRoom("room-1");
         verify(userRooms).remove("user-1", "room-1");
-        verify(roomOperations).sendEvent(MESSAGE, leaveMessageResponse);
-        ArgumentCaptor<Object> participantsCaptor = ArgumentCaptor.forClass(Object.class);
-        verify(roomOperations).sendEvent(eq(PARTICIPANTS_UPDATE), participantsCaptor.capture());
-        @SuppressWarnings("unchecked")
-        var participants = (java.util.List<UserResponse>) participantsCaptor.getValue();
-        assertEquals(List.of("user-2"), participants.stream().map(UserResponse::getId).toList());
-        verify(roomOperations, never()).sendEvent(eq("userLeft"), any());
+        verify(postJoinDispatcher).scheduleSystemMessage("room-1", "tester님이 퇴장하였습니다.");
+        verify(postJoinDispatcher).scheduleParticipantsUpdate("room-1");
     }
 }
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcherTest.java b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcherTest.java
new file mode 100644
index 00000000..63d42b7f
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/websocket/socketio/handler/RoomPostJoinDispatcherTest.java
@@ -0,0 +1,72 @@
+package com.ktb.chatapp.websocket.socketio.handler;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.corundumstudio.socketio.BroadcastOperations;
+import com.corundumstudio.socketio.SocketIOServer;
+import com.ktb.chatapp.model.Room;
+import com.ktb.chatapp.model.User;
+import com.ktb.chatapp.repository.MessageRepository;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import java.util.List;
+import java.util.Set;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.query.Query;
+
+@ExtendWith(MockitoExtension.class)
+class RoomPostJoinDispatcherTest {
+
+    @Mock private SocketIOServer socketIOServer;
+    @Mock private MessageRepository messageRepository;
+    @Mock private MongoTemplate mongoTemplate;
+    @Mock private MessageResponseMapper messageResponseMapper;
+    @Mock private BroadcastOperations roomOperations;
+
+    private RoomPostJoinDispatcher dispatcher;
+
+    @BeforeEach
+    void setUp() {
+        dispatcher = new RoomPostJoinDispatcher(
+                socketIOServer,
+                messageRepository,
+                mongoTemplate,
+                messageResponseMapper,
+                new SimpleMeterRegistry(),
+                1,
+                10,
+                25,
+                25);
+    }
+
+    @AfterEach
+    void tearDown() {
+        dispatcher.shutdown();
+    }
+
+    @Test
+    void repeatedParticipantUpdatesAreCoalescedIntoOneSnapshot() {
+        Room room = Room.builder().id("room-1").participantIds(Set.of("user-1")).build();
+        User user = User.builder().id("user-1").name("tester").email("tester@example.com").build();
+        when(mongoTemplate.findOne(any(Query.class), eq(Room.class))).thenReturn(room);
+        when(mongoTemplate.find(any(Query.class), eq(User.class))).thenReturn(List.of(user));
+        when(socketIOServer.getRoomOperations("room-1")).thenReturn(roomOperations);
+
+        dispatcher.scheduleParticipantsUpdate("room-1");
+        dispatcher.scheduleParticipantsUpdate("room-1");
+        dispatcher.scheduleParticipantsUpdate("room-1");
+
+        verify(roomOperations, org.mockito.Mockito.timeout(1_000).times(1))
+                .sendEvent(eq("participantsUpdate"), any());
+        verify(mongoTemplate, times(1)).findOne(any(Query.class), eq(Room.class));
+    }
+}
diff --git a/loadtest/load-test.js b/loadtest/load-test.js
index 9ffd1995..9ad567df 100644
--- a/loadtest/load-test.js
+++ b/loadtest/load-test.js
@@ -6,6 +6,8 @@ const yargs = require('yargs/yargs');
 const { hideBin } = require('yargs/helpers');
 const chalk = require('chalk');
 const Table = require('cli-table3');
+const fs = require('fs');
+const path = require('path');
 const { CLIENT_EMIT, SERVER_EMIT } = require('./socket-contract');
 
 // Parse command line arguments
@@ -60,6 +62,16 @@ const argv = yargs(hideBin(process.argv))
     type: 'number',
     default: 1000
   })
+  .option('start-index', {
+    description: 'Starting index for loadtest user identities',
+    type: 'number',
+    default: 0
+  })
+  .option('result-path', {
+    description: 'Write final machine-readable metrics to this JSON file',
+    type: 'string',
+    default: null
+  })
   .help()
   .alias('help', 'h')
   .argv;
@@ -262,16 +274,27 @@ class LoadTester {
           this.recordOperation('joinRoom', Date.now() - (socket.__loadtestJoinStart || Date.now()));
           this.log('info', `User ${userId} joined room ${roomId} with ${data.participants?.length || 0} participants`);
 
-          // Fetch previous messages before starting to send
-          socket.__loadtestPreviousStart = Date.now();
-          socket.emit(CLIENT_EMIT.FETCH_PREVIOUS_MESSAGES, { roomId: roomId, limit: 30 });
-
-          // Start sending messages
-          this.sendMessages(socket, userId, roomId);
+          if (Array.isArray(data.messages)) {
+            socket.__roomReadyRecorded = true;
+            this.recordOperation('roomReady', Date.now() - socket.__loadtestJoinStart);
+            socket.__messagesStarted = true;
+            this.sendMessages(socket, userId, roomId);
+          } else {
+            socket.__loadtestPreviousStart = Date.now();
+            socket.emit(CLIENT_EMIT.FETCH_PREVIOUS_MESSAGES, { roomId: roomId, limit: 30 });
+          }
         });
 
         socket.on(SERVER_EMIT.PREVIOUS_MESSAGES_LOADED, (data) => {
           this.recordOperation('fetchPreviousMessages', Date.now() - (socket.__loadtestPreviousStart || Date.now()));
+          if (!socket.__roomReadyRecorded) {
+            socket.__roomReadyRecorded = true;
+            this.recordOperation('roomReady', Date.now() - socket.__loadtestJoinStart);
+          }
+          if (!socket.__messagesStarted) {
+            socket.__messagesStarted = true;
+            this.sendMessages(socket, userId, roomId);
+          }
           this.metrics.previousMessagesFetched++;
           if (data.messages?.length) {
             this.metrics.messagesReceived += data.messages.length;
@@ -478,6 +501,61 @@ class LoadTester {
     return sorted[index];
   }
 
+  writeResult() {
+    if (!this.config.resultPath) return;
+
+    const summarize = operation => {
+      const latencies = operation.latencies || [];
+      const sum = latencies.reduce((total, value) => total + value, 0);
+      return {
+        count: operation.count,
+        errors: operation.errors,
+        errorRate: operation.count ? operation.errors / operation.count : 0,
+        averageMs: latencies.length ? sum / latencies.length : 0,
+        p50Ms: this.getPercentile(latencies, 50),
+        p95Ms: this.getPercentile(latencies, 95),
+        p99Ms: this.getPercentile(latencies, 99),
+        maxMs: latencies.length ? Math.max(...latencies) : 0
+      };
+    };
+
+    const resultPath = path.resolve(this.config.resultPath);
+    const operations = Object.fromEntries(
+      Object.entries(this.metrics.operations).map(([name, operation]) => [name, summarize(operation)])
+    );
+    const result = {
+      generatedAt: new Date().toISOString(),
+      elapsedMs: Date.now() - this.metrics.startTime,
+      config: {
+        users: this.config.totalUsers,
+        messagesPerUser: this.config.messages,
+        batchSize: this.config.batchSize,
+        batchDelayMs: this.config.batchDelay,
+        apiUrl: this.config.apiUrl,
+        socketUrl: this.config.socketUrl,
+        roomId: this.config.roomId,
+        startIndex: this.config.startIndex
+      },
+      totals: {
+        usersCreated: this.metrics.usersCreated,
+        connected: this.metrics.connected,
+        disconnected: this.metrics.disconnected,
+        authErrors: this.metrics.errorsAuth,
+        connectionErrors: this.metrics.errorsConnection,
+        messageErrors: this.metrics.errorsMessage
+      },
+      connection: summarize({
+        count: this.metrics.connectionTimes.length,
+        errors: this.metrics.errorsConnection,
+        latencies: this.metrics.connectionTimes
+      }),
+      operations
+    };
+
+    fs.writeFileSync(resultPath, `${JSON.stringify(result, null, 2)}\n`);
+    console.log(chalk.gray(`Metrics JSON: ${resultPath}`));
+  }
+
   async run() {
     const { totalUsers, rampUpTime, batchSize, batchDelay } = this.config;
     const totalBatches = Math.ceil(totalUsers / batchSize);
@@ -525,7 +603,7 @@ class LoadTester {
 
       // Spawn all users in this batch simultaneously
       for (let i = batchStart; i < batchEnd; i++) {
-        promises.push(this.simulateUser(i, roomId));
+        promises.push(this.simulateUser(this.config.startIndex + i, roomId));
       }
 
       // Wait before starting next batch (except for the last batch)
@@ -543,6 +621,7 @@ class LoadTester {
     clearInterval(this.metricsInterval);
     this.printMetrics();
     this.printOperationRanking();
+    this.writeResult();
 
     console.log(chalk.bold.green('\n✓ Load test completed!\n'));
     process.exit(0);
@@ -559,7 +638,9 @@ const tester = new LoadTester({
   duration: argv.duration,
   messages: argv.messages,
   batchSize: argv.batchSize,
-  batchDelay: argv.batchDelay
+  batchDelay: argv.batchDelay,
+  startIndex: argv.startIndex,
+  resultPath: argv.resultPath
 });
 
 tester.run().catch(error => {
diff --git a/loadtest/ramp-up-test.js b/loadtest/ramp-up-test.js
index 9fff80c2..c1398255 100644
--- a/loadtest/ramp-up-test.js
+++ b/loadtest/ramp-up-test.js
@@ -597,6 +597,19 @@ class RampUpLoadTester {
 
       this.sockets.push(socket);
 
+      let roomReadyRecorded = false;
+      const markRoomReady = () => {
+        if (roomReadyRecorded) return;
+        roomReadyRecorded = true;
+        onboardingStages.tti = Date.now() - onboardingStart;
+        this.metrics.onboardingCohorts.push({
+          arrivedAt,
+          activeUsersAtArrival,
+          stages: onboardingStages
+        });
+        this.startContinuousMessaging(socket, userId, roomId, token);
+      };
+
       socket.on('connect', () => {
         const connectionTime = Date.now() - connectStartTime;
         this.metrics.connected++;
@@ -611,24 +624,16 @@ class RampUpLoadTester {
 
       socket.on(SERVER_EMIT.JOIN_ROOM_SUCCESS, (data) => {
         this.log('info', `User ${userId} joined room ${roomId} via WebSocket`);
+        onboardingStages.join_ack = Date.now() - onboardingStart;
 
-        // TTI(도착~첫 메시지 성공 기준점)로 joinRoomSuccess 시점을 기록하고,
-        // 코호트를 도착 시점의 활성 사용자 수로 태깅해 온보딩 P95 산출에 쓴다.
-        onboardingStages.tti = Date.now() - onboardingStart;
-        this.metrics.onboardingCohorts.push({
-          arrivedAt,
-          activeUsersAtArrival,
-          stages: onboardingStages
-        });
-
-        // Step 5: WebSocket - Fetch Previous Messages
-        socket.emit(CLIENT_EMIT.FETCH_PREVIOUS_MESSAGES, {
-          roomId: roomId,
-          limit: 30
-        });
-
-        // Start continuous message sending
-        this.startContinuousMessaging(socket, userId, roomId, token);
+        if (Array.isArray(data.messages)) {
+          markRoomReady();
+        } else {
+          socket.emit(CLIENT_EMIT.FETCH_PREVIOUS_MESSAGES, {
+            roomId: roomId,
+            limit: 30
+          });
+        }
       });
 
       socket.on(SERVER_EMIT.JOIN_ROOM_ERROR, (error) => {
@@ -645,6 +650,7 @@ class RampUpLoadTester {
         if (messageCount > 0) {
           this.metrics.messagesReceived += messageCount;
         }
+        markRoomReady();
       });
 
       socket.on(SERVER_EMIT.MESSAGE, (data) => {
diff --git a/performance/room-join/fixture.js b/performance/room-join/fixture.js
index 111ec77d..37efba9f 100644
--- a/performance/room-join/fixture.js
+++ b/performance/room-join/fixture.js
@@ -8,7 +8,7 @@ const passwordHash = process.env.PASSWORD_HASH;
 const fixedMessages = integerEnv('FIXED_MESSAGES', 1000);
 const messageAxis = intList(process.env.MESSAGE_AXIS || '1000,10000,100000');
 const participantAxis = intList(process.env.PARTICIPANT_AXIS || '10,100,500');
-const maxVus = integerEnv('MAX_VUS', 100);
+const maxVus = integerEnv('MAX_VUS', 1200);
 
 if (!testId || !/^[a-zA-Z0-9][a-zA-Z0-9_-]{0,63}$/.test(testId)) fail('invalid or missing TEST_ID');
 if (action === 'create' && !passwordHash) fail('PASSWORD_HASH is required for create');

From 43ea501e5dde4ccd59e0f99c327abbfc22a3653f Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Tue, 11 Aug 2026 23:37:36 +0900
Subject: [PATCH 29/32] =?UTF-8?q?=EC=B1=84=ED=8C=85=EB=B0=A9=20=EC=83=9D?=
 =?UTF-8?q?=EC=84=B1=20=EC=95=88=EC=A0=95=ED=99=94=20=EB=B0=8F=20=EC=A7=84?=
 =?UTF-8?q?=EB=8B=A8=20=EA=B0=95=ED=99=94?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../com/ktb/chatapp/config/MongoConfig.java   |  20 +++
 .../chatapp/controller/RoomController.java    |   3 +-
 .../com/ktb/chatapp/service/RoomService.java  |   6 +-
 .../ktb/chatapp/config/MongoConfigTest.java   |  24 ++++
 apps/frontend/app/login/page.js               |  17 ---
 .../chat/rooms/__tests__/useRoomList.test.js  |  28 ++++-
 .../features/chat/rooms/useRoomList.js        |   5 +-
 apps/frontend/pages/chat/new.js               |  16 +--
 apps/frontend/pages/login.js                  |   1 +
 e2e/artillery/README.md                       |  18 +++
 e2e/artillery/ROOM_CREATION_DIAGNOSIS.md      |  84 +++++++++++++
 e2e/artillery/diagnose-room-creation.js       |  64 ++++++++++
 e2e/artillery/observation.js                  | 115 +++++++++++++++++-
 e2e/artillery/observation.test.js             |  39 +++++-
 14 files changed, 393 insertions(+), 47 deletions(-)
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/config/MongoConfigTest.java
 delete mode 100644 apps/frontend/app/login/page.js
 create mode 100644 apps/frontend/pages/login.js
 create mode 100644 e2e/artillery/ROOM_CREATION_DIAGNOSIS.md
 create mode 100644 e2e/artillery/diagnose-room-creation.js

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/MongoConfig.java b/apps/backend/src/main/java/com/ktb/chatapp/config/MongoConfig.java
index 19757fd5..e6866e42 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/config/MongoConfig.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/MongoConfig.java
@@ -1,9 +1,29 @@
 package com.ktb.chatapp.config;
 
+import com.ktb.chatapp.model.MessageType;
+import java.util.List;
+import java.util.Locale;
+import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.convert.converter.Converter;
 import org.springframework.data.mongodb.config.EnableMongoAuditing;
+import org.springframework.data.mongodb.core.convert.MongoCustomConversions;
+import org.springframework.data.convert.ReadingConverter;
 
 @Configuration
 @EnableMongoAuditing
 public class MongoConfig {
+
+    @Bean
+    MongoCustomConversions mongoCustomConversions() {
+        return new MongoCustomConversions(List.of(new MessageTypeReadConverter()));
+    }
+
+    @ReadingConverter
+    static class MessageTypeReadConverter implements Converter<String, MessageType> {
+        @Override
+        public MessageType convert(String source) {
+            return MessageType.valueOf(source.toLowerCase(Locale.ROOT));
+        }
+    }
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
index e9fdbf58..40b74f76 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
@@ -155,8 +155,7 @@ public ResponseEntity<?> createRoom(@Valid @RequestBody CreateRoomRequest create
                 );
             }
 
-            Room savedRoom = roomService.createRoom(createRoomRequest, principal.getName());
-            RoomResponse roomResponse = mapToRoomResponse(savedRoom, principal.getName());
+            RoomResponse roomResponse = roomService.createRoom(createRoomRequest, principal.getName());
 
             return ResponseEntity.status(201).body(
                 Map.of(
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
index 9e073fcf..b1ab821e 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
@@ -145,7 +145,7 @@ public HealthResponse getHealthStatus() {
         }
     }
 
-    public Room createRoom(CreateRoomRequest createRoomRequest, String name) {
+    public RoomResponse createRoom(CreateRoomRequest createRoomRequest, String name) {
         User creator = userRepository.findByEmail(name)
             .orElseThrow(() -> new RuntimeException("사용자를 찾을 수 없습니다: " + name));
 
@@ -160,16 +160,16 @@ public Room createRoom(CreateRoomRequest createRoomRequest, String name) {
         }
 
         Room savedRoom = roomRepository.save(room);
+        RoomResponse roomResponse = mapToRoomResponse(savedRoom, name);
         
         // Publish event for room created
         try {
-            RoomResponse roomResponse = mapToRoomResponse(savedRoom, name);
             eventPublisher.publishEvent(new RoomCreatedEvent(this, roomResponse));
         } catch (Exception e) {
             log.error("roomCreated 이벤트 발행 실패", e);
         }
         
-        return savedRoom;
+        return roomResponse;
     }
 
     public Optional<Room> findRoomById(String roomId) {
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/config/MongoConfigTest.java b/apps/backend/src/test/java/com/ktb/chatapp/config/MongoConfigTest.java
new file mode 100644
index 00000000..6667537c
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/config/MongoConfigTest.java
@@ -0,0 +1,24 @@
+package com.ktb.chatapp.config;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import com.ktb.chatapp.model.MessageType;
+import org.junit.jupiter.api.Test;
+
+class MongoConfigTest {
+
+    private final MongoConfig.MessageTypeReadConverter converter =
+            new MongoConfig.MessageTypeReadConverter();
+
+    @Test
+    void readsLegacyUppercaseMessageType() {
+        assertEquals(MessageType.text, converter.convert("TEXT"));
+        assertEquals(MessageType.system, converter.convert("SYSTEM"));
+    }
+
+    @Test
+    void keepsCurrentLowercaseMessageTypeCompatible() {
+        assertEquals(MessageType.ai, converter.convert("ai"));
+        assertEquals(MessageType.file, converter.convert("file"));
+    }
+}
diff --git a/apps/frontend/app/login/page.js b/apps/frontend/app/login/page.js
deleted file mode 100644
index 925c8c89..00000000
--- a/apps/frontend/app/login/page.js
+++ /dev/null
@@ -1,17 +0,0 @@
-'use client';
-
-import { useEffect } from 'react';
-import { useRouter } from 'next/navigation';
-
-const LoginRedirectPage = () => {
-  const router = useRouter();
-
-  useEffect(() => {
-    const queryString = window.location.search;
-    router.replace(queryString ? `/${queryString}` : '/');
-  }, [router]);
-
-  return null;
-};
-
-export default LoginRedirectPage;
diff --git a/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js b/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
index fd959b17..d3e29169 100644
--- a/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
@@ -13,9 +13,9 @@ vi.mock('@/services/axios', () => ({
 
 const roomsResponse = (rooms) => ({ data: { data: rooms } });
 
-const renderRoomList = () =>
-  renderHook(() =>
-    useRoomList({
+const renderRoomList = (overrides = {}) =>
+  renderHook(() => {
+    const props = {
       currentUser: { token: 'token-1' },
       router: { push: vi.fn() },
       connectionStatus: CONNECTION_STATUS.CONNECTED,
@@ -26,14 +26,32 @@ const renderRoomList = () =>
       setIsRetrying: vi.fn(),
       getRetryDelay: vi.fn(() => 1000),
       attemptConnection: vi.fn(() => Promise.resolve(true)),
-    })
-  );
+      ...overrides,
+    };
+    return useRoomList(props);
+  });
 
 describe('useRoomList', () => {
   beforeEach(() => {
     vi.clearAllMocks();
   });
 
+  it('loads rooms directly without a redundant health preflight', async () => {
+    const attemptConnection = vi.fn(() => Promise.resolve(true));
+    const setConnectionStatus = vi.fn();
+    axiosInstance.get.mockResolvedValue(roomsResponse([]));
+
+    const { result } = renderRoomList({ attemptConnection, setConnectionStatus });
+
+    await act(async () => {
+      await result.current.fetchRooms();
+    });
+
+    expect(attemptConnection).not.toHaveBeenCalled();
+    expect(axiosInstance.get).toHaveBeenCalledWith('/api/rooms');
+    expect(setConnectionStatus).toHaveBeenCalledWith(CONNECTION_STATUS.CONNECTED);
+  });
+
   it('replaces the list on refresh without leaving the refreshing flag on', async () => {
     axiosInstance.get.mockResolvedValue(roomsResponse([{ _id: 'room-1' }]));
 
diff --git a/apps/frontend/features/chat/rooms/useRoomList.js b/apps/frontend/features/chat/rooms/useRoomList.js
index a4dbf82c..fce4d5f1 100644
--- a/apps/frontend/features/chat/rooms/useRoomList.js
+++ b/apps/frontend/features/chat/rooms/useRoomList.js
@@ -57,8 +57,6 @@ export const useRoomList = ({
   }, [isRetrying, setConnectionStatus]);
 
   const loadRooms = useCallback(async () => {
-    await attemptConnection();
-
     const response = await axiosInstance.get('/api/rooms');
 
     if (!response?.data?.data) {
@@ -66,7 +64,8 @@ export const useRoomList = ({
     }
 
     setRooms(response.data.data);
-  }, [attemptConnection]);
+    setConnectionStatus(CONNECTION_STATUS.CONNECTED);
+  }, [setConnectionStatus]);
 
   const fetchRooms = useCallback(async () => {
     if (!currentUser?.token || isLoadingRef.current) {
diff --git a/apps/frontend/pages/chat/new.js b/apps/frontend/pages/chat/new.js
index c9914289..ff9758b1 100644
--- a/apps/frontend/pages/chat/new.js
+++ b/apps/frontend/pages/chat/new.js
@@ -28,19 +28,6 @@ function NewChatRoom() {
   const [submissionStage, setSubmissionStage] = useState('idle');
   const [error, setError] = useState('');
 
-  const joinRoom = async (roomId, password) => {
-    try {
-      setSubmissionStage('joining');
-      await api.post(`/api/rooms/${roomId}/join`, { password });
-
-      setSubmissionStage('navigating');
-      await router.push(`/chat/${roomId}`);
-    } catch (error) {
-      console.error('Room join error:', error);
-      throw error;
-    }
-  };
-
   const handleSubmit = async (e) => {
     e.preventDefault();
 
@@ -70,7 +57,8 @@ function NewChatRoom() {
       });
 
       const { data } = response.data;
-      await joinRoom(data._id, formData.hasPassword ? formData.password : undefined);
+      setSubmissionStage('navigating');
+      await router.push(`/chat/${data._id}`);
 
     } catch (error) {
       console.error('Room creation/join error:', error);
diff --git a/apps/frontend/pages/login.js b/apps/frontend/pages/login.js
new file mode 100644
index 00000000..dd7dc300
--- /dev/null
+++ b/apps/frontend/pages/login.js
@@ -0,0 +1 @@
+export { default } from './index';
diff --git a/e2e/artillery/README.md b/e2e/artillery/README.md
index 230102a3..53ebf756 100644
--- a/e2e/artillery/README.md
+++ b/e2e/artillery/README.md
@@ -32,8 +32,26 @@ pnpm --filter e2e exec artillery run artillery/artillery-config.yaml
 
 node e2e/artillery/report-observations.js \
   e2e/artillery/results/stage0-run1
+
+# 방 생성 후 입력창 노출 실패를 원인별로 분류
+node e2e/artillery/diagnose-room-creation.js \
+  e2e/artillery/results/stage0-run1
 ```
 
+`diagnose-room-creation.js`는 기존 시나리오의 5초 어서션을 변경하지 않는다. 실패한
+VU만 최대 15초 추가 관찰하여 늦은 렌더링인지 영구 실패인지 구분하고, 방 상세 REST,
+Socket.IO 연결, `joinRoomSuccess`, 브라우저 오류를 함께 저장한다. 결과는 실행 디렉터리의
+`room-creation-diagnosis.json`에 기록된다.
+
+백엔드는 `/actuator/prometheus`에 다음 입장 처리 지표를 제공한다.
+
+- `chat_room_join_total_duration_seconds`: 결과별 전체 `joinRoom` 처리시간
+- `chat_room_join_step_duration_seconds`: 사용자·방 조회, 참가자 갱신, 메시지 저장·조회,
+  참가자 직렬화, 성공 이벤트 전송의 단계별 처리시간
+
+`room_join_trace` 로그는 `roomId`, `socketId`, 결과, 전체 시간과 단계별 시간을 한 줄에
+남겨 브라우저 관측 결과와 연결할 수 있다.
+
 각 VU의 원시 JSON과 합산 `summary.json`이 지정한 실행 디렉터리에 저장됩니다. 표는
 호출 수, 성공/실패, 평균, p95, p99, 누적시간 및 같은 계층 내 누적시간 비중을 보여 줍니다.
 HTTP의 4xx/5xx는 실패로 집계되므로 의도된 로그인 실패 시나리오의 401은 행동 성공 여부와
diff --git a/e2e/artillery/ROOM_CREATION_DIAGNOSIS.md b/e2e/artillery/ROOM_CREATION_DIAGNOSIS.md
new file mode 100644
index 00000000..54a7b917
--- /dev/null
+++ b/e2e/artillery/ROOM_CREATION_DIAGNOSIS.md
@@ -0,0 +1,84 @@
+# 채팅방 생성 입력창 노출 실패 진단
+
+## 확정된 실행 경로
+
+`chatRoomCreationScenario`의 실패 어서션은 방 목록이 아니라 `/chat/{id}`의
+`chat-message-input`을 기다린다. 입력창이 나타나려면 다음 경로가 모두 완료돼야 한다.
+
+1. `POST /api/rooms`
+2. `POST /api/rooms/{roomId}/join`
+3. `/chat/{roomId}` 이동
+4. `GET /api/rooms/{roomId}`
+5. Socket.IO 연결
+6. `joinRoom` → `joinRoomSuccess`
+7. 프런트엔드 `setupSucceeded` 후 입력창 렌더링
+
+따라서 `/chat` 방 목록 LCP나 `room-list` 브로드캐스트는 직접 선행조건이 아니다. 시스템
+자원을 고갈시키는 간접 원인일 수는 있지만 별도 계측 없이 직접 원인으로 판단하지 않는다.
+
+## 현재 확보된 DB 증거
+
+기존 MongoDB profiler 자료인
+`results/e2e-api-priority-20260811-profiler-evidence.json`에서 방 참여 요청 한 번은 다음 작업을
+발생시켰다.
+
+- MongoDB 명령 10~11개
+- 메시지 컬렉션 전체 스캔 2회, 약 16,000개 문서 검사
+- 세션 컬렉션 전체 스캔 1회, 536개 문서 검사
+- 사용자 이메일 조회 전체 스캔 1회, 566개 문서 검사
+- 사용자 ID 단건 조회 4회
+
+저부하에서는 `room_create` 구간이 약 50ms였으므로 이 자료만으로 5초 초과의 단일 원인을
+확정할 수는 없다. 다만 동시 요청이 증가할 때 MongoDB 작업량이 선형 이상으로 증폭될 수 있는
+구체적인 병목 후보다. 특히 REST 참여 후 새 페이지에서 Socket.IO 참여가 다시 실행되므로 방
+하나 생성에 참여 경로가 두 번 수행된다.
+
+## 추가된 판별 수단
+
+- 실패 VU에서 원래 5초 어서션을 유지한 채 최대 15초까지 입력창을 추가 관찰한다.
+- REST 요청, 전체 Socket.IO 이벤트, 내비게이션, 브라우저 오류를 VU별 타임라인으로 저장한다.
+- `diagnose-room-creation.js`가 실패를 다음으로 자동 분류한다.
+  `late_visibility`, `room_detail_http_failure`, `room_detail_http_slow`,
+  `socket_connection`, `join_room_response`, `frontend_state_or_render`, `error_surface`.
+- 백엔드 Prometheus 지표가 `RoomJoinHandler`의 사용자·방 조회, 참가자 갱신, 입장 메시지 저장,
+  메시지 로드, 방 재조회, 참가자 직렬화, 성공 이벤트 전송 시간을 구간별로 기록한다.
+- `room_join_trace` 구조화 로그는 roomId와 socketId로 브라우저 타임라인과 연결한다.
+
+## 판정 순서와 수정 우선순위
+
+1. `late_visibility`가 대부분이면 타임아웃을 늘리지 말고 가장 큰 서버 단계의 p95를 줄인다.
+2. `room_detail_http_*`면 상세 응답 매핑과 MongoDB 조회·인덱스를 우선 수정한다.
+3. `join_room_response`면 `RoomJoinHandler` 단계 지표에서 가장 큰 DB/직렬화 구간을 수정한다.
+4. `frontend_state_or_render`면 `joinRoomSuccess` 이후 상태 전이와 React 렌더를 조사한다.
+5. DB가 원인으로 확인되면 메시지 `(roomId, timestamp)` 인덱스, 세션/사용자 조회 인덱스,
+   참가자 배치 조회, 중복 응답 매핑 제거 순으로 검증한다.
+
+## 적용한 수정
+
+- 메시지 조회 반환형을 `Page`에서 `Slice`로 변경해 매 요청마다 실행되던 별도 count 쿼리를
+  제거했다. MongoDB 실행계획상 기존 `(room, timestamp)` 인덱스는 역방향 스캔으로 정상
+  사용되므로 중복 인덱스는 추가하지 않았다.
+- 메시지 발신자, 방 참가자와 방 상세 참가자 조회를 ID별 반복 조회에서 `findAllById` 배치
+  조회로 변경했다.
+- 최대 30개 초기 메시지의 읽음 상태를 각각 `findById + save`하던 처리를 단일 MongoDB
+  `updateMulti`로 변경했다.
+- 방 생성 응답과 `roomCreated` 이벤트에 쓰는 `RoomResponse`를 한 번만 생성해 중복 사용자 조회와
+  최근 메시지 count를 제거했다.
+
+## 로컬 검증 상태
+
+관측 및 분류 단위 테스트는 통과했다. `/login`이 빈 클라이언트 리다이렉트를 렌더링하고 서버
+헬스 체크 중 로그인 폼을 숨기던 문제를 프런트 라우트에서 수정했다. 기존 Artillery 시나리오와
+액션은 변경하지 않았다.
+
+수정된 백엔드를 `performance/room-join` 스택에 재배포한 뒤 기존 10만 메시지 fixture에서
+HTTP 방 참여 경로를 확인했다.
+
+- 1 VU × 1회: 84.3ms, 오류 0건
+- 10 VU × 100회: 평균 38.5ms, p95 63.8ms, p99 89.0ms, 최대 94.2ms
+- 타임아웃과 계약 위반 모두 0건
+
+이 결과는 REST 참여 경로의 회귀가 없음을 확인한다. 원래 실패 지점인 브라우저 → Socket.IO →
+입력창 렌더링 전체 경로도 기존 시나리오 1 VU로 완주했다. `room_create`는 706.3ms였고 실패는
+0건이었다. 방 생성 POST는 47.9ms, 방 상세 GET p95는 28.8ms, Socket.IO `room_join` p95는
+90.3ms였다.
diff --git a/e2e/artillery/diagnose-room-creation.js b/e2e/artillery/diagnose-room-creation.js
new file mode 100644
index 00000000..6b9cb474
--- /dev/null
+++ b/e2e/artillery/diagnose-room-creation.js
@@ -0,0 +1,64 @@
+#!/usr/bin/env node
+const fs = require('fs');
+const path = require('path');
+const { classifyRoomCreateFailure, summarizeSamples } = require('./observation');
+
+const runDirectory = path.resolve(process.argv[2] || 'artillery/results');
+const files = fs.readdirSync(runDirectory)
+    .filter((file) => file.startsWith('vu-') && file.endsWith('.json'));
+if (files.length === 0) {
+    throw new Error(`No observation JSON files found in ${runDirectory}`);
+}
+
+const reports = files.map((file) => ({
+    file,
+    report: JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8')),
+}));
+const failures = reports.filter(({ report }) =>
+    report.samples?.actions?.some((sample) => sample.name === 'room_create' && !sample.success));
+const classifications = failures.map(({ file, report }) => ({
+    file,
+    classification: classifyRoomCreateFailure(report),
+    diagnostic: report.diagnostics?.find((item) => item.action === 'room_create') || null,
+    http: report.samples?.http?.filter((sample) => sample.action === 'room_create') || [],
+    socket: report.samples?.socket?.filter((sample) => sample.action === 'room_create') || [],
+    timeline: report.timeline || [],
+    browser: report.samples?.browser || [],
+}));
+const counts = classifications.reduce((result, item) => {
+    result[item.classification] = (result[item.classification] || 0) + 1;
+    return result;
+}, {});
+const relevantHttp = reports.flatMap(({ report }) => report.samples?.http || [])
+    .filter((sample) => [
+        'POST /api/rooms',
+        'POST /api/rooms/{roomId}/join',
+        'GET /api/rooms/{roomId}',
+    ].includes(sample.name));
+const relevantSocket = reports.flatMap(({ report }) => report.samples?.socket || [])
+    .filter((sample) => ['connection', 'room_join'].includes(sample.name));
+const unclassified = counts.unclassified || 0;
+const output = {
+    schemaVersion: 1,
+    generatedAt: new Date().toISOString(),
+    runDirectory,
+    virtualUsers: reports.length,
+    roomCreateFailures: failures.length,
+    classificationCounts: counts,
+    unclassifiedPct: failures.length === 0 ? 0 : Number(((unclassified / failures.length) * 100).toFixed(1)),
+    summary: {
+        http: summarizeSamples(relevantHttp),
+        socket: summarizeSamples(relevantSocket),
+    },
+    failures: classifications,
+};
+const outputPath = path.join(runDirectory, 'room-creation-diagnosis.json');
+fs.writeFileSync(outputPath, `${JSON.stringify(output, null, 2)}\n`);
+console.log(outputPath);
+console.log(JSON.stringify({
+    virtualUsers: output.virtualUsers,
+    roomCreateFailures: output.roomCreateFailures,
+    classificationCounts: output.classificationCounts,
+    unclassifiedPct: output.unclassifiedPct,
+    summary: output.summary,
+}, null, 2));
diff --git a/e2e/artillery/observation.js b/e2e/artillery/observation.js
index e3d19464..a8845fbb 100644
--- a/e2e/artillery/observation.js
+++ b/e2e/artillery/observation.js
@@ -5,6 +5,9 @@ const { randomUUID } = require('crypto');
 const OBJECT_ID = /^[a-f\d]{24}$/i;
 const UUID = /^[a-f\d]{8}-[a-f\d]{4}-[1-5][a-f\d]{3}-[89ab][a-f\d]{3}-[a-f\d]{12}$/i;
 const INTEGER = /^\d+$/;
+const CHAT_ROOM_URL = /\/chat\/[a-f\d]{24}(?:[/?#]|$)/i;
+const CHAT_INPUT_TEST_ID = 'chat-message-input';
+const LATE_VISIBILITY_TIMEOUT_MS = 15000;
 
 const SOCKET_PAIRS = {
     joinRoom: { response: 'joinRoomSuccess', metric: 'room_join' },
@@ -84,13 +87,70 @@ function summarizeSamples(samples) {
     }).sort((a, b) => b.totalDurationMs - a.totalDurationMs);
 }
 
+function classifyRoomCreateFailure(report) {
+    const diagnostic = report.diagnostics?.find((item) => item.action === 'room_create');
+    if (!diagnostic) return 'unclassified';
+    if (diagnostic.becameVisibleWithin15s) return 'late_visibility';
+
+    const detailRequests = report.samples?.http?.filter((sample) =>
+        sample.name === 'GET /api/rooms/{roomId}') || [];
+    if (detailRequests.some((sample) => sample.success === false || sample.status >= 400)) {
+        return 'room_detail_http_failure';
+    }
+    if (detailRequests.some((sample) => sample.durationMs >= 5000)) {
+        return 'room_detail_http_slow';
+    }
+
+    const socketSamples = report.samples?.socket || [];
+    if (!socketSamples.some((sample) => sample.name === 'connection' && sample.success)) {
+        return 'socket_connection';
+    }
+
+    const timeline = report.timeline || [];
+    const joinSent = timeline.some((item) => item.name === 'socket.sent' && item.event === 'joinRoom');
+    const joinSucceeded = timeline.some((item) =>
+        item.name === 'socket.received' && item.event === 'joinRoomSuccess');
+    if (joinSent && !joinSucceeded) return 'join_room_response';
+    if (joinSucceeded) return 'frontend_state_or_render';
+    if (diagnostic.errorSurface) return 'error_surface';
+    return 'unclassified';
+}
+
 function createObservation(page, vuContext) {
     const startedRequests = new Map();
     const pendingSocketEvents = new Map();
-    const samples = { documents: [], http: [], socket: [], actions: [], layoutShifts: [] };
+    const samples = {
+        documents: [], http: [], socket: [], actions: [], browser: [], layoutShifts: [],
+    };
+    const timeline = [];
+    const diagnostics = [];
     let currentAction = null;
     let socketSequence = 0;
 
+    const addTimelineEvent = (name, details = {}) => {
+        timeline.push({ name, at: new Date().toISOString(), ...details });
+    };
+
+    const onFrameNavigated = (frame) => {
+        if (frame !== page.mainFrame()) return;
+        addTimelineEvent('navigation', { url: frame.url() });
+    };
+    const onConsole = (message) => {
+        if (!['error', 'warning'].includes(message.type())) return;
+        samples.browser.push({
+            name: `console.${message.type()}`,
+            message: message.text(),
+            at: new Date().toISOString(),
+            url: page.url(),
+        });
+    };
+    const onPageError = (error) => {
+        samples.browser.push({
+            name: 'pageerror', message: error.message, stack: error.stack,
+            at: new Date().toISOString(), url: page.url(),
+        });
+    };
+
     const recordHttp = async (request, success) => {
         const started = startedRequests.get(request);
         startedRequests.delete(request);
@@ -138,6 +198,7 @@ function createObservation(page, vuContext) {
 
         webSocket.on('framesent', ({ payload }) => {
             const event = parseSocketEvent(payload);
+            if (event) addTimelineEvent('socket.sent', { event });
             const pair = SOCKET_PAIRS[event];
             if (!pair) return;
             const queue = pendingSocketEvents.get(pair.response) || [];
@@ -159,6 +220,7 @@ function createObservation(page, vuContext) {
                 }
             }
             const event = parseSocketEvent(payload);
+            if (event) addTimelineEvent('socket.received', { event });
             const queue = pendingSocketEvents.get(event);
             const pending = Array.isArray(queue) ? queue.shift() : null;
             if (pending) {
@@ -175,6 +237,44 @@ function createObservation(page, vuContext) {
     page.on('requestfailed', onRequestFailed);
     page.on('response', onResponse);
     page.on('websocket', onWebSocket);
+    page.on('framenavigated', onFrameNavigated);
+    page.on('console', onConsole);
+    page.on('pageerror', onPageError);
+
+    const diagnoseRoomCreateFailure = async (error, actionStartedAt) => {
+        const diagnostic = {
+            action: 'room_create',
+            error: error.message,
+            failedAt: new Date().toISOString(),
+            urlAtFailure: page.url(),
+            reachedChatRoomUrl: CHAT_ROOM_URL.test(page.url()),
+            inputVisibleAtFailure: false,
+            becameVisibleWithin15s: false,
+            inputVisibleAfterMs: null,
+            finalUrl: null,
+            errorSurface: null,
+        };
+        try {
+            diagnostic.inputVisibleAtFailure = await page.getByTestId(CHAT_INPUT_TEST_ID).isVisible();
+            if (!diagnostic.inputVisibleAtFailure && diagnostic.reachedChatRoomUrl) {
+                await page.getByTestId(CHAT_INPUT_TEST_ID).waitFor({
+                    state: 'visible', timeout: LATE_VISIBILITY_TIMEOUT_MS,
+                });
+            }
+            if (diagnostic.inputVisibleAtFailure || diagnostic.reachedChatRoomUrl) {
+                diagnostic.becameVisibleWithin15s = true;
+                diagnostic.inputVisibleAfterMs = Number((performance.now() - actionStartedAt).toFixed(1));
+                addTimelineEvent('chat-input.visible', { durationMs: diagnostic.inputVisibleAfterMs });
+            }
+        } catch {
+            diagnostic.errorSurface = await page.locator('[role="alert"], .chat-container').allTextContents()
+                .then((values) => values.join(' ').trim().slice(0, 1000))
+                .catch(() => null);
+        }
+        diagnostic.finalUrl = page.url();
+        diagnostic.reachedChatRoomUrl ||= CHAT_ROOM_URL.test(diagnostic.finalUrl);
+        diagnostics.push(diagnostic);
+    };
 
     void page.addInitScript(() => {
         window.__e2eLayoutShifts = [];
@@ -238,6 +338,7 @@ function createObservation(page, vuContext) {
         currentAction = name;
         const startedAt = performance.now();
         const wallStartedAt = new Date().toISOString();
+        addTimelineEvent('action.started', { action: name });
         try {
             const result = await callback();
             samples.actions.push({
@@ -252,8 +353,12 @@ function createObservation(page, vuContext) {
                 error: { name: error.name, message: error.message },
                 diagnostics: await collectPageDiagnostics(),
             });
+            if (name === 'room_create') {
+                await diagnoseRoomCreateFailure(error, startedAt);
+            }
             throw error;
         } finally {
+            addTimelineEvent('action.ended', { action: name });
             currentAction = previousAction;
         }
     }
@@ -265,6 +370,9 @@ function createObservation(page, vuContext) {
         page.off('requestfailed', onRequestFailed);
         page.off('response', onResponse);
         page.off('websocket', onWebSocket);
+        page.off('framenavigated', onFrameNavigated);
+        page.off('console', onConsole);
+        page.off('pageerror', onPageError);
 
         samples.layoutShifts = await page.evaluate(() => window.__e2eLayoutShifts || []).catch(() => []);
 
@@ -273,11 +381,13 @@ function createObservation(page, vuContext) {
         const runDirectory = path.join(outputRoot, runId);
         fs.mkdirSync(runDirectory, { recursive: true });
         const report = {
-            schemaVersion: 1,
+            schemaVersion: 2,
             runId,
             vuId: vuContext?._uid || vuContext?.vars?.$uuid || randomUUID(),
             generatedAt: new Date().toISOString(),
             samples,
+            timeline,
+            diagnostics,
             summary: {
                 actions: summarizeSamples(samples.actions),
                 documents: summarizeSamples(samples.documents),
@@ -318,4 +428,5 @@ module.exports = {
     normalizeDocumentPath,
     parseSocketEvent,
     summarizeSamples,
+    classifyRoomCreateFailure,
 };
diff --git a/e2e/artillery/observation.test.js b/e2e/artillery/observation.test.js
index 9d880f9a..ea7cf276 100644
--- a/e2e/artillery/observation.test.js
+++ b/e2e/artillery/observation.test.js
@@ -1,6 +1,12 @@
 const test = require('node:test');
 const assert = require('node:assert/strict');
-const { normalizeApiPath, normalizeDocumentPath, parseSocketEvent, summarizeSamples } = require('./observation');
+const {
+    normalizeApiPath,
+    normalizeDocumentPath,
+    parseSocketEvent,
+    summarizeSamples,
+    classifyRoomCreateFailure,
+} = require('./observation');
 
 test('normalizes dynamic API identifiers without changing static routes', () => {
     assert.equal(normalizeApiPath('http://localhost:5001/api/rooms'), '/api/rooms');
@@ -35,3 +41,34 @@ test('summarizes and ranks samples by cumulative duration', () => {
         averageMs: 20, p95Ms: 20, p99Ms: 20, totalDurationMs: 40, contributionPct: 57.1,
     });
 });
+
+test('classifies late visibility after the original assertion fails', () => {
+    assert.equal(classifyRoomCreateFailure({
+        diagnostics: [{ action: 'room_create', becameVisibleWithin15s: true }],
+    }), 'late_visibility');
+});
+
+test('classifies a missing joinRoomSuccess after socket join was sent', () => {
+    assert.equal(classifyRoomCreateFailure({
+        diagnostics: [{ action: 'room_create', becameVisibleWithin15s: false }],
+        samples: {
+            http: [{ name: 'GET /api/rooms/{roomId}', success: true, status: 200, durationMs: 20 }],
+            socket: [{ name: 'connection', success: true }],
+        },
+        timeline: [{ name: 'socket.sent', event: 'joinRoom' }],
+    }), 'join_room_response');
+});
+
+test('classifies render failure after joinRoomSuccess arrives', () => {
+    assert.equal(classifyRoomCreateFailure({
+        diagnostics: [{ action: 'room_create', becameVisibleWithin15s: false }],
+        samples: {
+            http: [],
+            socket: [{ name: 'connection', success: true }],
+        },
+        timeline: [
+            { name: 'socket.sent', event: 'joinRoom' },
+            { name: 'socket.received', event: 'joinRoomSuccess' },
+        ],
+    }), 'frontend_state_or_render');
+});

From bc50f4578d92424227846c2434eacb75d4271673 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=EC=9A=A9=EC=A3=BC?= <jake@yongjuui-MacBookPro.local>
Date: Wed, 12 Aug 2026 05:29:03 +0900
Subject: [PATCH 30/32] test(e2e): diagnose login fill failures under load

---
 apps/frontend/test/pages/index.test.js        |  30 +-
 e2e/actions/auth.actions.js                   |  32 +-
 e2e/actions/auth.actions.test.js              |  72 ++
 .../LOGIN_FILL_FAILURE_INVESTIGATION.md       | 101 +++
 e2e/artillery/README.md                       |  37 +
 e2e/artillery/aggregate-observations.js       |  11 +-
 e2e/artillery/analyze-login-fill-failures.js  | 639 ++++++++++++++++++
 .../analyze-login-fill-failures.test.js       | 336 +++++++++
 e2e/artillery/observation.js                  | 231 ++++++-
 e2e/artillery/observation.test.js             | 150 ++++
 e2e/artillery/report-observations.js          |   6 +-
 e2e/package.json                              |   3 +-
 12 files changed, 1618 insertions(+), 30 deletions(-)
 create mode 100644 e2e/actions/auth.actions.test.js
 create mode 100644 e2e/artillery/LOGIN_FILL_FAILURE_INVESTIGATION.md
 create mode 100644 e2e/artillery/analyze-login-fill-failures.js
 create mode 100644 e2e/artillery/analyze-login-fill-failures.test.js

diff --git a/apps/frontend/test/pages/index.test.js b/apps/frontend/test/pages/index.test.js
index 6af33ed1..dbd18e35 100644
--- a/apps/frontend/test/pages/index.test.js
+++ b/apps/frontend/test/pages/index.test.js
@@ -1,5 +1,5 @@
 import React from 'react';
-import { render, screen } from '@testing-library/react';
+import { render, screen, waitFor } from '@testing-library/react';
 import { describe, expect, it, vi } from 'vitest';
 import LoginPage from '../../pages/index';
 
@@ -27,4 +27,32 @@ describe('Login page', () => {
     expect(screen.getByTestId('login-password-input')).toBeEnabled();
     expect(screen.getByTestId('login-submit-button')).toBeEnabled();
   });
+
+  it('preserves the same editable inputs when the health check succeeds', async () => {
+    mocks.checkServerConnection.mockResolvedValueOnce(true);
+    render(<LoginPage />);
+    const emailInput = screen.getByTestId('login-email-input');
+    const passwordInput = screen.getByTestId('login-password-input');
+
+    await waitFor(() => expect(screen.queryByTestId('server-status-checking')).not.toBeInTheDocument());
+
+    expect(screen.getByTestId('login-email-input')).toBe(emailInput);
+    expect(screen.getByTestId('login-password-input')).toBe(passwordInput);
+    expect(emailInput).toBeEnabled();
+    expect(passwordInput).toBeEnabled();
+  });
+
+  it('preserves the same editable inputs when the health check fails', async () => {
+    mocks.checkServerConnection.mockRejectedValueOnce(new Error('health unavailable'));
+    render(<LoginPage />);
+    const emailInput = screen.getByTestId('login-email-input');
+    const passwordInput = screen.getByTestId('login-password-input');
+
+    await waitFor(() => expect(screen.getByTestId('server-status-message')).toBeInTheDocument());
+
+    expect(screen.getByTestId('login-email-input')).toBe(emailInput);
+    expect(screen.getByTestId('login-password-input')).toBe(passwordInput);
+    expect(emailInput).toBeEnabled();
+    expect(passwordInput).toBeEnabled();
+  });
 });
diff --git a/e2e/actions/auth.actions.js b/e2e/actions/auth.actions.js
index 3b05742b..65e8d6c7 100644
--- a/e2e/actions/auth.actions.js
+++ b/e2e/actions/auth.actions.js
@@ -1,5 +1,28 @@
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
 
+async function runLoginStep(step, testId, callback) {
+  try {
+    return await callback();
+  } catch (error) {
+    const originalMessage = error?.message || String(error);
+    const prefix = `[loginAction step=${step} locator=${testId}]`;
+
+    if (error && typeof error === 'object') {
+      const originalStack = error.stack;
+      error.message = `${prefix} ${originalMessage}`;
+      error.loginActionStep = step;
+      error.loginActionLocator = testId;
+      if (typeof originalStack === 'string') {
+        const firstFrame = originalStack.indexOf('\n');
+        error.stack = `${error.name || 'Error'}: ${error.message}${firstFrame >= 0 ? originalStack.slice(firstFrame) : ''}`;
+      }
+      throw error;
+    }
+
+    throw new Error(`${prefix} ${originalMessage}`);
+  }
+}
+
 /**
  * 로그인 액션
  * @param {import('@playwright/test').Page} page
@@ -17,9 +40,12 @@ async function openLoginAction(page) {
 
 async function loginAction(page, credentials, waitForRedirect = true, navigate = true) {
   if (navigate) await openLoginAction(page);
-  await page.getByTestId('login-email-input').fill(credentials.email);
-  await page.getByTestId('login-password-input').fill(credentials.password);
-  await page.getByTestId('login-submit-button').click();
+  await runLoginStep('email_fill', 'login-email-input', () =>
+    page.getByTestId('login-email-input').fill(credentials.email));
+  await runLoginStep('password_fill', 'login-password-input', () =>
+    page.getByTestId('login-password-input').fill(credentials.password));
+  await runLoginStep('submit_click', 'login-submit-button', () =>
+    page.getByTestId('login-submit-button').click());
   if (waitForRedirect) {
     await page.waitForURL(`${BASE_URL}/chat`);
   }
diff --git a/e2e/actions/auth.actions.test.js b/e2e/actions/auth.actions.test.js
new file mode 100644
index 00000000..5eaee6f5
--- /dev/null
+++ b/e2e/actions/auth.actions.test.js
@@ -0,0 +1,72 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { loginAction } = require('./auth.actions');
+
+function createPage(failAt = null, failure = null) {
+  const calls = [];
+  const locatorFor = (testId) => ({
+    fill: async (value) => {
+      calls.push({ testId, action: 'fill', value });
+      if (failAt === testId) throw failure;
+    },
+    click: async () => {
+      calls.push({ testId, action: 'click' });
+      if (failAt === testId) throw failure;
+    },
+  });
+  return {
+    calls,
+    getByTestId: locatorFor,
+    waitForURL: async () => calls.push({ action: 'waitForURL' }),
+  };
+}
+
+test('tags the email fill error without replacing the original error or cause', async () => {
+  const cause = new Error('browser disconnected');
+  const failure = new Error('locator.fill: Timeout 30000ms exceeded.', { cause });
+  failure.stack = 'TimeoutError: locator.fill: Timeout 30000ms exceeded.\n    at original-locator-frame';
+  const page = createPage('login-email-input', failure);
+
+  await assert.rejects(
+    loginAction(page, { email: 'bad@example.com', password: 'wrong' }, false, false),
+    (error) => {
+      assert.equal(error, failure);
+      assert.equal(error.cause, cause);
+      assert.match(error.message, /^\[loginAction step=email_fill locator=login-email-input\]/);
+      assert.match(error.stack, /original-locator-frame/);
+      assert.equal(error.loginActionStep, 'email_fill');
+      assert.equal(error.loginActionLocator, 'login-email-input');
+      return true;
+    }
+  );
+  assert.deepEqual(page.calls.map((call) => call.testId), ['login-email-input']);
+});
+
+test('tags the password fill step and does not retry or submit', async () => {
+  const failure = new Error('locator.fill: Timeout 30000ms exceeded.');
+  const page = createPage('login-password-input', failure);
+
+  await assert.rejects(
+    loginAction(page, { email: 'bad@example.com', password: 'wrong' }, false, false),
+    /step=password_fill locator=login-password-input/
+  );
+  assert.deepEqual(page.calls.map((call) => call.testId), [
+    'login-email-input',
+    'login-password-input',
+  ]);
+});
+
+test('tags the submit step and performs every login action exactly once', async () => {
+  const failure = new Error('locator.click: Timeout 30000ms exceeded.');
+  const page = createPage('login-submit-button', failure);
+
+  await assert.rejects(
+    loginAction(page, { email: 'bad@example.com', password: 'wrong' }, false, false),
+    /step=submit_click locator=login-submit-button/
+  );
+  assert.deepEqual(page.calls.map((call) => call.testId), [
+    'login-email-input',
+    'login-password-input',
+    'login-submit-button',
+  ]);
+});
diff --git a/e2e/artillery/LOGIN_FILL_FAILURE_INVESTIGATION.md b/e2e/artillery/LOGIN_FILL_FAILURE_INVESTIGATION.md
new file mode 100644
index 00000000..bec8182b
--- /dev/null
+++ b/e2e/artillery/LOGIN_FILL_FAILURE_INVESTIGATION.md
@@ -0,0 +1,101 @@
+# 로그인 `fill()` 실패 조사와 재검증
+
+## 현재 확정된 사실
+
+`st-58123cd5` HTML은 492 VU 중 `failedLoginScenario`의 `locator.fill` 실패 82건과
+401 응답 410건을 기록했다. 82는 줄 번호가 아니라 오류 수다. 시나리오는 로그인 폼 준비 후
+`loginAction()`에서 이메일, 비밀번호, 제출을 순서대로 실행하므로 이 오류는 로그인 POST 이전의
+이메일 또는 비밀번호 입력에서 발생했다.
+
+집계 HTML에는 locator와 VU별 원본이 없어 `410 + 82 = 492`의 개별 VU 상관관계와
+DOM 교체, 입력 불가, 부하 발생기 정체, 프런트엔드/Ingress 장애 중 하나를 확정할 수 없다.
+같은 실행에 502 148건, 504 755건과 전체 시나리오 실패 447건도 있어 로그인 코드만의
+문제로 단정하지 않는다.
+
+`e2e/artillery/scenarios/*.scenario.js`는 변경하지 않는다. Playwright의 30초 locator
+timeout도 늘리지 않고, 실패 재시도나 자동 재내비게이션도 추가하지 않는다.
+
+## 새 관측 자료
+
+공용 로그인 action은 원본 Error 객체, stack, cause를 유지하면서 오류 첫 줄에 다음 단계를 붙인다.
+
+- `email_fill` / `login-email-input`
+- `password_fill` / `login-password-input`
+- `submit_click` / `login-submit-button`
+
+각 VU JSON은 schema version 3으로 다음 자료를 포함한다.
+
+- 양쪽 로그인 input과 버튼의 count, connected, visible, enabled, readOnly, editable 상태
+- action 시작/종료 URL과 내비게이션 timeline
+- 최근 document, script, stylesheet, font, image 및 API 응답
+- 브라우저 console/page error와 Node 이벤트 루프 지연
+- Git SHA, 이미지 digest, Pod/노드/리소스, 부하 설정
+- 입력과 textarea를 마스킹한 실패 screenshot 및 전체 Error JSON
+
+다음 환경변수는 모든 분산 Pod에 동일하게 전달한다. `OBSERVATION_OUTPUT_DIR`은 Pod 종료 후에도
+남는 공유 PVC나 수집 대상 디렉터리여야 한다.
+
+```bash
+OBSERVATION_RUN_ID=login-fill-20260812-r1
+OBSERVATION_OUTPUT_DIR=/shared/e2e-observations
+GIT_SHA=<tested-commit>
+FRONTEND_IMAGE_DIGEST=sha256:<frontend-image>
+LOAD_IMAGE_DIGEST=sha256:<load-image>
+EXPECTED_TOTAL_VUS=500
+EXPECTED_FAILED_LOGIN_401=500
+EXPECTED_LOGIN_FILL_FAILURES=0
+VUS_PER_POD=4
+```
+
+Kubernetes Downward API로 `POD_NAME`, `POD_NAMESPACE`, `NODE_NAME`, `POD_CPU_REQUEST`,
+`POD_CPU_LIMIT`, `POD_MEMORY_REQUEST`, `POD_MEMORY_LIMIT`도 주입한다. 동일 run ID의
+`vu-*.json`, `run-metadata-*.json`, `artifacts/`, Artillery JSON/stdout과 인프라 evidence를
+모두 수집하기 전에는 분석하지 않는다.
+
+## 오프라인 분석
+
+분석기는 원본을 수정하지 않고 `analysis.json`, `failed-vus.csv`, `report.md`,
+`input-manifest.json`을 별도 출력 디렉터리에 만든다.
+
+```bash
+node e2e/artillery/analyze-login-fill-failures.js \
+  /shared/e2e-observations/login-fill-20260812-r1 \
+  --output-dir /shared/e2e-analysis/login-fill-20260812-r1 \
+  --expected-run-id login-fill-20260812-r1 \
+  --evidence /shared/e2e-observations/login-fill-20260812-r1/infrastructure-evidence.json \
+  --artillery-result /shared/e2e-observations/login-fill-20260812-r1/artillery-result.json \
+  --artillery-stdout /shared/e2e-observations/login-fill-20260812-r1/artillery.stdout.log
+```
+
+실행 metadata는 각 VU JSON에 내장된다. 배포 시스템이 별도의 통합 metadata JSON을 만든
+경우에만 `--metadata <file>`을 추가한다.
+
+기대 VU, 401, fill 실패 수는 CLI 값이 있으면 그것을 사용하고, 없으면 metadata 또는 VU JSON의
+`expectations`에서 읽는다. 하드코딩된 492/82/410 기본값은 없다. 기대 VU 수, 고유 VU ID,
+run ID, VU 파일 hash, 참조된 실패 artifact가 모두 맞지 않으면 manifest가 불완전해지고 CLI는
+exit code 2로 실패한다.
+
+분류 결과는 다음 중 하나다.
+
+- `email_fill`, `password_fill`: action 단계
+- `navigation`, `dom_replacement`, `not_editable`: 브라우저에서 직접 확인된 상태
+- `loadgen_stall`: editable DOM과 29초 이상 action에 event-loop lag 또는 부하 Pod 포화가 겹침
+- `frontend_or_ingress_failure`: 같은 실패 구간에 document/static 5xx 또는 인프라 포화가 겹침
+- `unclassified`: 위 증거가 부족함
+
+evidence의 event에는 `runId`, `scope`, `signal`, `startedAt`, `endedAt`이 필요하다.
+허용 scope는 `load_pod`, `load_generator`, `frontend`, `ingress`이며 VU 한정 자료는 `vuIds` 또는
+`vus.<vuId>`에 둔다. run ID나 시간 구간이 다르면 원인 판정에서 제외한다.
+
+## 재검증 게이트
+
+1. 1 VU: form 준비, email/password fill, 제출, 로그인 POST 1회 401, 오류 메시지 표시를 확인한다.
+2. 단일 Pod 4 VU: 위 결과가 4회이고 fill 실패와 이벤트 루프 정체가 없는지 확인한다.
+3. 492 VU: 기존 실행 규모를 재현해 82건의 분포와 원인을 비교한다.
+4. 500 VU: VU JSON 500개, 고유 VU ID 500개, 401 500건, fill 실패 0건, 중복 요청 0건을 확인한다.
+
+원인이 `navigation`이면 `/login` 리다이렉트가 fill 구간과 겹치는지 확인한 뒤 canonical `/` 직접
+이동만 적용한다. `dom_replacement`면 로그인 폼 subtree를 상태 변경 중에도 유지한다.
+`not_editable`이면 native input 연결과 disabled/readOnly 경로만 수정한다. `loadgen_stall`이면
+그때에만 `slowMo` 제거와 Pod당 브라우저/리소스를 조정한다. 5xx가 시간상 겹친 경우에만
+프런트엔드/Ingress 용량을 변경한다. `unclassified`에는 제품 코드 변경을 적용하지 않는다.
diff --git a/e2e/artillery/README.md b/e2e/artillery/README.md
index 53ebf756..533f71bf 100644
--- a/e2e/artillery/README.md
+++ b/e2e/artillery/README.md
@@ -62,6 +62,43 @@ HTTP의 4xx/5xx는 실패로 집계되므로 의도된 로그인 실패 시나
 `PHASE1_ARRIVAL_COUNT`는 지속 동시 VU가 아니라 phase 동안 생성할 총 사용자 수입니다.
 Stage 0을 3회 완료하고 Mongo profiler/실행계획 근거까지 확보하기 전에는 VU를 올리지 않습니다.
 
+### 로그인 fill 실패 관측
+
+분산 실행에서는 모든 Pod가 같은 `OBSERVATION_RUN_ID`와 공유 `OBSERVATION_OUTPUT_DIR`을 사용해야
+합니다. `PHASE1_ARRIVAL_COUNT`는 Pod별 생성 수이고 `EXPECTED_TOTAL_VUS`는 전체 Pod 합계입니다.
+
+```bash
+OBSERVATION_RUN_ID=login-fill-r1 \
+OBSERVATION_OUTPUT_DIR=/shared/e2e-observations \
+EXPECTED_TOTAL_VUS=500 \
+EXPECTED_FAILED_LOGIN_401=500 \
+EXPECTED_LOGIN_FILL_FAILURES=0 \
+GIT_SHA="$GIT_COMMIT" \
+FRONTEND_IMAGE_DIGEST="$FRONTEND_DIGEST" \
+LOAD_IMAGE_DIGEST="$LOAD_DIGEST" \
+pnpm --filter e2e exec artillery run artillery/artillery-config.yaml
+```
+
+실패 VU에는 email/password/submit 단계, 두 input의 DOM·editable 상태, URL, 최근 API·문서·정적
+리소스, 이벤트 루프 지연과 입력값이 마스킹된 screenshot이 기록됩니다. 분석기는 원본 수와
+artifact hash가 완전하지 않으면 성공하지 않습니다.
+
+```bash
+node e2e/artillery/analyze-login-fill-failures.js \
+  /shared/e2e-observations/login-fill-r1 \
+  --output-dir /shared/e2e-analysis/login-fill-r1 \
+  --expected-run-id login-fill-r1 \
+  --evidence /shared/e2e-observations/login-fill-r1/infrastructure-evidence.json \
+  --artillery-result /shared/e2e-observations/login-fill-r1/artillery-result.json \
+  --artillery-stdout /shared/e2e-observations/login-fill-r1/artillery.stdout.log
+```
+
+실행 metadata는 각 VU JSON에 포함됩니다. 별도의 통합 metadata JSON을 수집한 경우에만
+`--metadata <file>`을 추가합니다.
+
+세부 스키마, evidence 형식, `1 → 4 → 492 → 500 VU` 재검증 게이트는
+[`LOGIN_FILL_FAILURE_INVESTIGATION.md`](./LOGIN_FILL_FAILURE_INVESTIGATION.md)를 따릅니다.
+
 ## 환경 변수
 
   Artillery 실행 시 다음 환경 변수로 커스터마이징할 수 있습니다:
diff --git a/e2e/artillery/aggregate-observations.js b/e2e/artillery/aggregate-observations.js
index 3304c7f4..193e0fb0 100644
--- a/e2e/artillery/aggregate-observations.js
+++ b/e2e/artillery/aggregate-observations.js
@@ -12,12 +12,14 @@ const reports = runDirectories.flatMap((runDirectory) => fs.readdirSync(runDirec
     .filter((file) => file.startsWith('vu-') && file.endsWith('.json'))
     .map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8'))));
 
-const samples = { actions: [], documents: [], http: [], socket: [], layoutShifts: [] };
+const samples = { actions: [], documents: [], resources: [], http: [], socket: [], runtime: [], layoutShifts: [] };
 for (const report of reports) {
     samples.actions.push(...report.samples.actions);
     samples.documents.push(...(report.samples.documents || []));
+    samples.resources.push(...(report.samples.resources || []));
     samples.http.push(...report.samples.http);
     samples.socket.push(...report.samples.socket);
+    samples.runtime.push(...(report.samples.runtime || []));
     samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
 
@@ -28,8 +30,11 @@ const timestamps = Object.values(samples).flat()
 const rankedHttp = summarizeSamples(samples.http.filter((sample) =>
     sample.name !== 'GET /api/health' &&
     !(sample.name === 'POST /api/auth/login' && sample.status === 401)));
+const expectedTotal = Number(process.env.EXPECTED_TOTAL_VUS || 0);
 const expectedPerRun = Number(process.env.EXPECTED_VUS || process.env.PHASE1_ARRIVAL_COUNT || 0);
-const expectedVirtualUsers = expectedPerRun > 0 ? expectedPerRun * runDirectories.length : null;
+const expectedVirtualUsers = expectedTotal > 0
+    ? expectedTotal
+    : expectedPerRun > 0 ? expectedPerRun * runDirectories.length : null;
 const completedVirtualUsers = reports.filter((report) => report.samples.actions.length > 0 &&
     report.samples.actions.every((sample) => sample.success)).length;
 
@@ -54,9 +59,11 @@ const aggregate = {
     summary: {
         actions: summarizeSamples(samples.actions),
         documents: summarizeSamples(samples.documents),
+        resources: summarizeSamples(samples.resources),
         httpDiagnostic: summarizeSamples(samples.http),
         httpRanked: rankedHttp,
         socket: summarizeSamples(samples.socket),
+        runtime: summarizeSamples(samples.runtime),
         cls: {
             total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
             entries: samples.layoutShifts.length,
diff --git a/e2e/artillery/analyze-login-fill-failures.js b/e2e/artillery/analyze-login-fill-failures.js
new file mode 100644
index 00000000..021ad718
--- /dev/null
+++ b/e2e/artillery/analyze-login-fill-failures.js
@@ -0,0 +1,639 @@
+#!/usr/bin/env node
+const crypto = require('crypto');
+const fs = require('fs');
+const path = require('path');
+
+const LOGIN_ACTION = 'failed_login';
+const LOGIN_REQUEST = 'POST /api/auth/login';
+const EMAIL_SELECTOR = '[data-testid="login-email-input"]';
+const PASSWORD_SELECTOR = '[data-testid="login-password-input"]';
+const BUTTON_SELECTOR = '[data-testid="login-submit-button"]';
+const STALL_THRESHOLD_MS = 29000;
+
+function asArray(value) {
+    return Array.isArray(value) ? value : [];
+}
+
+function parseTime(value) {
+    const timestamp = Date.parse(value);
+    return Number.isFinite(timestamp) ? timestamp : null;
+}
+
+function pathname(rawUrl) {
+    if (!rawUrl) return null;
+    try {
+        return new URL(rawUrl, 'http://localhost').pathname;
+    } catch {
+        return null;
+    }
+}
+
+function detectFailureLocator(message = '') {
+    if (/login-email-input/i.test(message)) return 'login-email-input';
+    if (/login-password-input/i.test(message)) return 'login-password-input';
+    return 'unconfirmed';
+}
+
+function isFillError(message = '') {
+    return /locator\.fill|\.fill:\s*timeout|\bfill\(/i.test(message);
+}
+
+function findAction(report, name, success) {
+    return asArray(report.samples?.actions).find((sample) =>
+        sample.name === name && (success === undefined || sample.success === success));
+}
+
+function loginPostOutcome(report) {
+    const allLoginPosts = asArray(report.samples?.http)
+        .filter((sample) => sample.name === LOGIN_REQUEST);
+    const actionLoginPosts = allLoginPosts.filter((sample) => sample.action === LOGIN_ACTION);
+    const statuses = actionLoginPosts.map((sample) => sample.status ?? null);
+    let outcome = 'no_request';
+    if (actionLoginPosts.length > 0) {
+        if (statuses.some((status) => status === null)) outcome = 'transmission_failure';
+        else if (actionLoginPosts.length === 1 && statuses[0] === 401) outcome = '401';
+        else outcome = 'other_response';
+    }
+    return {
+        outcome,
+        requestCount: actionLoginPosts.length,
+        exact401Count: actionLoginPosts.filter((sample) => sample.status === 401).length,
+        statuses,
+        wrongActionRequestCount: allLoginPosts.length - actionLoginPosts.length,
+    };
+}
+
+function eventOverlapsAction(event, action, vuId) {
+    if (asArray(event.vuIds).length > 0 && !event.vuIds.includes(vuId)) return false;
+    const actionStart = parseTime(action?.startedAt);
+    const actionEnd = parseTime(action?.endedAt);
+    const eventStart = parseTime(event.startedAt);
+    const eventEnd = parseTime(event.endedAt) ?? eventStart;
+    if ([actionStart, actionEnd, eventStart, eventEnd].some((value) => value === null)) return false;
+    return eventStart <= actionEnd && eventEnd >= actionStart;
+}
+
+function evidenceForVu(evidence, vuId, action) {
+    const vuEvidence = evidence?.vus?.[vuId] || {};
+    const events = asArray(evidence?.events).filter((event) => eventOverlapsAction(event, action, vuId));
+    return { ...vuEvidence, events };
+}
+
+function hasLoadPodSaturation(events) {
+    const scopes = new Set(['load_pod', 'load-generator', 'load_generator']);
+    const signals = new Set(['saturation', 'cpu_throttling', 'memory_pressure', 'oom']);
+    return events.some((event) => scopes.has(event.scope) && signals.has(event.signal));
+}
+
+function hasFrontendIngressSaturation(events) {
+    const scopes = new Set(['frontend', 'ingress']);
+    const signals = new Set([
+        'saturation', 'cpu_throttling', 'memory_pressure', 'oom', 'restart',
+        'http_499', 'http_502', 'http_504', 'document_delay', 'static_delay', 'api_delay',
+    ]);
+    return events.some((event) => scopes.has(event.scope) && signals.has(event.signal));
+}
+
+function hasRuntimeStall(report, action) {
+    return asArray(report.samples?.runtime).some((sample) =>
+        sample.name === 'event_loop_lag' && sample.durationMs >= 1000 &&
+        eventOverlapsAction(sample, action, report.vuId));
+}
+
+function hasDeliveryFailure(diagnostics, action) {
+    return [...asArray(diagnostics.recentDocuments), ...asArray(diagnostics.recentResources)]
+        .some((sample) => (sample.status === 502 || sample.status === 504 || sample.status >= 500) &&
+            (sample.action === LOGIN_ACTION || eventOverlapsAction(sample, action, null)));
+}
+
+function actionNavigations(report, action) {
+    const start = parseTime(action?.startedAt);
+    const end = parseTime(action?.endedAt);
+    if (start === null || end === null) return [];
+    return asArray(report.timeline).filter((item) => item.name === 'navigation' &&
+        parseTime(item.at) !== null && parseTime(item.at) >= start && parseTime(item.at) <= end);
+}
+
+function urlAtActionStart(report, action) {
+    const start = parseTime(action?.startedAt);
+    if (start === null) return null;
+    return asArray(report.timeline)
+        .filter((item) => item.name === 'navigation' && parseTime(item.at) !== null && parseTime(item.at) <= start)
+        .sort((left, right) => parseTime(right.at) - parseTime(left.at))[0]?.url || null;
+}
+
+function domStateForLocator(locator, diagnostics, vuEvidence) {
+    if (locator === 'login-email-input') return diagnostics.ui?.[EMAIL_SELECTOR] || null;
+    if (locator === 'login-password-input') {
+        return vuEvidence.passwordDom || diagnostics.ui?.[PASSWORD_SELECTOR] || null;
+    }
+    return null;
+}
+
+function describeEvent(event) {
+    return [event.scope, event.signal, event.detail].filter(Boolean).join(': ');
+}
+
+function classifyFailedVu(report, sourceFile, evidence = null) {
+    const action = findAction(report, LOGIN_ACTION, false);
+    if (!action) return null;
+
+    const message = action.error?.message || '';
+    const failureLocator = action.error?.loginActionLocator || detectFailureLocator(message);
+    const failureStep = action.error?.loginActionStep || (failureLocator === 'login-email-input' ? 'email_fill'
+        : failureLocator === 'login-password-input' ? 'password_fill' : 'unconfirmed');
+    const diagnostics = action.diagnostics || {};
+    const vuEvidence = evidenceForVu(evidence, report.vuId, action);
+    const startUrl = urlAtActionStart(report, action);
+    const failureUrl = diagnostics.url || null;
+    const navigations = actionNavigations(report, action);
+    const changedNavigation = startUrl ? navigations.find((item) => pathname(item.url) !== pathname(startUrl)) ||
+        (failureUrl && pathname(failureUrl) !== pathname(startUrl) ? { url: failureUrl } : null) : null;
+    const emailDom = diagnostics.ui?.[EMAIL_SELECTOR] || null;
+    const targetDom = domStateForLocator(failureLocator, diagnostics, vuEvidence);
+    const formWasReady = Boolean(findAction(report, 'login_form_ready_for_failed_login', true));
+    const detachCount = (message.match(/detach(?:ed|ing)?/gi) || []).length;
+    const repeatedDetach = vuEvidence.repeatedDetach === true || detachCount >= 2;
+    const loadingVisible = vuEvidence.loadingVisible === true;
+    const loadPodSaturation = hasLoadPodSaturation(vuEvidence.events);
+    const frontendIngressSaturation = hasFrontendIngressSaturation(vuEvidence.events);
+    const runtimeStall = hasRuntimeStall(report, action);
+    const deliveryFailure = hasDeliveryFailure(diagnostics, action);
+    const domMaintained = targetDom?.count > 0 && targetDom.visible === true && targetDom.enabled === true &&
+        targetDom.connected !== false && targetDom.editable !== false && targetDom.readOnly !== true;
+    const notEditable = targetDom?.count > 0 && (
+        targetDom.enabled === false || targetDom.editable === false || targetDom.readOnly === true
+    );
+
+    let directCause = 'unclassified';
+    let mechanism = null;
+    let confidence = 'unconfirmed_hypothesis';
+    if (changedNavigation) {
+        directCause = 'navigation';
+        confidence = 'confirmed_fact';
+    } else if (formWasReady && (emailDom?.count === 0 || targetDom?.count === 0 || loadingVisible || repeatedDetach)) {
+        directCause = 'dom_replacement';
+        mechanism = repeatedDetach ? 'hydration_remount' : loadingVisible ? 'auth_loading_render' : 'locator_disappeared';
+        confidence = repeatedDetach ? 'strong_indication' : 'confirmed_fact';
+    } else if (notEditable) {
+        directCause = 'not_editable';
+        mechanism = targetDom.readOnly === true ? 'readonly' : targetDom.enabled === false ? 'disabled' : 'non_editable_element';
+        confidence = 'confirmed_fact';
+    } else if (action.durationMs >= STALL_THRESHOLD_MS && domMaintained && (loadPodSaturation || runtimeStall)) {
+        directCause = 'loadgen_stall';
+        mechanism = runtimeStall ? 'event_loop_lag' : 'pod_saturation';
+        confidence = 'strong_indication';
+    } else if (deliveryFailure || frontendIngressSaturation) {
+        directCause = 'frontend_or_ingress_failure';
+        mechanism = deliveryFailure ? 'overlapping_5xx' : 'infrastructure_saturation';
+        confidence = 'strong_indication';
+    }
+
+    let upstreamCause = 'unconfirmed';
+    if (loadPodSaturation) upstreamCause = 'load_pod_saturation';
+    else if (frontendIngressSaturation) upstreamCause = 'frontend_or_ingress_saturation';
+
+    const hypotheses = [];
+    if (directCause === 'unclassified' && action.durationMs >= STALL_THRESHOLD_MS && domMaintained) {
+        hypotheses.push('loadgen_stall_requires_runtime_or_pod_evidence');
+    }
+    if (failureLocator === 'login-password-input' && !targetDom) {
+        hypotheses.push('password_dom_state_requires_trace_screenshot_or_call_log');
+    }
+    if (upstreamCause === 'unconfirmed' && directCause === 'dom_replacement') {
+        hypotheses.push('auth_initialization_or_hydration_requires_runtime_evidence');
+    }
+
+    const loginPost = loginPostOutcome(report);
+    const errorMessageDisplayed = asArray(report.samples?.actions).some((sample) => sample.name === 'register')
+        ? 'confirmed_by_scenario_progression' : 'unconfirmed';
+    const sequence = {
+        formReady: formWasReady ? 'completed' : 'unconfirmed',
+        emailFill: failureStep === 'email_fill' ? 'failed'
+            : failureStep === 'password_fill' || loginPost.requestCount > 0 ? 'completed' : 'unconfirmed',
+        passwordFill: failureStep === 'password_fill' ? 'failed'
+            : failureStep === 'email_fill' ? 'not_reached'
+                : loginPost.requestCount > 0 ? 'completed' : 'unconfirmed',
+        submitClick: loginPost.requestCount > 0 ? 'completed'
+            : isFillError(message) ? 'not_reached' : 'unconfirmed',
+        loginPost: loginPost.outcome,
+        response401: loginPost.exact401Count === 1 ? 'received' : 'not_confirmed',
+        errorMessage: errorMessageDisplayed,
+    };
+    const podEvidence = vuEvidence.events.filter((event) =>
+        ['load_pod', 'load-generator', 'load_generator', 'frontend'].includes(event.scope)).map(describeEvent);
+    const ingressEvidence = vuEvidence.events.filter((event) => event.scope === 'ingress').map(describeEvent);
+    return {
+        vuId: report.vuId,
+        sourceFile,
+        failureLocator,
+        failureStep,
+        isFillFailure: isFillError(message),
+        errorMessage: message,
+        actionDurationMs: action.durationMs,
+        sequence,
+        urlAtActionStart: startUrl,
+        urlAtFailure: failureUrl,
+        navigationUrls: navigations.map((item) => item.url),
+        domState: {
+            email: diagnostics.ui?.[EMAIL_SELECTOR] || null,
+            password: vuEvidence.passwordDom || diagnostics.ui?.[PASSWORD_SELECTOR] || null,
+            button: diagnostics.ui?.[BUTTON_SELECTOR] || null,
+            loadingVisible: vuEvidence.loadingVisible ?? null,
+            repeatedDetach,
+        },
+        recentHttp: asArray(diagnostics.recentHttp),
+        loginPost,
+        errorMessageDisplayed,
+        podEvidence,
+        ingressEvidence,
+        directCause,
+        mechanism,
+        upstreamCause,
+        confidence,
+        hypotheses,
+    };
+}
+
+function countBy(items, property) {
+    return items.reduce((counts, item) => {
+        const key = item[property];
+        counts[key] = (counts[key] || 0) + 1;
+        return counts;
+    }, {});
+}
+
+function sampleWindow(reports) {
+    const timestamps = reports.flatMap((report) => [
+        ...asArray(report.samples?.actions).flatMap((sample) => [sample.startedAt, sample.endedAt]),
+        ...asArray(report.samples?.http).flatMap((sample) => [sample.startedAt, sample.endedAt]),
+    ]).filter((value) => parseTime(value) !== null).sort();
+    return { startedAt: timestamps[0] || null, endedAt: timestamps.at(-1) || null };
+}
+
+function expectedValue(explicitValue, metadata, reports, metadataPaths) {
+    if (explicitValue !== undefined && explicitValue !== null) return explicitValue;
+    const candidates = [metadata, ...reports.map((report) => report.metadata)].filter(Boolean);
+    for (const candidate of candidates) {
+        for (const pathParts of metadataPaths) {
+            const value = pathParts.reduce((current, part) => current?.[part], candidate);
+            if (Number.isInteger(value) && value >= 0) return value;
+        }
+    }
+    return null;
+}
+
+function analyzeReports(entries, options = {}) {
+    const reports = entries.map((entry) => entry.report);
+    const embeddedMetadata = reports.find((report) => report.metadata)?.metadata || null;
+    const effectiveMetadata = options.metadata || embeddedMetadata;
+    const expectedVus = expectedValue(options.expectedVus, options.metadata, reports, [
+        ['expectations', 'totalVus'], ['workload', 'arrivalCount'], ['vuBatch', 'arrivalCount'], ['expectedVus'],
+    ]);
+    const expectedFillFailures = expectedValue(options.expectedFillFailures, options.metadata, reports, [
+        ['expectations', 'fillFailures'],
+    ]);
+    const expected401Vus = expectedValue(options.expected401Vus, options.metadata, reports, [
+        ['expectations', 'failedLogin401'],
+    ]);
+    const allRunIdsPresent = reports.every((report) => typeof report.runId === 'string' && report.runId.length > 0);
+    const allVuIdsPresent = reports.every((report) => typeof report.vuId === 'string' && report.vuId.length > 0);
+    const runIds = [...new Set(reports.map((report) => report.runId).filter(Boolean))];
+    const runId = allRunIdsPresent && runIds.length === 1 ? runIds[0] : null;
+    const evidenceMatchesRun = !options.evidence || Boolean(runId && options.evidence.runId === runId);
+    const evidence = evidenceMatchesRun ? options.evidence : null;
+    const failed = entries.map((entry) => classifyFailedVu(entry.report, entry.sourceFile, evidence)).filter(Boolean);
+    const allDetails = entries.map((entry) => ({
+        vuId: entry.report.vuId,
+        failedLoginAction: Boolean(findAction(entry.report, LOGIN_ACTION, false)),
+        fillFailure: failed.find((detail) => detail.vuId === entry.report.vuId)?.isFillFailure || false,
+        loginPost: loginPostOutcome(entry.report),
+    }));
+    const exact401Vus = allDetails.filter((detail) =>
+        detail.loginPost.requestCount === 1 && detail.loginPost.exact401Count === 1);
+    const errorMessageConfirmedVus = reports.filter((report) =>
+        asArray(report.samples?.actions).some((sample) => sample.name === 'register')).length;
+    const failedFillVus = failed.filter((detail) => detail.isFillFailure);
+    const uniqueVuIds = new Set(reports.map((report) => report.vuId).filter(Boolean));
+    const expectationsKnown = [expectedVus, expectedFillFailures, expected401Vus]
+        .every((value) => Number.isInteger(value));
+    const relationConfirmed = expectationsKnown && Boolean(runId) && allVuIdsPresent && reports.length === expectedVus &&
+        uniqueVuIds.size === expectedVus &&
+        failedFillVus.length === expectedFillFailures && exact401Vus.length === expected401Vus &&
+        failedFillVus.every((detail) => detail.loginPost.outcome === 'no_request') &&
+        allDetails.filter((detail) => !detail.fillFailure).every((detail) =>
+            detail.loginPost.requestCount === 1 && detail.loginPost.exact401Count === 1);
+
+    return {
+        schemaVersion: 1,
+        generatedAt: new Date().toISOString(),
+        runId,
+        window: sampleWindow(reports),
+        expectations: { expectedVus, expectedFillFailures, expected401Vus },
+        validity: {
+            oneRunId: Boolean(runId),
+            allRunIdsPresent,
+            runIds,
+            expectedRunIdMatches: options.expectedRunId ? runId === options.expectedRunId : null,
+            vuFiles: reports.length,
+            uniqueVuIds: uniqueVuIds.size,
+            allVuIdsPresent,
+            expectedVuKnown: Number.isInteger(expectedVus),
+            expectedVuCountMatches: Number.isInteger(expectedVus) ? reports.length === expectedVus : null,
+            evidenceProvided: Boolean(options.evidence),
+            evidenceMatchesRun,
+            metadataProvided: Boolean(effectiveMetadata),
+            metadataMatchesRun: options.metadata ? options.metadata.runId === runId : embeddedMetadata ? true : null,
+        },
+        summary: {
+            failedLoginActions: failed.length,
+            fillFailures: failedFillVus.length,
+            failureSteps: countBy(failed, 'failureStep'),
+            directCauses: countBy(failed, 'directCause'),
+            upstreamCauses: countBy(failed, 'upstreamCause'),
+            loginPostOutcomesForFailures: countBy(failed.map((detail) => detail.loginPost), 'outcome'),
+            exact401Vus: exact401Vus.length,
+            expectedOutcomeRelationConfirmed: relationConfirmed,
+            errorMessageConfirmedVus,
+            allExpectedFailedLoginOutcomesConfirmed: relationConfirmed &&
+                errorMessageConfirmedVus === expected401Vus,
+            relation410Plus82Confirmed: relationConfirmed,
+        },
+        metadata: options.metadata
+            ? options.metadata.runId === runId ? options.metadata : null
+            : embeddedMetadata,
+        excludedEvidence: options.evidence && !evidenceMatchesRun
+            ? { runId: options.evidence.runId, reason: 'run_id_mismatch' } : null,
+        failedVus: failed,
+    };
+}
+
+function hashFile(filePath) {
+    return crypto.createHash('sha256').update(fs.readFileSync(filePath)).digest('hex');
+}
+
+function loadRunDirectory(runDirectory) {
+    const files = fs.readdirSync(runDirectory).filter((file) => /^vu-.*\.json$/.test(file)).sort();
+    if (files.length === 0) throw new Error(`No vu-*.json files found in ${runDirectory}`);
+    return files.map((file) => {
+        const filePath = path.join(runDirectory, file);
+        return {
+            sourceFile: file,
+            sha256: hashFile(filePath),
+            report: JSON.parse(fs.readFileSync(filePath, 'utf8')),
+        };
+    });
+}
+
+function buildInputManifest(entries, runDirectory, expectedVus, supportingFiles = []) {
+    const root = path.resolve(runDirectory);
+    const runIds = [...new Set(entries.map((entry) => entry.report.runId).filter(Boolean))];
+    const vuIds = entries.map((entry) => entry.report.vuId).filter(Boolean);
+    const artifactFiles = [];
+    const missingArtifacts = [];
+    const artifactWriteErrors = [];
+
+    for (const entry of entries) {
+        const reportArtifacts = asArray(entry.report.artifacts);
+        if (asArray(entry.report.samples?.actions).some((action) => action.success === false)) {
+            for (const requiredKind of ['error', 'screenshot']) {
+                if (!reportArtifacts.some((artifact) => artifact.kind === requiredKind)) {
+                    missingArtifacts.push({
+                        vuId: entry.report.vuId, kind: requiredKind, path: null, reason: 'missing_reference',
+                    });
+                }
+            }
+        }
+        for (const artifact of reportArtifacts) {
+            if (artifact.writeError) {
+                artifactWriteErrors.push({ vuId: entry.report.vuId, kind: artifact.kind, error: artifact.writeError });
+                continue;
+            }
+            if (!artifact.path) continue;
+            const artifactPath = path.resolve(root, artifact.path);
+            if (artifactPath !== root && !artifactPath.startsWith(`${root}${path.sep}`)) {
+                missingArtifacts.push({ vuId: entry.report.vuId, path: artifact.path, reason: 'outside_run_directory' });
+                continue;
+            }
+            if (!fs.existsSync(artifactPath)) {
+                missingArtifacts.push({ vuId: entry.report.vuId, path: artifact.path, reason: 'missing' });
+                continue;
+            }
+            artifactFiles.push({
+                vuId: entry.report.vuId,
+                kind: artifact.kind,
+                path: artifact.path,
+                sha256: hashFile(artifactPath),
+            });
+        }
+    }
+
+    const expectedVuKnown = Number.isInteger(expectedVus);
+    const uniqueVuIds = new Set(vuIds).size;
+    const executionIdentities = entries.map((entry) => entry.report.metadata).filter(Boolean).map((metadata) => ({
+        targetUrl: metadata.targetUrl || null,
+        gitSha: metadata.gitSha || null,
+        frontendImage: metadata.images?.frontend || null,
+        loadImage: metadata.images?.loadGenerator || null,
+    }));
+    const allMetadataPresent = executionIdentities.length === entries.length;
+    const identityFieldsComplete = allMetadataPresent && executionIdentities.every((identity) =>
+        Object.values(identity).every(Boolean));
+    const uniqueExecutionIdentities = new Set(executionIdentities.map((identity) => JSON.stringify(identity)));
+    const oneExecutionIdentity = identityFieldsComplete && uniqueExecutionIdentities.size === 1;
+    const supportingKinds = new Set(supportingFiles.map((source) => source.kind));
+    const missingSupportingKinds = ['artillery_result', 'artillery_stdout', 'evidence']
+        .filter((kind) => !supportingKinds.has(kind));
+    const complete = expectedVuKnown && entries.length === expectedVus && uniqueVuIds === expectedVus &&
+        vuIds.length === entries.length && runIds.length === 1 &&
+        missingArtifacts.length === 0 && artifactWriteErrors.length === 0 && missingSupportingKinds.length === 0 &&
+        oneExecutionIdentity;
+    return {
+        schemaVersion: 1,
+        generatedAt: new Date().toISOString(),
+        runDirectory: root,
+        expectedVus,
+        observationFiles: entries.map(({ sourceFile, sha256, report }) => ({
+            path: sourceFile, sha256, vuId: report.vuId, runId: report.runId,
+        })),
+        artifactFiles,
+        supportingFiles,
+        validity: {
+            complete,
+            expectedVuKnown,
+            observationFiles: entries.length,
+            uniqueVuIds,
+            allVuIdsPresent: vuIds.length === entries.length,
+            oneRunId: runIds.length === 1,
+            runIds,
+            allMetadataPresent,
+            identityFieldsComplete,
+            oneExecutionIdentity,
+            executionIdentities: [...uniqueExecutionIdentities].map((identity) => JSON.parse(identity)),
+            missingArtifacts,
+            artifactWriteErrors,
+            missingSupportingKinds,
+        },
+    };
+}
+
+function csvCell(value) {
+    const text = value === null || value === undefined ? ''
+        : typeof value === 'string' ? value : JSON.stringify(value);
+    return `"${text.replaceAll('"', '""')}"`;
+}
+
+function renderCsv(details) {
+    const columns = [
+        'vuId', 'failureLocator', 'failureStep', 'urlAtFailure', 'domState', 'loginPost',
+        'podEvidence', 'ingressEvidence', 'directCause', 'mechanism', 'upstreamCause', 'confidence', 'hypotheses',
+    ];
+    return `${columns.map(csvCell).join(',')}\n${details.map((detail) =>
+        columns.map((column) => csvCell(detail[column])).join(',')).join('\n')}\n`;
+}
+
+function tableCell(value) {
+    const text = value === null || value === undefined ? ''
+        : typeof value === 'string' ? value : JSON.stringify(value);
+    return text.replaceAll('|', '\\|').replaceAll('\n', ' ');
+}
+
+function renderMarkdown(analysis, sources) {
+    const { summary, validity } = analysis;
+    const facts = [
+        `원본 VU 파일: ${validity.vuFiles}개 (고유 VU ID ${validity.uniqueVuIds}개)`,
+        `실행 ID: ${analysis.runId || '미확정'}`,
+        `failed_login action 실패: ${summary.failedLoginActions}건`,
+        `fill 오류로 판별: ${summary.fillFailures}건`,
+        `정확한 failed_login POST /api/auth/login 401 VU: ${summary.exact401Vus}건`,
+        `로그인 오류 UI 확인 후 다음 시나리오로 진행한 VU: ${summary.errorMessageConfirmedVus}건`,
+        `기대 401 + fill 실패 = 전체 VU 관계: ${summary.relation410Plus82Confirmed ? '확정' : '미확정'}`,
+        `입력 manifest: ${validity.inputManifestComplete ? '완전' : '불완전'}`,
+    ];
+    const unresolved = [];
+    if (!validity.expectedVuKnown) unresolved.push('metadata 또는 CLI에 기대 VU 수가 없다.');
+    else if (!validity.expectedVuCountMatches) unresolved.push(`대상 ${analysis.expectations.expectedVus} VU와 원본 수가 일치하지 않는다.`);
+    if (!validity.metadataProvided) unresolved.push('실행 시각·대상 URL·배치·부하 Pod 수 metadata가 없다.');
+    if (!validity.inputManifestComplete) unresolved.push('VU·artifact·Artillery 원본·stdout·인프라 evidence manifest가 불완전하다.');
+    if (!validity.evidenceProvided) unresolved.push('동시간대 프런트엔드·Ingress·부하 Pod 로그/지표 evidence가 없다.');
+    if (validity.evidenceProvided && !validity.evidenceMatchesRun) unresolved.push('evidence 실행 ID가 달라 대상 판정에서 제외했다.');
+    if (analysis.failedVus.some((detail) => detail.failureLocator === 'login-password-input' && !detail.domState.password)) {
+        unresolved.push('비밀번호 실패 VU의 DOM 상태를 확정할 trace·screenshot·call log가 부족하다.');
+    }
+
+    const lines = [
+        '# 로그인 fill 실패 분석', '',
+        '## 확정 사실', '',
+        ...facts.map((fact) => `- ${fact}`), '',
+        '## 원인별 집계', '',
+        `- 실패 단계: ${JSON.stringify(summary.failureSteps)}`,
+        `- 직접 원인: ${JSON.stringify(summary.directCauses)}`,
+        `- 상위 원인: ${JSON.stringify(summary.upstreamCauses)}`,
+        `- 실패 VU 로그인 POST: ${JSON.stringify(summary.loginPostOutcomesForFailures)}`, '',
+        '## 강한 정황', '',
+        ...(analysis.failedVus.filter((detail) => detail.confidence === 'strong_indication')
+            .map((detail) => `- ${detail.vuId}: ${detail.directCause}${detail.mechanism ? ` (${detail.mechanism})` : ''}`)),
+    ];
+    if (!lines.at(-1)) lines.push('- 없음');
+    lines.push('', '## 미확정 가설 및 추가 자료', '');
+    lines.push(...(unresolved.length ? unresolved : ['추가 미확정 항목 없음']).map((item) => `- ${item}`));
+    lines.push('', '## 입력 자료 무결성', '', ...sources.map((source) =>
+        `- ${source.kind}: ${source.path} (SHA-256 ${source.sha256})`));
+    lines.push('', '## 실패 VU 상세', '',
+        '| VU ID | 실패 locator | URL | DOM 상태 | 로그인 POST | Pod·Ingress 증거 | 직접 원인 | 상위 원인 |',
+        '|---|---|---|---|---|---|---|---|');
+    for (const detail of analysis.failedVus) {
+        lines.push(`| ${[
+            detail.vuId, detail.failureLocator, detail.urlAtFailure, detail.domState, detail.loginPost,
+            [...detail.podEvidence, ...detail.ingressEvidence], detail.directCause, detail.upstreamCause,
+        ].map(tableCell).join(' | ')} |`);
+    }
+    return `${lines.join('\n')}\n`;
+}
+
+function parseArgs(argv) {
+    const args = { runDirectory: null };
+    for (let index = 0; index < argv.length; index += 1) {
+        const value = argv[index];
+        if (!value.startsWith('--') && !args.runDirectory) {
+            args.runDirectory = path.resolve(value);
+            continue;
+        }
+        const key = value.slice(2);
+        const next = argv[index + 1];
+        if (!next || next.startsWith('--')) throw new Error(`Missing value for --${key}`);
+        args[key] = next;
+        index += 1;
+    }
+    if (!args.runDirectory || !args['output-dir']) {
+        throw new Error('Usage: node analyze-login-fill-failures.js <run-directory> --output-dir <directory> --evidence <json> --artillery-result <file> --artillery-stdout <file> [--expected-run-id <id>] [--metadata <json>]');
+    }
+    for (const required of ['evidence', 'artillery-result', 'artillery-stdout']) {
+        if (!args[required]) throw new Error(`Missing required --${required}`);
+    }
+    return args;
+}
+
+function readJson(filePath) {
+    return JSON.parse(fs.readFileSync(path.resolve(filePath), 'utf8'));
+}
+
+function main(argv) {
+    const args = parseArgs(argv);
+    const entries = loadRunDirectory(args.runDirectory);
+    const metadata = args.metadata ? readJson(args.metadata) : null;
+    const evidence = args.evidence ? readJson(args.evidence) : null;
+    const analysis = analyzeReports(entries, {
+        expectedRunId: args['expected-run-id'],
+        expectedVus: args['expected-vus'] ? Number(args['expected-vus']) : undefined,
+        expectedFillFailures: args['expected-fill-failures'] ? Number(args['expected-fill-failures']) : undefined,
+        expected401Vus: args['expected-401-vus'] ? Number(args['expected-401-vus']) : undefined,
+        metadata,
+        evidence,
+    });
+    const sources = entries.map(({ sourceFile, sha256 }) => ({
+        kind: 'observation', path: path.join(args.runDirectory, sourceFile), sha256,
+    }));
+    for (const [key, kind] of [['artillery-result', 'artillery_result'], ['artillery-stdout', 'artillery_stdout'], ['metadata', 'metadata'], ['evidence', 'evidence']]) {
+        if (!args[key]) continue;
+        const sourcePath = path.resolve(args[key]);
+        sources.push({ kind, path: sourcePath, sha256: hashFile(sourcePath) });
+    }
+    const manifest = buildInputManifest(
+        entries,
+        args.runDirectory,
+        analysis.expectations.expectedVus,
+        sources.filter((source) => source.kind !== 'observation')
+    );
+    analysis.validity.inputManifestComplete = manifest.validity.complete;
+    analysis.sources = sources;
+
+    const outputDirectory = path.resolve(args['output-dir']);
+    fs.mkdirSync(outputDirectory, { recursive: true });
+    fs.writeFileSync(path.join(outputDirectory, 'analysis.json'), `${JSON.stringify(analysis, null, 2)}\n`);
+    fs.writeFileSync(path.join(outputDirectory, 'failed-vus.csv'), renderCsv(analysis.failedVus));
+    fs.writeFileSync(path.join(outputDirectory, 'input-manifest.json'), `${JSON.stringify(manifest, null, 2)}\n`);
+    fs.writeFileSync(path.join(outputDirectory, 'report.md'), renderMarkdown(analysis, sources));
+    console.log(JSON.stringify({
+        outputDirectory,
+        runId: analysis.runId,
+        validity: analysis.validity,
+        summary: analysis.summary,
+    }, null, 2));
+    if (!analysis.validity.oneRunId || analysis.validity.expectedRunIdMatches === false ||
+        !analysis.validity.evidenceMatchesRun || analysis.validity.metadataMatchesRun === false ||
+        !manifest.validity.complete) process.exitCode = 2;
+}
+
+if (require.main === module) main(process.argv.slice(2));
+
+module.exports = {
+    analyzeReports,
+    buildInputManifest,
+    classifyFailedVu,
+    detectFailureLocator,
+    isFillError,
+    loginPostOutcome,
+    renderCsv,
+    renderMarkdown,
+};
diff --git a/e2e/artillery/analyze-login-fill-failures.test.js b/e2e/artillery/analyze-login-fill-failures.test.js
new file mode 100644
index 00000000..32ffa64e
--- /dev/null
+++ b/e2e/artillery/analyze-login-fill-failures.test.js
@@ -0,0 +1,336 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const { spawnSync } = require('child_process');
+const {
+    analyzeReports,
+    buildInputManifest,
+    classifyFailedVu,
+    detectFailureLocator,
+    loginPostOutcome,
+} = require('./analyze-login-fill-failures');
+
+const RUN_ID = 'target-492';
+const STARTED_AT = '2026-08-12T00:00:01.000Z';
+const ENDED_AT = '2026-08-12T00:00:31.000Z';
+
+function failedReport(vuId, locator = 'login-email-input', dom = { count: 1, visible: true, enabled: true }) {
+    return {
+        runId: RUN_ID,
+        vuId,
+        samples: {
+            actions: [
+                {
+                    name: 'login_form_ready_for_failed_login', success: true, durationMs: 100,
+                    startedAt: '2026-08-12T00:00:00.000Z', endedAt: '2026-08-12T00:00:00.100Z',
+                },
+                {
+                    name: 'failed_login', success: false, durationMs: 30000,
+                    startedAt: STARTED_AT, endedAt: ENDED_AT,
+                    error: {
+                        name: 'TimeoutError',
+                        message: `locator.fill: Timeout 30000ms exceeded. waiting for getByTestId('${locator}')`,
+                    },
+                    diagnostics: {
+                        url: 'https://chat.example.com/login',
+                        ui: {
+                            '[data-testid="login-email-input"]': locator === 'login-email-input' ? dom : {
+                                count: 1, visible: true, enabled: true,
+                            },
+                            '[data-testid="login-submit-button"]': { count: 1, visible: true, enabled: true },
+                        },
+                        recentHttp: [],
+                    },
+                },
+            ],
+            http: [],
+        },
+        timeline: [
+            { name: 'navigation', at: '2026-08-12T00:00:00.000Z', url: 'https://chat.example.com/login' },
+        ],
+    };
+}
+
+function successfulFailedLoginReport(vuId, action = 'failed_login') {
+    return {
+        runId: RUN_ID,
+        vuId,
+        samples: {
+            actions: [
+                {
+                    name: 'failed_login', success: true, durationMs: 50,
+                    startedAt: STARTED_AT, endedAt: '2026-08-12T00:00:01.050Z',
+                },
+                {
+                    name: 'register', success: true, durationMs: 50,
+                    startedAt: '2026-08-12T00:00:02.000Z', endedAt: '2026-08-12T00:00:02.050Z',
+                },
+            ],
+            http: [{
+                name: 'POST /api/auth/login', action, status: 401, success: false,
+                startedAt: STARTED_AT, endedAt: '2026-08-12T00:00:01.040Z',
+            }],
+        },
+        timeline: [],
+    };
+}
+
+test('detects only the locator named by the Playwright error', () => {
+    assert.equal(detectFailureLocator("waiting for getByTestId('login-email-input')"), 'login-email-input');
+    assert.equal(detectFailureLocator('locator.fill waiting for [data-testid="login-password-input"]'), 'login-password-input');
+    assert.equal(detectFailureLocator('locator.fill: Timeout 30000ms exceeded'), 'unconfirmed');
+});
+
+test('uses only failed_login POST /api/auth/login for the 401 outcome', () => {
+    const report = successfulFailedLoginReport('vu-1', 'login');
+    assert.deepEqual(loginPostOutcome(report), {
+        outcome: 'no_request', requestCount: 0, exact401Count: 0, statuses: [], wrongActionRequestCount: 1,
+    });
+});
+
+test('classifies a locator that disappeared after form readiness as DOM replacement', () => {
+    const detail = classifyFailedVu(failedReport('vu-1', 'login-email-input', {
+        count: 0, visible: false, enabled: false,
+    }), 'vu-1.json');
+    assert.equal(detail.failureStep, 'email_fill');
+    assert.equal(detail.directCause, 'dom_replacement');
+    assert.equal(detail.mechanism, 'locator_disappeared');
+    assert.deepEqual(detail.sequence, {
+        formReady: 'completed',
+        emailFill: 'failed',
+        passwordFill: 'not_reached',
+        submitClick: 'not_reached',
+        loginPost: 'no_request',
+        response401: 'not_confirmed',
+        errorMessage: 'unconfirmed',
+    });
+});
+
+test('uses the observed email locator disappearance to classify form replacement during password fill', () => {
+    const report = failedReport('vu-1', 'login-password-input');
+    report.samples.actions[1].diagnostics.ui['[data-testid="login-email-input"]'] = {
+        count: 0, visible: false, enabled: false,
+    };
+    const detail = classifyFailedVu(report, 'vu-1.json');
+    assert.equal(detail.failureStep, 'password_fill');
+    assert.equal(detail.directCause, 'dom_replacement');
+});
+
+test('classifies a changed URL during fill as navigation before DOM state', () => {
+    const report = failedReport('vu-1', 'login-email-input', { count: 0, visible: false, enabled: false });
+    report.samples.actions[1].diagnostics.url = 'https://chat.example.com/register';
+    report.timeline.push({
+        name: 'navigation', at: '2026-08-12T00:00:10.000Z', url: 'https://chat.example.com/register',
+    });
+    const detail = classifyFailedVu(report, 'vu-1.json');
+    assert.equal(detail.directCause, 'navigation');
+});
+
+test('requires overlapping load Pod saturation evidence to classify an execution stall', () => {
+    const report = failedReport('vu-1');
+    const withoutEvidence = classifyFailedVu(report, 'vu-1.json');
+    assert.equal(withoutEvidence.directCause, 'unclassified');
+    assert.deepEqual(withoutEvidence.hypotheses, ['loadgen_stall_requires_runtime_or_pod_evidence']);
+
+    const withEvidence = classifyFailedVu(report, 'vu-1.json', {
+        vus: {},
+        events: [{
+            scope: 'load_pod', signal: 'cpu_throttling', detail: '91%',
+            startedAt: '2026-08-12T00:00:05.000Z', endedAt: '2026-08-12T00:00:25.000Z',
+        }],
+    });
+    assert.equal(withEvidence.directCause, 'loadgen_stall');
+    assert.equal(withEvidence.upstreamCause, 'load_pod_saturation');
+});
+
+test('classifies a tagged non-editable password input without relying on aggregate error text', () => {
+    const report = failedReport('vu-1', 'login-password-input');
+    report.samples.actions[1].error = {
+        name: 'TimeoutError',
+        message: '[loginAction step=password_fill locator=login-password-input] locator.fill timed out',
+        loginActionStep: 'password_fill',
+        loginActionLocator: 'login-password-input',
+    };
+    report.samples.actions[1].diagnostics.ui['[data-testid="login-password-input"]'] = {
+        count: 1, visible: true, enabled: true, connected: true, editable: false, readOnly: true,
+    };
+
+    const detail = classifyFailedVu(report, 'vu-1.json');
+    assert.equal(detail.failureStep, 'password_fill');
+    assert.equal(detail.failureLocator, 'login-password-input');
+    assert.equal(detail.directCause, 'not_editable');
+    assert.equal(detail.mechanism, 'readonly');
+});
+
+test('uses overlapping event-loop lag to classify a maintained editable locator as loadgen stall', () => {
+    const report = failedReport('vu-1');
+    report.samples.runtime = [{
+        name: 'event_loop_lag', durationMs: 1500,
+        startedAt: '2026-08-12T00:00:05.000Z', endedAt: '2026-08-12T00:00:06.500Z',
+    }];
+    const detail = classifyFailedVu(report, 'vu-1.json');
+    assert.equal(detail.directCause, 'loadgen_stall');
+    assert.equal(detail.mechanism, 'event_loop_lag');
+});
+
+test('classifies an overlapping document 504 when no closer DOM cause exists', () => {
+    const report = failedReport('vu-1');
+    report.samples.actions[1].diagnostics.recentDocuments = [{ status: 504, action: 'failed_login' }];
+    const detail = classifyFailedVu(report, 'vu-1.json');
+    assert.equal(detail.directCause, 'frontend_or_ingress_failure');
+    assert.equal(detail.mechanism, 'overlapping_5xx');
+});
+
+test('does not infer password DOM state from email diagnostics', () => {
+    const detail = classifyFailedVu(failedReport('vu-1', 'login-password-input'), 'vu-1.json');
+    assert.equal(detail.failureStep, 'password_fill');
+    assert.equal(detail.domState.password, null);
+    assert.equal(detail.directCause, 'unclassified');
+    assert.ok(detail.hypotheses.includes('password_dom_state_requires_trace_screenshot_or_call_log'));
+});
+
+test('confirms N plus M only when failed fill VUs have no request and every other VU has one exact 401', () => {
+    const entries = [
+        { sourceFile: 'vu-1.json', report: failedReport('vu-1') },
+        { sourceFile: 'vu-2.json', report: successfulFailedLoginReport('vu-2') },
+        { sourceFile: 'vu-3.json', report: successfulFailedLoginReport('vu-3') },
+    ];
+    const analysis = analyzeReports(entries, {
+        expectedVus: 3, expectedFillFailures: 1, expected401Vus: 2,
+    });
+    assert.equal(analysis.summary.relation410Plus82Confirmed, true);
+
+    entries[2] = { sourceFile: 'vu-3.json', report: successfulFailedLoginReport('vu-3', 'login') };
+    const actionMismatch = analyzeReports(entries, {
+        expectedVus: 3, expectedFillFailures: 1, expected401Vus: 2,
+    });
+    assert.equal(actionMismatch.summary.relation410Plus82Confirmed, false);
+});
+
+test('derives expected counts from embedded run metadata instead of hardcoded defaults', () => {
+    const entries = [
+        { sourceFile: 'vu-1.json', report: failedReport('vu-1') },
+        { sourceFile: 'vu-2.json', report: successfulFailedLoginReport('vu-2') },
+        { sourceFile: 'vu-3.json', report: successfulFailedLoginReport('vu-3') },
+    ];
+    for (const entry of entries) {
+        entry.report.metadata = {
+            workload: { arrivalCount: 3 },
+            expectations: { fillFailures: 1, failedLogin401: 2 },
+        };
+    }
+
+    const analysis = analyzeReports(entries);
+    assert.deepEqual(analysis.expectations, {
+        expectedVus: 3, expectedFillFailures: 1, expected401Vus: 2,
+    });
+    assert.equal(analysis.summary.expectedOutcomeRelationConfirmed, true);
+    assert.equal(analysis.summary.allExpectedFailedLoginOutcomesConfirmed, true);
+});
+
+test('marks the input manifest incomplete when a referenced failure artifact is missing', () => {
+    const temporaryRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'login-fill-manifest-'));
+    try {
+        const report = failedReport('vu-1');
+        report.artifacts = [{ kind: 'screenshot', path: 'artifacts/missing.png' }];
+        const manifest = buildInputManifest([{
+            sourceFile: 'vu-1.json', sha256: 'abc', report,
+        }], temporaryRoot, 1);
+        assert.equal(manifest.validity.complete, false);
+        assert.deepEqual(manifest.validity.missingArtifacts, [
+            { vuId: 'vu-1', kind: 'error', path: null, reason: 'missing_reference' },
+            { vuId: 'vu-1', path: 'artifacts/missing.png', reason: 'missing' },
+        ]);
+    } finally {
+        fs.rmSync(temporaryRoot, { recursive: true, force: true });
+    }
+});
+
+test('excludes evidence whose run ID differs from the observation run', () => {
+    const analysis = analyzeReports([{ sourceFile: 'vu-1.json', report: failedReport('vu-1') }], {
+        expectedVus: 1,
+        expectedFillFailures: 1,
+        expected401Vus: 0,
+        evidence: {
+            runId: 'another-run',
+            events: [{
+                scope: 'load_pod', signal: 'saturation',
+                startedAt: STARTED_AT, endedAt: ENDED_AT,
+            }],
+        },
+    });
+    assert.equal(analysis.validity.evidenceMatchesRun, false);
+    assert.equal(analysis.failedVus[0].directCause, 'unclassified');
+    assert.deepEqual(analysis.excludedEvidence, { runId: 'another-run', reason: 'run_id_mismatch' });
+});
+
+test('does not confirm the relation when run or VU identifiers are missing', () => {
+    const missingRunId = failedReport('vu-1');
+    delete missingRunId.runId;
+    const missingVuId = successfulFailedLoginReport('vu-2');
+    delete missingVuId.vuId;
+    const analysis = analyzeReports([
+        { sourceFile: 'vu-1.json', report: missingRunId },
+        { sourceFile: 'vu-2.json', report: missingVuId },
+    ], { expectedVus: 2, expectedFillFailures: 1, expected401Vus: 1 });
+    assert.equal(analysis.validity.oneRunId, false);
+    assert.equal(analysis.validity.allVuIdsPresent, false);
+    assert.equal(analysis.summary.relation410Plus82Confirmed, false);
+});
+
+test('CLI writes JSON, CSV, and Markdown without changing the observation input', () => {
+    const temporaryRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'login-fill-analysis-'));
+    const runDirectory = path.join(temporaryRoot, RUN_ID);
+    const outputDirectory = path.join(temporaryRoot, 'output');
+    fs.mkdirSync(runDirectory);
+    const sourcePath = path.join(runDirectory, 'vu-1.json');
+    const evidencePath = path.join(temporaryRoot, 'evidence.json');
+    const artilleryResultPath = path.join(temporaryRoot, 'artillery-result.json');
+    const artilleryStdoutPath = path.join(temporaryRoot, 'artillery.stdout.log');
+    const report = failedReport('vu-1');
+    report.metadata = {
+        targetUrl: 'https://chat.example.com',
+        gitSha: 'abc123',
+        images: { frontend: 'sha256:frontend', loadGenerator: 'sha256:load' },
+        workload: { arrivalCount: 1 },
+        expectations: { totalVus: 1, fillFailures: 1, failedLogin401: 0 },
+    };
+    report.artifacts = [
+        { kind: 'error', path: 'artifacts/vu-1.error.json' },
+        { kind: 'screenshot', path: 'artifacts/vu-1.png', masked: true },
+    ];
+    const source = `${JSON.stringify(report, null, 2)}\n`;
+    fs.mkdirSync(path.join(runDirectory, 'artifacts'));
+    fs.writeFileSync(path.join(runDirectory, 'artifacts/vu-1.error.json'), '{}\n');
+    fs.writeFileSync(path.join(runDirectory, 'artifacts/vu-1.png'), 'masked screenshot fixture');
+    fs.writeFileSync(sourcePath, source);
+    fs.writeFileSync(evidencePath, `${JSON.stringify({ runId: RUN_ID, events: [], vus: {} })}\n`);
+    fs.writeFileSync(artilleryResultPath, '{}\n');
+    fs.writeFileSync(artilleryStdoutPath, 'artillery output\n');
+    try {
+        const result = spawnSync(process.execPath, [
+            path.resolve(__dirname, 'analyze-login-fill-failures.js'),
+            runDirectory,
+            '--output-dir', outputDirectory,
+            '--expected-run-id', RUN_ID,
+            '--expected-vus', '1',
+            '--expected-fill-failures', '1',
+            '--expected-401-vus', '0',
+            '--evidence', evidencePath,
+            '--artillery-result', artilleryResultPath,
+            '--artillery-stdout', artilleryStdoutPath,
+        ], { encoding: 'utf8' });
+        assert.equal(result.status, 0, result.stderr);
+        assert.equal(fs.readFileSync(sourcePath, 'utf8'), source);
+        assert.equal(JSON.parse(fs.readFileSync(path.join(outputDirectory, 'analysis.json')))
+            .summary.relation410Plus82Confirmed, true);
+        assert.match(fs.readFileSync(path.join(outputDirectory, 'failed-vus.csv'), 'utf8'), /vu-1/);
+        assert.equal(JSON.parse(fs.readFileSync(path.join(outputDirectory, 'input-manifest.json')))
+            .validity.complete, true);
+        assert.match(fs.readFileSync(path.join(outputDirectory, 'report.md'), 'utf8'), /확정 사실/);
+    } finally {
+        fs.rmSync(temporaryRoot, { recursive: true, force: true });
+    }
+});
diff --git a/e2e/artillery/observation.js b/e2e/artillery/observation.js
index a8845fbb..aa8d3e83 100644
--- a/e2e/artillery/observation.js
+++ b/e2e/artillery/observation.js
@@ -8,6 +8,9 @@ const INTEGER = /^\d+$/;
 const CHAT_ROOM_URL = /\/chat\/[a-f\d]{24}(?:[/?#]|$)/i;
 const CHAT_INPUT_TEST_ID = 'chat-message-input';
 const LATE_VISIBILITY_TIMEOUT_MS = 15000;
+const EVENT_LOOP_SAMPLE_MS = 1000;
+const DEFAULT_RUN_ID = new Date().toISOString().replace(/[:.]/g, '-');
+const RESOURCE_TYPES = new Set(['document', 'script', 'stylesheet', 'font', 'image']);
 
 const SOCKET_PAIRS = {
     joinRoom: { response: 'joinRoomSuccess', metric: 'room_join' },
@@ -15,6 +18,102 @@ const SOCKET_PAIRS = {
     chatMessage: { response: 'message', metric: 'message_send' },
 };
 
+function numberFromEnvironment(value) {
+    if (value === undefined || value === null || value === '') return null;
+    const number = Number(value);
+    return Number.isFinite(number) ? number : null;
+}
+
+function sanitizeArtifactSegment(value) {
+    return String(value || 'unknown').replace(/[^a-zA-Z0-9._-]/g, '_').slice(0, 120);
+}
+
+function createRunMetadata(environment = process.env) {
+    return {
+        targetUrl: environment.BASE_URL || null,
+        gitSha: environment.GIT_SHA || environment.COMMIT_SHA || null,
+        images: {
+            frontend: environment.FRONTEND_IMAGE_DIGEST || null,
+            loadGenerator: environment.LOAD_IMAGE_DIGEST || null,
+        },
+        workload: {
+            durationSeconds: numberFromEnvironment(environment.PHASE1_DURATION),
+            arrivalCount: numberFromEnvironment(environment.PHASE1_ARRIVAL_COUNT),
+            virtualUsersPerPod: numberFromEnvironment(environment.VUS_PER_POD),
+        },
+        expectations: {
+            totalVus: numberFromEnvironment(environment.EXPECTED_TOTAL_VUS),
+            failedLogin401: numberFromEnvironment(environment.EXPECTED_FAILED_LOGIN_401),
+            fillFailures: numberFromEnvironment(environment.EXPECTED_LOGIN_FILL_FAILURES),
+        },
+        pod: {
+            name: environment.POD_NAME || environment.HOSTNAME || null,
+            namespace: environment.POD_NAMESPACE || null,
+            nodeName: environment.NODE_NAME || null,
+            cpuRequest: environment.POD_CPU_REQUEST || null,
+            cpuLimit: environment.POD_CPU_LIMIT || null,
+            memoryRequest: environment.POD_MEMORY_REQUEST || null,
+            memoryLimit: environment.POD_MEMORY_LIMIT || null,
+        },
+        runtime: {
+            nodeVersion: process.version,
+            platform: process.platform,
+            arch: process.arch,
+            pid: process.pid,
+        },
+    };
+}
+
+function serializeError(error) {
+    return {
+        name: error?.name || 'Error',
+        message: error?.message || String(error),
+        stack: typeof error?.stack === 'string' ? error.stack : null,
+        cause: error?.cause ? {
+            name: error.cause.name || null,
+            message: error.cause.message || String(error.cause),
+            stack: typeof error.cause.stack === 'string' ? error.cause.stack : null,
+        } : null,
+        loginActionStep: error?.loginActionStep || null,
+        loginActionLocator: error?.loginActionLocator || null,
+    };
+}
+
+async function inspectLocator(locator) {
+    const count = await locator.count().catch(() => 0);
+    if (count === 0) {
+        return {
+            count: 0, connected: false, visible: false, enabled: false,
+            readOnly: null, editable: false, tagName: null, inputType: null,
+        };
+    }
+
+    const [visible, enabled, element] = await Promise.all([
+        locator.isVisible().catch(() => false),
+        locator.isEnabled().catch(() => false),
+        locator.evaluate((node) => {
+            const tagName = node.tagName?.toLowerCase() || null;
+            const supportsReadOnly = tagName === 'input' || tagName === 'textarea';
+            const disabled = 'disabled' in node ? Boolean(node.disabled) : false;
+            const readOnly = supportsReadOnly ? Boolean(node.readOnly) : null;
+            const editable = supportsReadOnly
+                ? !disabled && !readOnly
+                : Boolean(node.isContentEditable);
+            return {
+                connected: Boolean(node.isConnected),
+                tagName,
+                inputType: tagName === 'input' ? node.type || null : null,
+                readOnly,
+                editable,
+            };
+        }, undefined, { timeout: 1000 }).catch(() => ({
+            connected: false, tagName: null, inputType: null, readOnly: null, editable: false,
+        })),
+    ]);
+
+    return { count, visible, enabled, ...element };
+}
+
 function percentile(values, ratio) {
     if (values.length === 0) return 0;
     const sorted = [...values].sort((a, b) => a - b);
@@ -120,13 +219,37 @@ function createObservation(page, vuContext) {
     const startedRequests = new Map();
     const pendingSocketEvents = new Map();
     const samples = {
-        documents: [], http: [], socket: [], actions: [], browser: [], layoutShifts: [],
+        documents: [], resources: [], http: [], socket: [], actions: [], browser: [], runtime: [], layoutShifts: [],
     };
     const timeline = [];
     const diagnostics = [];
+    const artifacts = [];
+    const runId = process.env.OBSERVATION_RUN_ID || DEFAULT_RUN_ID;
+    const outputRoot = process.env.OBSERVATION_OUTPUT_DIR || path.resolve(__dirname, 'results');
+    const runDirectory = path.join(outputRoot, runId);
+    const vuId = vuContext?._uid || vuContext?.vars?.$uuid || randomUUID();
+    const artifactPrefix = `${process.pid}-${sanitizeArtifactSegment(vuId)}-${randomUUID()}`;
+    const runMetadata = createRunMetadata();
     let currentAction = null;
     let socketSequence = 0;
 
+    let expectedEventLoopSampleAt = performance.now() + EVENT_LOOP_SAMPLE_MS;
+    const eventLoopTimer = setInterval(() => {
+        const sampledAt = performance.now();
+        const lagMs = Math.max(0, sampledAt - expectedEventLoopSampleAt);
+        const endedAt = new Date();
+        samples.runtime.push({
+            name: 'event_loop_lag',
+            action: currentAction,
+            durationMs: lagMs,
+            startedAt: new Date(endedAt.getTime() - lagMs).toISOString(),
+            endedAt: endedAt.toISOString(),
+            success: lagMs < EVENT_LOOP_SAMPLE_MS,
+        });
+        expectedEventLoopSampleAt = sampledAt + EVENT_LOOP_SAMPLE_MS;
+    }, EVENT_LOOP_SAMPLE_MS);
+    eventLoopTimer.unref?.();
+
     const addTimelineEvent = (name, details = {}) => {
         timeline.push({ name, at: new Date().toISOString(), ...details });
     };
@@ -177,18 +300,23 @@ function createObservation(page, vuContext) {
     const onRequestFailed = (request) => { void recordHttp(request, false); };
     const onResponse = (response) => {
         const request = response.request();
-        if (request.resourceType() !== 'document') return;
+        const resourceType = request.resourceType();
+        if (!RESOURCE_TYPES.has(resourceType)) return;
         const timing = request.timing();
         const ttfbMs = timing?.responseStart;
-        samples.documents.push({
+        const sample = {
             name: `GET ${normalizeDocumentPath(response.url())}`,
+            action: currentAction,
+            resourceType,
             durationMs: Number.isFinite(ttfbMs) && ttfbMs >= 0 ? ttfbMs : 0,
             startedAt: new Date(Date.now() - Math.max(ttfbMs || 0, 0)).toISOString(),
             endedAt: new Date().toISOString(),
             success: response.status() < 400,
             status: response.status(),
             redirectedFrom: request.redirectedFrom()?.url() || null,
-        });
+        };
+        if (resourceType === 'document') samples.documents.push(sample);
+        else samples.resources.push(sample);
     };
     const onWebSocket = (webSocket) => {
         const connectionKey = `connection:${socketSequence += 1}`;
@@ -310,48 +438,90 @@ function createObservation(page, vuContext) {
 
     async function collectPageDiagnostics() {
         const selectors = [
-            '[data-testid="login-email-input"]', '[data-testid="login-submit-button"]',
+            '[data-testid="login-email-input"]', '[data-testid="login-password-input"]',
+            '[data-testid="login-submit-button"]',
             '[data-testid="chat-room-name-input"]', '[data-testid="chat-message-input"]',
             '[data-testid="message-submission-status"]', '[data-testid="file-message-container"]',
         ];
         const ui = {};
         for (const selector of selectors) {
             const locator = page.locator(selector).first();
-            ui[selector] = {
-                count: await locator.count().catch(() => 0),
-                visible: await locator.isVisible().catch(() => false),
-                enabled: await locator.isEnabled().catch(() => false),
-                text: await locator.textContent().catch(() => null),
-            };
+            ui[selector] = await inspectLocator(locator);
         }
+        const documentState = await page.evaluate(() => ({
+            readyState: document.readyState,
+            visibilityState: document.visibilityState,
+            url: window.location.href,
+        })).catch(() => null);
         return {
             url: page.url(),
+            document: documentState,
             ui,
             recentHttp: samples.http.slice(-10).map(({ name, status, success, action, startedAt, endedAt }) =>
                 ({ name, status, success, action, startedAt, endedAt })),
+            recentDocuments: samples.documents.slice(-10),
+            recentResources: samples.resources.slice(-20),
+            recentBrowserErrors: samples.browser.slice(-10),
+            recentEventLoopLag: samples.runtime.slice(-10),
             pendingSocketEvents: [...pendingSocketEvents.keys()].map(String),
         };
     }
 
+    async function writeFailureArtifacts(actionName, error) {
+        const artifactsDirectory = path.join(runDirectory, 'artifacts');
+        const baseName = `${artifactPrefix}-${sanitizeArtifactSegment(actionName)}`;
+        const written = [];
+
+        try {
+            fs.mkdirSync(artifactsDirectory, { recursive: true });
+            const errorFile = path.join(artifactsDirectory, `${baseName}.error.json`);
+            fs.writeFileSync(errorFile, `${JSON.stringify(serializeError(error), null, 2)}\n`);
+            written.push({ kind: 'error', path: path.relative(runDirectory, errorFile) });
+        } catch (artifactError) {
+            written.push({ kind: 'error', writeError: artifactError.message });
+        }
+
+        try {
+            fs.mkdirSync(artifactsDirectory, { recursive: true });
+            const screenshotFile = path.join(artifactsDirectory, `${baseName}.png`);
+            await page.screenshot({
+                path: screenshotFile,
+                fullPage: true,
+                mask: [page.locator('input'), page.locator('textarea')],
+                timeout: 5000,
+            });
+            written.push({ kind: 'screenshot', path: path.relative(runDirectory, screenshotFile), masked: true });
+        } catch (artifactError) {
+            written.push({ kind: 'screenshot', writeError: artifactError.message, masked: true });
+        }
+
+        artifacts.push(...written.map((artifact) => ({ action: actionName, ...artifact })));
+        return written;
+    }
+
     async function action(name, callback) {
         const previousAction = currentAction;
         currentAction = name;
         const startedAt = performance.now();
         const wallStartedAt = new Date().toISOString();
-        addTimelineEvent('action.started', { action: name });
+        const urlAtStart = page.url();
+        addTimelineEvent('action.started', { action: name, url: urlAtStart });
         try {
             const result = await callback();
             samples.actions.push({
                 name, durationMs: performance.now() - startedAt, startedAt: wallStartedAt,
-                endedAt: new Date().toISOString(), success: true,
+                endedAt: new Date().toISOString(), success: true, urlAtStart, urlAtEnd: page.url(),
             });
             return result;
         } catch (error) {
+            const pageDiagnostics = await collectPageDiagnostics();
+            const failureArtifacts = await writeFailureArtifacts(name, error);
             samples.actions.push({
                 name, durationMs: performance.now() - startedAt, startedAt: wallStartedAt,
-                endedAt: new Date().toISOString(), success: false,
-                error: { name: error.name, message: error.message },
-                diagnostics: await collectPageDiagnostics(),
+                endedAt: new Date().toISOString(), success: false, urlAtStart, urlAtEnd: page.url(),
+                error: serializeError(error),
+                diagnostics: pageDiagnostics,
+                artifacts: failureArtifacts,
             });
             if (name === 'room_create') {
                 await diagnoseRoomCreateFailure(error, startedAt);
@@ -365,6 +535,7 @@ function createObservation(page, vuContext) {
 
     async function finish() {
         await new Promise((resolve) => setImmediate(resolve));
+        clearInterval(eventLoopTimer);
         page.off('request', onRequest);
         page.off('requestfinished', onRequestFinished);
         page.off('requestfailed', onRequestFailed);
@@ -376,30 +547,43 @@ function createObservation(page, vuContext) {
 
         samples.layoutShifts = await page.evaluate(() => window.__e2eLayoutShifts || []).catch(() => []);
 
-        const runId = process.env.OBSERVATION_RUN_ID || new Date().toISOString().replace(/[:.]/g, '-');
-        const outputRoot = process.env.OBSERVATION_OUTPUT_DIR || path.resolve(__dirname, 'results');
-        const runDirectory = path.join(outputRoot, runId);
         fs.mkdirSync(runDirectory, { recursive: true });
+        const metadata = {
+            ...runMetadata,
+            runtimeSnapshot: {
+                memoryUsage: process.memoryUsage(),
+                resourceUsage: typeof process.resourceUsage === 'function' ? process.resourceUsage() : null,
+            },
+        };
         const report = {
-            schemaVersion: 2,
+            schemaVersion: 3,
             runId,
-            vuId: vuContext?._uid || vuContext?.vars?.$uuid || randomUUID(),
+            vuId,
             generatedAt: new Date().toISOString(),
+            metadata,
             samples,
             timeline,
             diagnostics,
+            artifacts,
             summary: {
                 actions: summarizeSamples(samples.actions),
                 documents: summarizeSamples(samples.documents),
+                resources: summarizeSamples(samples.resources),
                 http: summarizeSamples(samples.http),
                 socket: summarizeSamples(samples.socket),
+                runtime: summarizeSamples(samples.runtime),
                 cls: {
                     total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
                     entries: samples.layoutShifts.length,
                 },
             },
         };
-        const filename = `vu-${process.pid}-${randomUUID()}.json`;
+        const metadataFilename = `run-metadata-${sanitizeArtifactSegment(runMetadata.pod.name)}-${process.pid}.json`;
+        const metadataPath = path.join(runDirectory, metadataFilename);
+        if (!fs.existsSync(metadataPath)) {
+            fs.writeFileSync(metadataPath, `${JSON.stringify({ runId, ...runMetadata }, null, 2)}\n`);
+        }
+        const filename = `vu-${artifactPrefix}.json`;
         fs.writeFileSync(path.join(runDirectory, filename), `${JSON.stringify(report, null, 2)}\n`);
         printSummary(report.summary);
         return report;
@@ -424,9 +608,12 @@ function printSummary(summary) {
 
 module.exports = {
     createObservation,
+    createRunMetadata,
+    inspectLocator,
     normalizeApiPath,
     normalizeDocumentPath,
     parseSocketEvent,
+    serializeError,
     summarizeSamples,
     classifyRoomCreateFailure,
 };
diff --git a/e2e/artillery/observation.test.js b/e2e/artillery/observation.test.js
index ea7cf276..0d0dfa31 100644
--- a/e2e/artillery/observation.test.js
+++ b/e2e/artillery/observation.test.js
@@ -1,9 +1,17 @@
 const test = require('node:test');
 const assert = require('node:assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const { EventEmitter } = require('events');
 const {
+    createObservation,
+    createRunMetadata,
+    inspectLocator,
     normalizeApiPath,
     normalizeDocumentPath,
     parseSocketEvent,
+    serializeError,
     summarizeSamples,
     classifyRoomCreateFailure,
 } = require('./observation');
@@ -42,6 +50,148 @@ test('summarizes and ranks samples by cumulative duration', () => {
     });
 });
 
+test('records run identity and workload metadata from the execution environment', () => {
+    const metadata = createRunMetadata({
+        BASE_URL: 'https://chat.example.com',
+        GIT_SHA: 'abc123',
+        FRONTEND_IMAGE_DIGEST: 'sha256:frontend',
+        LOAD_IMAGE_DIGEST: 'sha256:load',
+        PHASE1_DURATION: '60',
+        PHASE1_ARRIVAL_COUNT: '500',
+        VUS_PER_POD: '4',
+        EXPECTED_TOTAL_VUS: '500',
+        EXPECTED_FAILED_LOGIN_401: '500',
+        EXPECTED_LOGIN_FILL_FAILURES: '0',
+        POD_NAME: 'load-1',
+        POD_NAMESPACE: 'loadtest',
+        NODE_NAME: 'worker-1',
+    });
+
+    assert.equal(metadata.targetUrl, 'https://chat.example.com');
+    assert.equal(metadata.gitSha, 'abc123');
+    assert.deepEqual(metadata.images, {
+        frontend: 'sha256:frontend',
+        loadGenerator: 'sha256:load',
+    });
+    assert.deepEqual(metadata.workload, {
+        durationSeconds: 60,
+        arrivalCount: 500,
+        virtualUsersPerPod: 4,
+    });
+    assert.deepEqual(metadata.expectations, { totalVus: 500, failedLogin401: 500, fillFailures: 0 });
+    assert.equal(metadata.pod.name, 'load-1');
+});
+
+test('inspects input editability without reading or serializing its value', async () => {
+    const locator = {
+        count: async () => 1,
+        isVisible: async () => true,
+        isEnabled: async () => true,
+        evaluate: async (callback) => callback({
+            tagName: 'INPUT', type: 'password', disabled: false, readOnly: false,
+            isConnected: true, isContentEditable: false, value: 'must-not-be-recorded',
+        }),
+    };
+
+    const state = await inspectLocator(locator);
+    assert.deepEqual(state, {
+        count: 1,
+        visible: true,
+        enabled: true,
+        connected: true,
+        tagName: 'input',
+        inputType: 'password',
+        readOnly: false,
+        editable: true,
+    });
+    assert.doesNotMatch(JSON.stringify(state), /must-not-be-recorded/);
+});
+
+test('serializes the full tagged error without dropping its cause', () => {
+    const cause = new Error('browser disconnected');
+    const error = new Error('fill timed out', { cause });
+    error.loginActionStep = 'password_fill';
+    error.loginActionLocator = 'login-password-input';
+
+    const serialized = serializeError(error);
+    assert.equal(serialized.message, 'fill timed out');
+    assert.match(serialized.stack, /fill timed out/);
+    assert.equal(serialized.cause.message, 'browser disconnected');
+    assert.equal(serialized.loginActionStep, 'password_fill');
+    assert.equal(serialized.loginActionLocator, 'login-password-input');
+});
+
+test('writes a schema v3 failed-login report with masked failure artifacts', async () => {
+    const temporaryRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'observation-v3-'));
+    const environmentKeys = [
+        'OBSERVATION_RUN_ID', 'OBSERVATION_OUTPUT_DIR', 'BASE_URL', 'GIT_SHA',
+        'FRONTEND_IMAGE_DIGEST', 'LOAD_IMAGE_DIGEST', 'EXPECTED_TOTAL_VUS',
+    ];
+    const previousEnvironment = Object.fromEntries(environmentKeys.map((key) => [key, process.env[key]]));
+    Object.assign(process.env, {
+        OBSERVATION_RUN_ID: 'run-v3',
+        OBSERVATION_OUTPUT_DIR: temporaryRoot,
+        BASE_URL: 'https://chat.example.com',
+        GIT_SHA: 'abc123',
+        FRONTEND_IMAGE_DIGEST: 'sha256:frontend',
+        LOAD_IMAGE_DIGEST: 'sha256:load',
+        EXPECTED_TOTAL_VUS: '1',
+    });
+
+    const emitter = new EventEmitter();
+    const locator = {
+        first() { return this; },
+        count: async () => 1,
+        isVisible: async () => true,
+        isEnabled: async () => true,
+        evaluate: async (callback) => callback({
+            tagName: 'INPUT', type: 'password', disabled: false, readOnly: false,
+            isConnected: true, isContentEditable: false, value: 'never-record-this',
+        }),
+    };
+    const page = {
+        on: emitter.on.bind(emitter),
+        off: emitter.off.bind(emitter),
+        addInitScript: async () => {},
+        mainFrame: () => null,
+        url: () => 'https://chat.example.com/',
+        locator: () => locator,
+        evaluate: async () => [],
+        screenshot: async ({ path: screenshotPath, mask, timeout }) => {
+            assert.equal(mask.length, 2);
+            assert.equal(timeout, 5000);
+            fs.writeFileSync(screenshotPath, 'masked screenshot');
+        },
+    };
+
+    try {
+        const observation = createObservation(page, { _uid: 'vu-1', vars: {} });
+        const failure = new Error('[loginAction step=password_fill locator=login-password-input] fill timeout');
+        failure.loginActionStep = 'password_fill';
+        failure.loginActionLocator = 'login-password-input';
+        await assert.rejects(observation.action('failed_login', async () => { throw failure; }));
+        const report = await observation.finish();
+
+        assert.equal(report.schemaVersion, 3);
+        assert.equal(report.metadata.gitSha, 'abc123');
+        assert.equal(report.samples.actions[0].diagnostics.ui['[data-testid="login-password-input"]'].editable, true);
+        assert.deepEqual(report.artifacts.map((artifact) => artifact.kind), ['error', 'screenshot']);
+        const reportText = JSON.stringify(report);
+        assert.doesNotMatch(reportText, /never-record-this/);
+        for (const artifact of report.artifacts) {
+            assert.equal(fs.existsSync(path.join(temporaryRoot, 'run-v3', artifact.path)), true);
+        }
+        assert.equal(fs.readdirSync(path.join(temporaryRoot, 'run-v3'))
+            .filter((file) => file.startsWith('vu-') && file.endsWith('.json')).length, 1);
+    } finally {
+        for (const key of environmentKeys) {
+            if (previousEnvironment[key] === undefined) delete process.env[key];
+            else process.env[key] = previousEnvironment[key];
+        }
+        fs.rmSync(temporaryRoot, { recursive: true, force: true });
+    }
+});
+
 test('classifies late visibility after the original assertion fails', () => {
     assert.equal(classifyRoomCreateFailure({
         diagnostics: [{ action: 'room_create', becameVisibleWithin15s: true }],
diff --git a/e2e/artillery/report-observations.js b/e2e/artillery/report-observations.js
index e754f3a5..66a5717f 100644
--- a/e2e/artillery/report-observations.js
+++ b/e2e/artillery/report-observations.js
@@ -8,12 +8,14 @@ const files = fs.readdirSync(runDirectory).filter((file) => file.startsWith('vu-
 if (files.length === 0) throw new Error(`No observation JSON files found in ${runDirectory}`);
 
 const reports = files.map((file) => JSON.parse(fs.readFileSync(path.join(runDirectory, file), 'utf8')));
-const samples = { actions: [], documents: [], http: [], socket: [], layoutShifts: [] };
+const samples = { actions: [], documents: [], resources: [], http: [], socket: [], runtime: [], layoutShifts: [] };
 for (const report of reports) {
     samples.actions.push(...report.samples.actions);
     samples.documents.push(...(report.samples.documents || []));
+    samples.resources.push(...(report.samples.resources || []));
     samples.http.push(...report.samples.http);
     samples.socket.push(...report.samples.socket);
+    samples.runtime.push(...(report.samples.runtime || []));
     samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
 
@@ -25,8 +27,10 @@ const aggregate = {
     summary: {
         actions: summarizeSamples(samples.actions),
         documents: summarizeSamples(samples.documents),
+        resources: summarizeSamples(samples.resources),
         http: summarizeSamples(samples.http),
         socket: summarizeSamples(samples.socket),
+        runtime: summarizeSamples(samples.runtime),
         cls: {
             total: Number(samples.layoutShifts.reduce((sum, entry) => sum + entry.value, 0).toFixed(4)),
             entries: samples.layoutShifts.length,
diff --git a/e2e/package.json b/e2e/package.json
index a4bd6996..fa2d8a13 100644
--- a/e2e/package.json
+++ b/e2e/package.json
@@ -5,7 +5,8 @@
   "scripts": {
     "test": "playwright test --grep @smoke",
     "test:full": "playwright test",
-    "test:ui": "playwright test --ui"
+    "test:ui": "playwright test --ui",
+    "test:artillery-unit": "node --test actions/*.test.js artillery/*.test.js"
   },
   "devDependencies": {
     "@playwright/test": "1.61.0",

From f1e8341736ad59d935623d3c3bb5ef511427cd6b Mon Sep 17 00:00:00 2001
From: jeongminju <wjdalswn0405@naver.com>
Date: Wed, 12 Aug 2026 05:44:19 +0900
Subject: [PATCH 31/32] =?UTF-8?q?feat(upload):=20S3=20=EB=B3=B4=EC=A1=B0?=
 =?UTF-8?q?=20=EB=AF=B8=EB=9F=AC=20=EC=97=85=EB=A1=9C=EB=93=9C=20=EC=B6=94?=
 =?UTF-8?q?=EA=B0=80?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 apps/backend/DEPLOY.md                        |  4 +-
 apps/backend/README.md                        |  2 +-
 apps/backend/docs/S3_DIRECT_UPLOAD.md         | 37 +++++++
 .../java/com/ktb/chatapp/config/S3Config.java | 10 +-
 .../chatapp/controller/FileController.java    |  2 +-
 .../ProfileUploadIntentController.java        | 45 +++++++++
 .../controller/UploadIntentController.java    | 45 +++++++++
 .../chatapp/controller/UserController.java    |  2 +-
 .../dto/MirrorUploadResultRequest.java        | 11 +++
 .../ktb/chatapp/dto/UploadIntentRequest.java  |  9 ++
 .../ktb/chatapp/dto/UploadIntentResponse.java |  8 ++
 .../com/ktb/chatapp/model/PendingUpload.java  | 31 ++++++
 .../repository/PendingUploadRepository.java   | 12 +++
 .../ktb/chatapp/service/LocalFileService.java |  9 +-
 .../service/UploadIntentException.java        | 12 +++
 .../chatapp/service/UploadIntentService.java  | 91 +++++++++++++++++
 .../com/ktb/chatapp/service/UploadPolicy.java | 30 ++++++
 .../com/ktb/chatapp/service/UserService.java  |  3 +
 .../storage/MirrorUploadPresigner.java        |  7 ++
 .../ktb/chatapp/storage/PresignedUpload.java  |  7 ++
 .../storage/S3MirrorUploadPresigner.java      | 31 ++++++
 .../java/com/ktb/chatapp/util/FileUtil.java   |  6 ++
 .../src/main/resources/application.properties |  7 +-
 .../service/UploadIntentServiceTest.java      | 77 +++++++++++++++
 .../ktb/chatapp/service/UploadPolicyTest.java | 32 ++++++
 .../ktb/chatapp/service/UserServiceTest.java  |  2 +-
 apps/frontend/.env.example                    |  3 +-
 apps/frontend/Dockerfile                      |  3 +
 apps/frontend/README.md                       |  5 +
 .../frontend/components/ProfileImageUpload.js | 57 ++++++++---
 .../frontend/lib/api/__tests__/client.test.js | 19 ++++
 apps/frontend/lib/api/client.js               |  3 +-
 .../services/__tests__/fileService.test.js    | 99 ++++++++++++++++---
 apps/frontend/services/fileService.js         | 59 +++++++++--
 34 files changed, 732 insertions(+), 48 deletions(-)
 create mode 100644 apps/backend/docs/S3_DIRECT_UPLOAD.md
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileUploadIntentController.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/controller/UploadIntentController.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/dto/MirrorUploadResultRequest.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentRequest.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentResponse.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/model/PendingUpload.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/repository/PendingUploadRepository.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentException.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentService.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/service/UploadPolicy.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/storage/MirrorUploadPresigner.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/storage/PresignedUpload.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/storage/S3MirrorUploadPresigner.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/service/UploadIntentServiceTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/service/UploadPolicyTest.java

diff --git a/apps/backend/DEPLOY.md b/apps/backend/DEPLOY.md
index 28913277..6d05e718 100644
--- a/apps/backend/DEPLOY.md
+++ b/apps/backend/DEPLOY.md
@@ -117,8 +117,8 @@ tail -f logs/app.log
    REDIS_HOST=localhost
    REDIS_PORT=6379
 
-   # 파일 저장소 (AWS 자격 증명은 IAM Role 또는 AWS 기본 환경 변수 사용)
-   FILE_STORAGE_TYPE=s3
+   # 원본 파일은 서버에 유지하고, 선택적 S3 mirror는 IAM Role로 PUT만 수행
+   FILE_STORAGE_TYPE=local
    S3_BUCKET=your-s3-bucket
    AWS_REGION=ap-northeast-2
    S3_ENDPOINT=
diff --git a/apps/backend/README.md b/apps/backend/README.md
index e09767b5..55d0e65f 100644
--- a/apps/backend/README.md
+++ b/apps/backend/README.md
@@ -91,7 +91,7 @@ make verify-java
 | `MONGO_URI` | ✅ | 없음 | MongoDB 연결 문자열              |
 | `REDIS_HOST` | ✅ | 없음 | Redis 호스트                    |
 | `REDIS_PORT` | ✅ | 없음 | Redis 포트                      |
-| `FILE_STORAGE_TYPE` | ❌ | `s3` | 파일 저장 방식. 현재 `s3`만 지원 |
+| `FILE_STORAGE_TYPE` | ❌ | `local` | 원본 파일 저장 방식. S3 mirror 모드에서도 `local` 유지 |
 | `S3_BUCKET` | ✅ | 없음 | 이미지 및 첨부 파일을 저장할 S3 버킷 |
 | `AWS_REGION` | ❌ | `ap-northeast-2` | S3 버킷 리전 |
 | `S3_ENDPOINT` | ❌ | 빈 값 | AWS S3 호환 스토리지의 사용자 지정 엔드포인트 |
diff --git a/apps/backend/docs/S3_DIRECT_UPLOAD.md b/apps/backend/docs/S3_DIRECT_UPLOAD.md
new file mode 100644
index 00000000..9cb350dd
--- /dev/null
+++ b/apps/backend/docs/S3_DIRECT_UPLOAD.md
@@ -0,0 +1,37 @@
+# S3 PutObject mirror upload
+
+`NEXT_PUBLIC_FILE_UPLOAD_MODE=mirror`는 기존 multipart 업로드가 성공한 뒤 같은 파일을
+S3 `pending/mirror/`에 best-effort로 복제한다. 서비스 원본과 응답 계약은 기존 서버
+저장소를 유지하며, S3 실패는 사용자 업로드 성공을 되돌리지 않는다. 기본값은 `server`다.
+
+백엔드는 `FILE_STORAGE_TYPE=local`, `S3_BUCKET`, `AWS_REGION`을 사용한다. IAM Role에는
+`s3:PutObject`만 필요하고 애플리케이션은 미러 객체에 GET, HEAD, Copy, Delete를 호출하지
+않는다. AWS 자격증명은 이미지나 환경변수에 넣지 않는다.
+
+## Bucket CORS
+
+`https://frontend.example.com`을 실제 운영 origin으로 바꾸고 `*`는 사용하지 않는다.
+
+```json
+[
+  {
+    "AllowedOrigins": ["https://frontend.example.com"],
+    "AllowedMethods": ["PUT"],
+    "AllowedHeaders": ["content-type", "x-amz-*"],
+    "ExposeHeaders": ["ETag"],
+    "MaxAgeSeconds": 300
+  }
+]
+```
+
+`pending/` prefix 객체를 1일 후 삭제하는 lifecycle rule을 설정한다. PUT URL은 5분,
+MongoDB intent는 15분 후 만료된다.
+
+## Rollout
+
+1. 기존 서버 저장소가 정상인지 확인한다.
+2. 운영 이미지를 `--build-arg NEXT_PUBLIC_FILE_UPLOAD_MODE=mirror`로 빌드한다.
+3. `mirror_presign_success`, `mirror_put_success`, `mirror_put_failure`를 관찰한다.
+4. 문제가 생기면 프론트만 `server`로 다시 빌드한다.
+
+이 모드는 백엔드 업로드 부하를 제거하거나 S3를 영구 원본으로 사용하지 않는다.
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java b/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
index 16d5eb25..b4e0186d 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/S3Config.java
@@ -6,6 +6,7 @@
 import org.springframework.context.annotation.Configuration;
 import software.amazon.awssdk.regions.Region;
 import software.amazon.awssdk.services.s3.S3Client;
+import software.amazon.awssdk.services.s3.S3Configuration;
 import software.amazon.awssdk.services.s3.presigner.S3Presigner;
 
 @Configuration
@@ -28,8 +29,13 @@ S3Client s3Client(
     @Bean
     S3Presigner s3Presigner(
             @Value("${aws.s3.region}") String region,
-            @Value("${aws.s3.endpoint:}") String endpoint) {
-        var builder = S3Presigner.builder().region(Region.of(region));
+            @Value("${aws.s3.endpoint:}") String endpoint,
+            @Value("${aws.s3.path-style-access:false}") boolean pathStyleAccess) {
+        var builder = S3Presigner.builder()
+                .region(Region.of(region))
+                .serviceConfiguration(S3Configuration.builder()
+                        .pathStyleAccessEnabled(pathStyleAccess)
+                        .build());
         if (!endpoint.isBlank()) {
             builder.endpointOverride(URI.create(endpoint));
         }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/FileController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/FileController.java
index db90ecba..586d8c2f 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/FileController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/FileController.java
@@ -45,7 +45,7 @@ public class FileController {
     /**
      * 파일 업로드
      */
-    @Operation(summary = "파일 업로드", description = "파일을 업로드합니다. 최대 50MB까지 가능합니다.")
+    @Operation(summary = "파일 업로드", description = "server 모드에서 파일을 업로드합니다. 최대 5MB입니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "파일 업로드 성공"),
         @ApiResponse(responseCode = "400", description = "잘못된 파일",
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileUploadIntentController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileUploadIntentController.java
new file mode 100644
index 00000000..96aafb41
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/ProfileUploadIntentController.java
@@ -0,0 +1,45 @@
+package com.ktb.chatapp.controller;
+
+import com.ktb.chatapp.dto.StandardResponse;
+import com.ktb.chatapp.dto.MirrorUploadResultRequest;
+import com.ktb.chatapp.dto.UploadIntentRequest;
+import com.ktb.chatapp.model.PendingUpload;
+import com.ktb.chatapp.service.UploadIntentException;
+import com.ktb.chatapp.service.UploadIntentService;
+import jakarta.validation.Valid;
+import java.security.Principal;
+import lombok.RequiredArgsConstructor;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/api/users/profile-image")
+@RequiredArgsConstructor
+public class ProfileUploadIntentController {
+    private final UploadIntentService service;
+
+    @PostMapping("/presign")
+    public ResponseEntity<?> create(@Valid @RequestBody UploadIntentRequest request, Principal principal) {
+        return ResponseEntity.ok(service.create(principal.getName(), request, PendingUpload.Purpose.PROFILE));
+    }
+
+    @PostMapping("/mirror-result")
+    public ResponseEntity<Void> result(@Valid @RequestBody MirrorUploadResultRequest request, Principal principal) {
+        service.recordResult(principal.getName(), request, PendingUpload.Purpose.PROFILE);
+        return ResponseEntity.noContent().build();
+    }
+
+    @ExceptionHandler(UploadIntentException.class)
+    ResponseEntity<?> handle(UploadIntentException ex) {
+        return ResponseEntity.status(ex.status()).body(StandardResponse.error(ex.getMessage()));
+    }
+
+    @ExceptionHandler(IllegalArgumentException.class)
+    ResponseEntity<?> handleInvalid(IllegalArgumentException ex) {
+        return ResponseEntity.badRequest().body(StandardResponse.error(ex.getMessage()));
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/UploadIntentController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/UploadIntentController.java
new file mode 100644
index 00000000..f131db80
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/UploadIntentController.java
@@ -0,0 +1,45 @@
+package com.ktb.chatapp.controller;
+
+import com.ktb.chatapp.dto.StandardResponse;
+import com.ktb.chatapp.dto.MirrorUploadResultRequest;
+import com.ktb.chatapp.dto.UploadIntentRequest;
+import com.ktb.chatapp.model.PendingUpload;
+import com.ktb.chatapp.service.UploadIntentException;
+import com.ktb.chatapp.service.UploadIntentService;
+import jakarta.validation.Valid;
+import java.security.Principal;
+import lombok.RequiredArgsConstructor;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/api/files/upload")
+@RequiredArgsConstructor
+public class UploadIntentController {
+    private final UploadIntentService service;
+
+    @PostMapping("/presign")
+    public ResponseEntity<?> create(@Valid @RequestBody UploadIntentRequest request, Principal principal) {
+        return ResponseEntity.ok(service.create(principal.getName(), request, PendingUpload.Purpose.CHAT));
+    }
+
+    @PostMapping("/mirror-result")
+    public ResponseEntity<Void> result(@Valid @RequestBody MirrorUploadResultRequest request, Principal principal) {
+        service.recordResult(principal.getName(), request, PendingUpload.Purpose.CHAT);
+        return ResponseEntity.noContent().build();
+    }
+
+    @ExceptionHandler(UploadIntentException.class)
+    ResponseEntity<?> handle(UploadIntentException ex) {
+        return ResponseEntity.status(ex.status()).body(StandardResponse.error(ex.getMessage()));
+    }
+
+    @ExceptionHandler(IllegalArgumentException.class)
+    ResponseEntity<?> handleInvalid(IllegalArgumentException ex) {
+        return ResponseEntity.badRequest().body(StandardResponse.error(ex.getMessage()));
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/UserController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/UserController.java
index 2146d6e2..f9306f9f 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/UserController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/UserController.java
@@ -103,7 +103,7 @@ public ResponseEntity<?> updateCurrentUserProfile(
     /**
      * 프로필 이미지 업로드
      */
-    @Operation(summary = "프로필 이미지 업로드", description = "프로필 이미지를 업로드합니다. 최대 5MB까지 가능합니다.")
+    @Operation(summary = "프로필 이미지 업로드", description = "server 모드에서 프로필 이미지를 업로드합니다. 최대 5MB입니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "이미지 업로드 성공",
             content = @Content(schema = @Schema(implementation = ProfileImageResponse.class))),
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/dto/MirrorUploadResultRequest.java b/apps/backend/src/main/java/com/ktb/chatapp/dto/MirrorUploadResultRequest.java
new file mode 100644
index 00000000..04542989
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/dto/MirrorUploadResultRequest.java
@@ -0,0 +1,11 @@
+package com.ktb.chatapp.dto;
+
+import jakarta.validation.constraints.Min;
+import jakarta.validation.constraints.Max;
+import jakarta.validation.constraints.NotBlank;
+
+public record MirrorUploadResultRequest(
+        @NotBlank String uploadId,
+        boolean success,
+        @Min(0) @Max(599) int status,
+        @Min(0) long durationMs) {}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentRequest.java b/apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentRequest.java
new file mode 100644
index 00000000..85bc365d
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentRequest.java
@@ -0,0 +1,9 @@
+package com.ktb.chatapp.dto;
+
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Positive;
+
+public record UploadIntentRequest(
+        @NotBlank String originalFilename,
+        @NotBlank String contentType,
+        @Positive long size) {}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentResponse.java b/apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentResponse.java
new file mode 100644
index 00000000..92af7483
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/dto/UploadIntentResponse.java
@@ -0,0 +1,8 @@
+package com.ktb.chatapp.dto;
+
+import java.time.Instant;
+import java.util.Map;
+
+public record UploadIntentResponse(
+        String uploadId, String key, String uploadUrl, String method,
+        Map<String, String> headers, Instant expiresAt) {}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/model/PendingUpload.java b/apps/backend/src/main/java/com/ktb/chatapp/model/PendingUpload.java
new file mode 100644
index 00000000..2fc9d8e3
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/model/PendingUpload.java
@@ -0,0 +1,31 @@
+package com.ktb.chatapp.model;
+
+import java.time.Instant;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.index.Indexed;
+import org.springframework.data.mongodb.core.mapping.Document;
+
+@Data
+@Builder
+@NoArgsConstructor
+@AllArgsConstructor
+@Document(collection = "pending_uploads")
+public class PendingUpload {
+    @Id private String id;
+    @Indexed private String userId;
+    private Purpose purpose;
+    @Indexed(unique = true) private String pendingKey;
+    private String originalFilename;
+    private String contentType;
+    private long expectedSize;
+    private Status status;
+    private Instant createdAt;
+    @Indexed(expireAfter = "0s") private Instant expiresAt;
+
+    public enum Purpose { CHAT, PROFILE }
+    public enum Status { PENDING, COMPLETED, FAILED }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/PendingUploadRepository.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/PendingUploadRepository.java
new file mode 100644
index 00000000..a6710b2a
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/PendingUploadRepository.java
@@ -0,0 +1,12 @@
+package com.ktb.chatapp.repository;
+
+import com.ktb.chatapp.model.PendingUpload;
+import java.time.Instant;
+import java.util.Optional;
+import org.springframework.data.mongodb.repository.MongoRepository;
+
+public interface PendingUploadRepository extends MongoRepository<PendingUpload, String> {
+    Optional<PendingUpload> findByIdAndUserIdAndPurpose(String id, String userId, PendingUpload.Purpose purpose);
+    long countByUserIdAndStatusAndExpiresAtAfter(String userId, PendingUpload.Status status, Instant now);
+    long countByUserIdAndCreatedAtAfter(String userId, Instant since);
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/LocalFileService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/LocalFileService.java
index 7c27e8a5..ee83cc78 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/LocalFileService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/LocalFileService.java
@@ -18,17 +18,19 @@ public class LocalFileService implements FileService {
 
     private final StoragePort storagePort;
     private final FileRepository fileRepository;
+    private final UploadPolicy uploadPolicy;
 
-    public LocalFileService(StoragePort storagePort, FileRepository fileRepository) {
+    public LocalFileService(StoragePort storagePort, FileRepository fileRepository, UploadPolicy uploadPolicy) {
         this.storagePort = storagePort;
         this.fileRepository = fileRepository;
+        this.uploadPolicy = uploadPolicy;
     }
 
     @Override
     public FileUploadResult uploadFile(MultipartFile file, String uploaderId) {
         try {
             // 파일 보안 검증
-            FileUtil.validateFile(file);
+            uploadPolicy.validate(file.getOriginalFilename(), file.getContentType(), file.getSize(), false);
 
             // 안전한 파일명 생성
             String originalFilename = file.getOriginalFilename();
@@ -75,7 +77,8 @@ public FileUploadResult uploadFile(MultipartFile file, String uploaderId) {
     public String storeFile(MultipartFile file, String subDirectory) {
         try {
             // 파일 보안 검증
-            FileUtil.validateFile(file);
+            uploadPolicy.validate(file.getOriginalFilename(), file.getContentType(), file.getSize(),
+                    "profiles".equals(subDirectory));
 
             // 안전한 파일명 생성
             String originalFilename = file.getOriginalFilename();
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentException.java b/apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentException.java
new file mode 100644
index 00000000..5c7a897a
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentException.java
@@ -0,0 +1,12 @@
+package com.ktb.chatapp.service;
+
+import org.springframework.http.HttpStatus;
+
+public class UploadIntentException extends RuntimeException {
+    private final HttpStatus status;
+    public UploadIntentException(HttpStatus status, String message) {
+        super(message);
+        this.status = status;
+    }
+    public HttpStatus status() { return status; }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentService.java
new file mode 100644
index 00000000..490f7b20
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/UploadIntentService.java
@@ -0,0 +1,91 @@
+package com.ktb.chatapp.service;
+
+import com.ktb.chatapp.dto.MirrorUploadResultRequest;
+import com.ktb.chatapp.dto.UploadIntentRequest;
+import com.ktb.chatapp.dto.UploadIntentResponse;
+import com.ktb.chatapp.model.PendingUpload;
+import com.ktb.chatapp.model.User;
+import com.ktb.chatapp.repository.PendingUploadRepository;
+import com.ktb.chatapp.repository.UserRepository;
+import com.ktb.chatapp.storage.PresignedUpload;
+import com.ktb.chatapp.storage.MirrorUploadPresigner;
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.Timer;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.UUID;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.http.HttpStatus;
+import org.springframework.security.core.userdetails.UsernameNotFoundException;
+import org.springframework.stereotype.Service;
+
+@Service
+@RequiredArgsConstructor
+@Slf4j
+public class UploadIntentService {
+    private static final Duration URL_TTL = Duration.ofMinutes(5);
+    private static final Duration PENDING_TTL = Duration.ofMinutes(15);
+    private static final int MAX_PENDING = 5;
+
+    private final PendingUploadRepository pendingRepository;
+    private final UserRepository userRepository;
+    private final MirrorUploadPresigner presigner;
+    private final UploadPolicy policy;
+    private final MeterRegistry meterRegistry;
+
+    public UploadIntentResponse create(String email, UploadIntentRequest request, PendingUpload.Purpose purpose) {
+        User user = user(email);
+        policy.validate(request.originalFilename(), request.contentType(), request.size(),
+                purpose == PendingUpload.Purpose.PROFILE);
+        if (pendingRepository.countByUserIdAndCreatedAtAfter(user.getId(), Instant.now().minusSeconds(60)) >= 20) {
+            throw new UploadIntentException(HttpStatus.TOO_MANY_REQUESTS, "업로드 요청은 분당 20회까지 가능합니다.");
+        }
+        if (pendingRepository.countByUserIdAndStatusAndExpiresAtAfter(
+                user.getId(), PendingUpload.Status.PENDING, Instant.now()) >= MAX_PENDING) {
+            throw new UploadIntentException(HttpStatus.TOO_MANY_REQUESTS, "동시에 진행할 수 있는 업로드는 최대 5개입니다.");
+        }
+
+        String id = UUID.randomUUID().toString();
+        String pendingKey = "pending/mirror/" + purpose.name().toLowerCase() + "/" + UUID.randomUUID();
+        Instant now = Instant.now();
+        PendingUpload pending = PendingUpload.builder().id(id).userId(user.getId()).purpose(purpose)
+                .pendingKey(pendingKey).originalFilename(request.originalFilename())
+                .contentType(request.contentType()).expectedSize(request.size()).status(PendingUpload.Status.PENDING)
+                .createdAt(now).expiresAt(now.plus(PENDING_TTL)).build();
+        pendingRepository.save(pending);
+        try {
+            PresignedUpload signed = presigner.presign(pendingKey, request.contentType(), request.size(), URL_TTL);
+            meterRegistry.counter("mirror_presign_success", "purpose", purpose.name().toLowerCase()).increment();
+            return new UploadIntentResponse(id, pendingKey, signed.url().toString(), "PUT",
+                    signed.headers(), signed.expiresAt());
+        } catch (RuntimeException ex) {
+            pendingRepository.deleteById(id);
+            throw ex;
+        }
+    }
+
+    public void recordResult(String email, MirrorUploadResultRequest request, PendingUpload.Purpose purpose) {
+        User user = user(email);
+        PendingUpload pending = owned(request.uploadId(), user.getId(), purpose);
+        pending.setStatus(request.success() ? PendingUpload.Status.COMPLETED : PendingUpload.Status.FAILED);
+        pendingRepository.save(pending);
+        meterRegistry.counter(request.success() ? "mirror_put_success" : "mirror_put_failure",
+                "purpose", purpose.name().toLowerCase(), "status", Integer.toString(request.status())).increment();
+        Timer.builder("mirror_put_latency").tag("purpose", purpose.name().toLowerCase())
+                .register(meterRegistry).record(Duration.ofMillis(request.durationMs()));
+        if (!request.success()) {
+            log.warn("S3 미러 업로드 실패 purpose={} size={} mime={} status={} durationMs={}",
+                    purpose, pending.getExpectedSize(), pending.getContentType(), request.status(), request.durationMs());
+        }
+    }
+
+    private PendingUpload owned(String id, String userId, PendingUpload.Purpose purpose) {
+        return pendingRepository.findByIdAndUserIdAndPurpose(id, userId, purpose)
+                .orElseThrow(() -> new UploadIntentException(HttpStatus.NOT_FOUND, "업로드 요청을 찾을 수 없습니다."));
+    }
+    private User user(String email) {
+        return userRepository.findByEmail(email.toLowerCase())
+                .orElseThrow(() -> new UsernameNotFoundException("사용자를 찾을 수 없습니다."));
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/UploadPolicy.java b/apps/backend/src/main/java/com/ktb/chatapp/service/UploadPolicy.java
new file mode 100644
index 00000000..6e4d986f
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/UploadPolicy.java
@@ -0,0 +1,30 @@
+package com.ktb.chatapp.service;
+
+import com.ktb.chatapp.util.FileUtil;
+import java.nio.charset.StandardCharsets;
+import org.springframework.stereotype.Component;
+import org.springframework.util.StringUtils;
+import org.springframework.http.HttpStatus;
+
+@Component
+public class UploadPolicy {
+    public static final long MAX_SIZE = 5L * 1024 * 1024;
+
+    public void validate(String originalFilename, String contentType, long size, boolean profileImage) {
+        if (!StringUtils.hasText(originalFilename) || originalFilename.getBytes(StandardCharsets.UTF_8).length > 255) {
+            throw new IllegalArgumentException("파일명이 올바르지 않습니다.");
+        }
+        if (size <= 0) {
+            throw new IllegalArgumentException("파일이 비어있습니다.");
+        }
+        if (size > MAX_SIZE) {
+            throw new UploadIntentException(HttpStatus.PAYLOAD_TOO_LARGE, "파일 크기는 5MB를 초과할 수 없습니다.");
+        }
+        if (!FileUtil.isAllowedType(originalFilename, contentType)) {
+            throw new IllegalArgumentException("지원하지 않는 파일 형식입니다.");
+        }
+        if (profileImage && !contentType.startsWith("image/")) {
+            throw new IllegalArgumentException("이미지 파일만 업로드할 수 있습니다.");
+        }
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/UserService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/UserService.java
index 9c47c24c..79379369 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/UserService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/UserService.java
@@ -26,6 +26,7 @@ public class UserService {
     private final UserRepository userRepository;
     private final FileService fileService;
     private final StoragePort storagePort;
+    private final UploadPolicy uploadPolicy;
 
     @Value("${app.profile.image.max-size:5242880}") // 5MB
     private long maxProfileImageSize;
@@ -110,6 +111,8 @@ private void validateProfileImageFile(MultipartFile file) {
             throw new IllegalArgumentException("이미지가 제공되지 않았습니다.");
         }
 
+        uploadPolicy.validate(file.getOriginalFilename(), file.getContentType(), file.getSize(), true);
+
         // 파일 크기 검증
         if (file.getSize() > maxProfileImageSize) {
             throw new IllegalArgumentException("파일 크기는 5MB를 초과할 수 없습니다.");
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/storage/MirrorUploadPresigner.java b/apps/backend/src/main/java/com/ktb/chatapp/storage/MirrorUploadPresigner.java
new file mode 100644
index 00000000..d55cfcfa
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/storage/MirrorUploadPresigner.java
@@ -0,0 +1,7 @@
+package com.ktb.chatapp.storage;
+
+import java.time.Duration;
+
+public interface MirrorUploadPresigner {
+    PresignedUpload presign(String key, String contentType, long size, Duration ttl);
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/storage/PresignedUpload.java b/apps/backend/src/main/java/com/ktb/chatapp/storage/PresignedUpload.java
new file mode 100644
index 00000000..56043161
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/storage/PresignedUpload.java
@@ -0,0 +1,7 @@
+package com.ktb.chatapp.storage;
+
+import java.net.URI;
+import java.time.Instant;
+import java.util.Map;
+
+public record PresignedUpload(URI url, Map<String, String> headers, Instant expiresAt) {}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/storage/S3MirrorUploadPresigner.java b/apps/backend/src/main/java/com/ktb/chatapp/storage/S3MirrorUploadPresigner.java
new file mode 100644
index 00000000..8f34a046
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/storage/S3MirrorUploadPresigner.java
@@ -0,0 +1,31 @@
+package com.ktb.chatapp.storage;
+
+import java.net.URI;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.Map;
+import lombok.RequiredArgsConstructor;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Component;
+import software.amazon.awssdk.services.s3.model.PutObjectRequest;
+import software.amazon.awssdk.services.s3.presigner.S3Presigner;
+import software.amazon.awssdk.services.s3.presigner.model.PutObjectPresignRequest;
+
+@Component
+@RequiredArgsConstructor
+public class S3MirrorUploadPresigner implements MirrorUploadPresigner {
+    private final S3Presigner presigner;
+
+    @Value("${aws.s3.bucket}")
+    private String bucket;
+
+    @Override
+    public PresignedUpload presign(String key, String contentType, long size, Duration ttl) {
+        PutObjectRequest request = PutObjectRequest.builder()
+                .bucket(bucket).key(key).contentType(contentType).contentLength(size).build();
+        var signed = presigner.presignPutObject(PutObjectPresignRequest.builder()
+                .signatureDuration(ttl).putObjectRequest(request).build());
+        return new PresignedUpload(URI.create(signed.url().toString()),
+                Map.of("Content-Type", contentType), Instant.now().plus(ttl));
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/util/FileUtil.java b/apps/backend/src/main/java/com/ktb/chatapp/util/FileUtil.java
index 4c892dcc..56306055 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/util/FileUtil.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/util/FileUtil.java
@@ -87,6 +87,12 @@ public static void validateFile(MultipartFile file) {
         }
     }
 
+    public static boolean isAllowedType(String filename, String contentType) {
+        if (filename == null || contentType == null) return false;
+        List<String> extensions = ALLOWED_TYPES.get(contentType);
+        return extensions != null && extensions.contains(getFileExtension(filename).toLowerCase());
+    }
+
     /**
      * 파일 타입 한글명 반환
      */
diff --git a/apps/backend/src/main/resources/application.properties b/apps/backend/src/main/resources/application.properties
index 3418acb4..61d49719 100644
--- a/apps/backend/src/main/resources/application.properties
+++ b/apps/backend/src/main/resources/application.properties
@@ -18,12 +18,13 @@ spring.data.redis.port=${REDIS_PORT:6379}
 spring.data.redis.password=${REDIS_PASSWORD:}
 
 # File Upload Configuration
-# Files are stored only in S3. Credentials use the AWS default credentials chain.
-file.storage.type=${FILE_STORAGE_TYPE:s3}
-aws.s3.bucket=${S3_BUCKET}
+# Original files stay in local storage. Optional mirror PUTs use the AWS default credentials chain.
+file.storage.type=${FILE_STORAGE_TYPE:local}
+aws.s3.bucket=${S3_BUCKET:}
 aws.s3.region=${AWS_REGION:ap-northeast-2}
 aws.s3.endpoint=${S3_ENDPOINT:}
 aws.s3.path-style-access=${S3_PATH_STYLE_ACCESS:false}
+# Bucket CORS/lifecycle must be provisioned before rollout. See docs/S3_DIRECT_UPLOAD.md.
 
 # Multipart upload limits
 spring.servlet.multipart.max-file-size=5MB
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/UploadIntentServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/UploadIntentServiceTest.java
new file mode 100644
index 00000000..890d14af
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/UploadIntentServiceTest.java
@@ -0,0 +1,77 @@
+package com.ktb.chatapp.service;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyLong;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.ktb.chatapp.dto.UploadIntentRequest;
+import com.ktb.chatapp.dto.MirrorUploadResultRequest;
+import com.ktb.chatapp.model.PendingUpload;
+import com.ktb.chatapp.model.User;
+import com.ktb.chatapp.repository.PendingUploadRepository;
+import com.ktb.chatapp.repository.UserRepository;
+import com.ktb.chatapp.storage.PresignedUpload;
+import com.ktb.chatapp.storage.MirrorUploadPresigner;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import java.net.URI;
+import java.time.Instant;
+import java.util.Map;
+import java.util.Optional;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class UploadIntentServiceTest {
+    @Mock PendingUploadRepository pendingRepository;
+    @Mock UserRepository userRepository;
+    @Mock MirrorUploadPresigner presigner;
+
+    private UploadIntentService service;
+    private User user;
+    private SimpleMeterRegistry meterRegistry;
+
+    @BeforeEach
+    void setUp() {
+        meterRegistry = new SimpleMeterRegistry();
+        service = new UploadIntentService(
+                pendingRepository, userRepository, presigner, new UploadPolicy(), meterRegistry);
+        user = User.builder().id("user-1").email("user@example.com").build();
+        when(userRepository.findByEmail("user@example.com")).thenReturn(Optional.of(user));
+    }
+
+    @Test
+    void createReturnsFiveMinutePresignedPutForOwnedPendingObject() {
+        when(presigner.presign(any(), any(), anyLong(), any())).thenReturn(
+                new PresignedUpload(URI.create("https://s3.test/pending"),
+                        Map.of("Content-Type", "image/png"), Instant.now().plusSeconds(300)));
+
+        var response = service.create("USER@example.com",
+                new UploadIntentRequest("image.png", "image/png", 100), PendingUpload.Purpose.CHAT);
+
+        assertThat(response.method()).isEqualTo("PUT");
+        assertThat(response.uploadUrl()).isEqualTo("https://s3.test/pending");
+        assertThat(response.key()).startsWith("pending/mirror/chat/");
+        verify(pendingRepository).save(any(PendingUpload.class));
+    }
+
+    @Test
+    void recordResultClosesPendingIntentAndRecordsPutMetric() {
+        PendingUpload pending = PendingUpload.builder().id("upload-1").userId("user-1")
+                .purpose(PendingUpload.Purpose.CHAT).expectedSize(100).contentType("image/png")
+                .status(PendingUpload.Status.PENDING).build();
+        when(pendingRepository.findByIdAndUserIdAndPurpose(
+                "upload-1", "user-1", PendingUpload.Purpose.CHAT)).thenReturn(Optional.of(pending));
+
+        service.recordResult("user@example.com",
+                new MirrorUploadResultRequest("upload-1", true, 200, 42), PendingUpload.Purpose.CHAT);
+
+        assertThat(pending.getStatus()).isEqualTo(PendingUpload.Status.COMPLETED);
+        assertThat(meterRegistry.get("mirror_put_success").counter().count()).isEqualTo(1);
+        verify(pendingRepository).save(pending);
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/UploadPolicyTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/UploadPolicyTest.java
new file mode 100644
index 00000000..2ae9166f
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/UploadPolicyTest.java
@@ -0,0 +1,32 @@
+package com.ktb.chatapp.service;
+
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.Assertions.assertThatNoException;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.junit.jupiter.api.Test;
+
+class UploadPolicyTest {
+    private final UploadPolicy policy = new UploadPolicy();
+
+    @Test
+    void acceptsEveryDocumentTypeAtFiveMegabytes() {
+        assertThatNoException().isThrownBy(() -> policy.validate("report.docx",
+                "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+                UploadPolicy.MAX_SIZE, false));
+    }
+
+    @Test
+    void rejectsOversizeAndMimeExtensionMismatch() {
+        assertThatThrownBy(() -> policy.validate("photo.jpg", "image/jpeg", UploadPolicy.MAX_SIZE + 1, false))
+                .isInstanceOf(UploadIntentException.class);
+        assertThatIllegalArgumentException().isThrownBy(() ->
+                policy.validate("photo.pdf", "image/jpeg", 10, false));
+    }
+
+    @Test
+    void profilePurposeOnlyAcceptsImages() {
+        assertThatIllegalArgumentException().isThrownBy(() ->
+                policy.validate("clip.mp4", "video/mp4", 10, true));
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/UserServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/UserServiceTest.java
index 46595baa..5ec5da1a 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/service/UserServiceTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/UserServiceTest.java
@@ -48,7 +48,7 @@ class UserServiceTest {
      */
     @BeforeEach
     void setUp() {
-        userService = new UserService(userRepository, fileService, new LocalStorage(uploadDir.toString()));
+        userService = new UserService(userRepository, fileService, new LocalStorage(uploadDir.toString()), new UploadPolicy());
         ReflectionTestUtils.setField(userService, "maxProfileImageSize", 5242880L);
     }
 
diff --git a/apps/frontend/.env.example b/apps/frontend/.env.example
index 35b55696..68580a95 100644
--- a/apps/frontend/.env.example
+++ b/apps/frontend/.env.example
@@ -1,3 +1,4 @@
 PORT=3000
 NEXT_PUBLIC_API_URL=http://localhost:5001
-NEXT_PUBLIC_SOCKET_URL=http://localhost:5002
\ No newline at end of file
+NEXT_PUBLIC_SOCKET_URL=http://localhost:5002
+NEXT_PUBLIC_FILE_UPLOAD_MODE=server
diff --git a/apps/frontend/Dockerfile b/apps/frontend/Dockerfile
index f50a3b6c..531ba7dc 100644
--- a/apps/frontend/Dockerfile
+++ b/apps/frontend/Dockerfile
@@ -2,6 +2,9 @@
 FROM node:22-alpine AS builder
 WORKDIR /repo
 
+ARG NEXT_PUBLIC_FILE_UPLOAD_MODE=server
+ENV NEXT_PUBLIC_FILE_UPLOAD_MODE=$NEXT_PUBLIC_FILE_UPLOAD_MODE
+
 ENV PNPM_HOME="/pnpm"
 ENV PATH="$PNPM_HOME:$PATH"
 
diff --git a/apps/frontend/README.md b/apps/frontend/README.md
index 47bfccf8..a3379269 100644
--- a/apps/frontend/README.md
+++ b/apps/frontend/README.md
@@ -37,6 +37,7 @@ cp .env.example .env.local
 PORT=3000
 NEXT_PUBLIC_API_URL=http://localhost:5001
 NEXT_PUBLIC_SOCKET_URL=http://localhost:5002
+NEXT_PUBLIC_FILE_UPLOAD_MODE=server
 ```
 
 > `.env.local`은 프로덕션 빌드(`build:production`)에서도 `.env.production`보다
@@ -45,6 +46,7 @@ NEXT_PUBLIC_SOCKET_URL=http://localhost:5002
 **환경 변수 설명:**
 - `NEXT_PUBLIC_API_URL`: 백엔드 REST API 서버 주소
 - `NEXT_PUBLIC_SOCKET_URL`: Socket.IO 서버 주소
+- `NEXT_PUBLIC_FILE_UPLOAD_MODE`: `server`(기본 multipart) 또는 `mirror`(multipart 성공 후 S3 보조 복제)
 
 서버환경에서 실행시 Route 53 에 등록한 도메인을 입력하세요. 예: `https://chat.goorm-ktb-[번호].goorm.team`
 
@@ -116,6 +118,9 @@ make deploy DEPLOY_PATH=/opt/ktb-chat-frontend
 
 ```bash
 docker build -t chat-app-frontend .
+
+# 운영 S3 직접 업로드 이미지
+docker build --build-arg NEXT_PUBLIC_FILE_UPLOAD_MODE=mirror -t chat-app-frontend .
 ```
 
 > **참고**: `NEXT_PUBLIC_*` 환경 변수는 빌드 시점에 코드에 인라인됩니다.
diff --git a/apps/frontend/components/ProfileImageUpload.js b/apps/frontend/components/ProfileImageUpload.js
index 5a8e8dfc..5ea3a805 100644
--- a/apps/frontend/components/ProfileImageUpload.js
+++ b/apps/frontend/components/ProfileImageUpload.js
@@ -55,20 +55,12 @@ const ProfileImageUpload = ({ currentImage, onImageChange }) => {
         throw new Error('인증 정보가 없습니다.');
       }
 
-      // FormData 생성
       const formData = new FormData();
       formData.append('profileImage', file);
-
-      // 파일 업로드 요청
-      const response = await api.post(
-        '/api/users/profile-image',
-        formData,
-        {
-          headers: {
-            'Content-Type': 'multipart/form-data',
-          },
-        }
-      );
+      const response = await api.post('/api/users/profile-image', formData, {
+        headers: { 'Content-Type': 'multipart/form-data' },
+        maxRetries: 0,
+      });
 
       const data = response.data;
 
@@ -76,6 +68,12 @@ const ProfileImageUpload = ({ currentImage, onImageChange }) => {
         throw new Error(data?.message || '이미지 업로드에 실패했습니다.');
       }
 
+      if (process.env.NEXT_PUBLIC_FILE_UPLOAD_MODE === 'mirror') {
+        mirrorProfileImage(file).catch((mirrorError) => {
+          console.warn('S3 profile mirror upload failed', mirrorError?.message || mirrorError);
+        });
+      }
+
       const updatedUser = {
         ...user,
         profileImage: data.imageUrl
@@ -214,4 +212,39 @@ const ProfileImageUpload = ({ currentImage, onImageChange }) => {
   );
 };
 
+const mirrorProfileImage = async (file) => {
+  let uploadId;
+  const startedAt = Date.now();
+  try {
+    const intent = await api.post('/api/users/profile-image/presign', {
+      originalFilename: file.name,
+      contentType: file.type,
+      size: file.size,
+    }, { maxRetries: 0 });
+    uploadId = intent.data.uploadId;
+    const putResponse = await fetch(intent.data.uploadUrl, {
+      method: 'PUT',
+      headers: intent.data.headers,
+      body: file,
+      credentials: 'omit',
+    });
+    if (!putResponse.ok) {
+      const error = new Error('S3 mirror upload failed');
+      error.status = putResponse.status;
+      throw error;
+    }
+    await api.post('/api/users/profile-image/mirror-result', {
+      uploadId, success: true, status: putResponse.status, durationMs: Date.now() - startedAt,
+    }, { maxRetries: 0 });
+  } catch (error) {
+    if (uploadId) {
+      await api.post('/api/users/profile-image/mirror-result', {
+        uploadId, success: false, status: error.status || error.response?.status || 0,
+        durationMs: Date.now() - startedAt,
+      }, { maxRetries: 0 }).catch(() => {});
+    }
+    throw error;
+  }
+};
+
 export default ProfileImageUpload;
diff --git a/apps/frontend/lib/api/__tests__/client.test.js b/apps/frontend/lib/api/__tests__/client.test.js
index 78b18e93..43455012 100644
--- a/apps/frontend/lib/api/__tests__/client.test.js
+++ b/apps/frontend/lib/api/__tests__/client.test.js
@@ -130,4 +130,23 @@ describe('api client', () => {
       },
     });
   });
+
+  it('respects a per-request zero retry limit', async () => {
+    const client = createApiClient({
+      baseURL: 'http://api.test',
+      getSession: () => null,
+    });
+    let attempts = 0;
+    client.defaults.adapter = async (config) => {
+      attempts += 1;
+      const error = new Error('Service unavailable');
+      error.config = config;
+      error.response = { config, data: {}, headers: {}, status: 503, statusText: 'Unavailable' };
+      throw error;
+    };
+
+    await expect(client.post('/api/files/upload/presign', {}, { maxRetries: 0 }))
+      .rejects.toMatchObject({ status: 503 });
+    expect(attempts).toBe(1);
+  });
 });
diff --git a/apps/frontend/lib/api/client.js b/apps/frontend/lib/api/client.js
index 362e895e..483790d5 100644
--- a/apps/frontend/lib/api/client.js
+++ b/apps/frontend/lib/api/client.js
@@ -69,6 +69,7 @@ export const createApiClient = ({
     async (error) => {
       const config = error.config || {};
       config.retryCount = config.retryCount || 0;
+      const maxRetries = config.maxRetries ?? RETRY_CONFIG.maxRetries;
 
       if (isCanceledRequest(error)) {
         return Promise.reject(error);
@@ -92,7 +93,7 @@ export const createApiClient = ({
         return Promise.reject(error);
       }
 
-      if (isRetryableError(error) && config.retryCount < RETRY_CONFIG.maxRetries) {
+      if (isRetryableError(error) && config.retryCount < maxRetries) {
         config.retryCount++;
         const delay = getRetryDelay(config.retryCount);
 
diff --git a/apps/frontend/services/__tests__/fileService.test.js b/apps/frontend/services/__tests__/fileService.test.js
index b3cf075a..da8e7e50 100644
--- a/apps/frontend/services/__tests__/fileService.test.js
+++ b/apps/frontend/services/__tests__/fileService.test.js
@@ -1,24 +1,98 @@
-import { afterEach, describe, expect, it, vi } from 'vitest';
-import fileService from '../fileService';
+import axios from 'axios';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import api from '../axios';
+
+vi.mock('axios', async (importOriginal) => {
+  const actual = await importOriginal();
+  return {
+    ...actual,
+    default: { ...actual.default, put: vi.fn(), isAxiosError: actual.default.isAxiosError },
+  };
+});
+
+vi.mock('../axios', () => ({
+  default: { post: vi.fn() },
+}));
 
 vi.mock('../../components/Toast', () => ({
-  Toast: {
-    error: vi.fn(),
-  },
+  Toast: { error: vi.fn() },
 }));
 
-describe('fileService', () => {
-  afterEach(() => {
-    vi.restoreAllMocks();
+describe('fileService mirror upload', () => {
+  beforeEach(() => {
+    vi.resetModules();
+    vi.clearAllMocks();
+    process.env.NEXT_PUBLIC_FILE_UPLOAD_MODE = 'mirror';
+  });
+
+  it('keeps the multipart result and mirrors without credentials or retries', async () => {
+    api.post
+      .mockResolvedValueOnce({ data: { success: true, file: { filename: 'server.png' } } })
+      .mockResolvedValueOnce({ data: {
+        uploadId: 'upload-1', uploadUrl: 'https://s3.test/pending/file',
+        headers: { 'Content-Type': 'image/png' },
+      } })
+      .mockResolvedValueOnce({ status: 204 });
+    axios.put.mockResolvedValue({ status: 200 });
+    const { default: fileService } = await import('../fileService');
+    const file = new File(['image'], 'file.png', { type: 'image/png' });
+
+    const result = await fileService.uploadFile(file, vi.fn());
+
+    expect(result.data.file.filename).toBe('server.png');
+    await vi.waitFor(() => expect(api.post).toHaveBeenCalledTimes(3));
+    expect(api.post).toHaveBeenNthCalledWith(2, '/api/files/upload/presign', {
+      originalFilename: 'file.png', contentType: 'image/png', size: file.size,
+    }, expect.objectContaining({ maxRetries: 0 }));
+    expect(axios.put).toHaveBeenCalledWith('https://s3.test/pending/file', file,
+      expect.objectContaining({ withCredentials: false }));
+    expect(api.post).toHaveBeenNthCalledWith(3, '/api/files/upload/mirror-result',
+      expect.objectContaining({ uploadId: 'upload-1', success: true, status: 200 }),
+      expect.objectContaining({ maxRetries: 0 }));
+    expect(result.success).toBe(true);
+  });
+
+  it('keeps the multipart result when the mirror PUT fails', async () => {
+    api.post
+      .mockResolvedValueOnce({ data: { success: true, file: { filename: 'server.png' } } })
+      .mockResolvedValueOnce({ data: {
+        uploadId: 'upload-1', uploadUrl: 'https://s3.test/pending/file',
+        headers: { 'Content-Type': 'image/png' },
+      } })
+      .mockResolvedValueOnce({ status: 204 });
+    axios.put.mockRejectedValue(Object.assign(new Error('forbidden'), { response: { status: 403 } }));
+    vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const { default: fileService } = await import('../fileService');
+
+    const result = await fileService.uploadFile(
+      new File(['image'], 'file.png', { type: 'image/png' }), vi.fn());
+
+    expect(result.success).toBe(true);
+    expect(result.data.file.filename).toBe('server.png');
+    await vi.waitFor(() => expect(api.post).toHaveBeenCalledWith(
+      '/api/files/upload/mirror-result',
+      expect.objectContaining({ uploadId: 'upload-1', success: false, status: 403 }),
+      expect.objectContaining({ maxRetries: 0 })
+    ));
+  });
+
+  it('does not call S3 in server mode', async () => {
+    process.env.NEXT_PUBLIC_FILE_UPLOAD_MODE = 'server';
+    api.post.mockResolvedValueOnce({ data: { success: true, file: { filename: 'server.png' } } });
+    const { default: fileService } = await import('../fileService');
+
+    await fileService.uploadFile(new File(['image'], 'file.png', { type: 'image/png' }), vi.fn());
+
+    expect(api.post).toHaveBeenCalledTimes(1);
+    expect(axios.put).not.toHaveBeenCalled();
   });
 
-  it('handles upload size limit errors without logging console errors', () => {
+  it('handles upload size limit errors without logging console errors', async () => {
     const consoleError = vi.spyOn(console, 'error').mockImplementation(() => {});
+    const { default: fileService } = await import('../fileService');
 
     const result = fileService.handleUploadError(
-      Object.assign(new Error('파일 크기는 5MB를 초과할 수 없습니다.'), {
-        status: 413,
-      })
+      Object.assign(new Error('파일 크기는 5MB를 초과할 수 없습니다.'), { status: 413 })
     );
 
     expect(result).toEqual({
@@ -26,5 +100,6 @@ describe('fileService', () => {
       message: '파일 크기는 5MB를 초과할 수 없습니다.',
     });
     expect(consoleError).not.toHaveBeenCalled();
+    consoleError.mockRestore();
   });
 });
diff --git a/apps/frontend/services/fileService.js b/apps/frontend/services/fileService.js
index f77a1d11..b5c16045 100644
--- a/apps/frontend/services/fileService.js
+++ b/apps/frontend/services/fileService.js
@@ -5,22 +5,20 @@ import { Toast } from '../components/Toast';
 class FileService {
   constructor() {
     this.baseUrl = process.env.NEXT_PUBLIC_API_URL;
-    this.uploadLimit = 50 * 1024 * 1024; // 50MB
-    this.retryAttempts = 3;
-    this.retryDelay = 1000;
+    this.uploadLimit = 5 * 1024 * 1024;
     this.activeUploads = new Map();
 
     this.allowedTypes = {
       image: {
         extensions: ['.jpg', '.jpeg', '.png', '.gif', '.webp'],
         mimeTypes: ['image/jpeg', 'image/png', 'image/gif', 'image/webp'],
-        maxSize: 10 * 1024 * 1024,
+        maxSize: this.uploadLimit,
         name: '이미지'
       },
       document: {
         extensions: ['.pdf'],
         mimeTypes: ['application/pdf'],
-        maxSize: 20 * 1024 * 1024,
+        maxSize: this.uploadLimit,
         name: 'PDF 문서'
       }
     };
@@ -97,16 +95,17 @@ class FileService {
         headers: {
           'Content-Type': 'multipart/form-data'
         },
-        // 업로드는 한도가 50MB 라 공통 타임아웃으로는 정상 전송도 끊긴다.
+        // multipart 본문 전송은 공통 API 타임아웃보다 길 수 있다.
         timeout: 30000,
         cancelToken: source.token,
+        maxRetries: 0,
         withCredentials: true,
         onUploadProgress: (progressEvent) => {
-          if (onProgress) {
+          if (onProgress && progressEvent.total) {
             const percentCompleted = Math.round(
               (progressEvent.loaded * 100) / progressEvent.total
             );
-            onProgress(percentCompleted);
+            this.reportProgress(file.name, percentCompleted, onProgress);
           }
         }
       });
@@ -121,6 +120,11 @@ class FileService {
       }
 
       const fileData = response.data.file;
+      if (process.env.NEXT_PUBLIC_FILE_UPLOAD_MODE === 'mirror') {
+        this.uploadMirror(file).catch((error) => {
+          console.warn('S3 mirror upload failed', error?.message || error);
+        });
+      }
       return {
         success: true,
         data: {
@@ -149,6 +153,45 @@ class FileService {
       return this.handleUploadError(error);
     }
   }
+
+  async uploadMirror(file) {
+    let uploadId;
+    const startedAt = Date.now();
+    try {
+      const intent = await axiosInstance.post('/api/files/upload/presign', {
+        originalFilename: file.name,
+        contentType: file.type,
+        size: file.size
+      }, { maxRetries: 0 });
+      uploadId = intent.data.uploadId;
+
+      // S3 요청에는 앱 인증 정보나 쿠키를 절대 전달하지 않는다.
+      const putResponse = await axios.put(intent.data.uploadUrl, file, {
+        headers: intent.data.headers,
+        withCredentials: false,
+        transformRequest: [(data) => data],
+        timeout: 30000
+      });
+      await axiosInstance.post('/api/files/upload/mirror-result', {
+        uploadId, success: true, status: putResponse.status, durationMs: Date.now() - startedAt
+      }, { maxRetries: 0 });
+    } catch (error) {
+      if (uploadId) {
+        await axiosInstance.post('/api/files/upload/mirror-result', {
+          uploadId, success: false, status: error.response?.status || 0,
+          durationMs: Date.now() - startedAt
+        }, { maxRetries: 0 }).catch(() => {});
+      }
+      throw error;
+    }
+  }
+  reportProgress(fileName, percent, callback) {
+    const upload = this.activeUploads.get(fileName);
+    const lastPercent = upload?.lastProgress ?? -10;
+    if (percent !== 100 && percent - lastPercent < 10) return;
+    if (upload) upload.lastProgress = percent;
+    callback(percent);
+  }
   getFileUrl(filename, forPreview = false) {
     if (!filename) return '';
 

From 86e85a15a11c9d76a6320e9026a32bcb167605f9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=EC=9A=A9=EC=A3=BC?= <jake@yongjuui-MacBookPro.local>
Date: Wed, 12 Aug 2026 08:15:57 +0900
Subject: [PATCH 32/32] fix(chat): stabilize room list under E2E load

---
 .../config/RoomPaginationIndexMigration.java  |  82 +++++++++++
 .../chatapp/controller/RoomController.java    |  31 +++-
 .../com/ktb/chatapp/dto/PageMetadata.java     |  16 +--
 .../exception/InvalidRoomCursorException.java |  12 ++
 .../repository/RoomRepositoryCustom.java      |   4 +
 .../repository/RoomRepositoryImpl.java        |  22 +++
 .../ktb/chatapp/service/RoomCursorCodec.java  |  47 ++++++
 .../com/ktb/chatapp/service/RoomService.java  | 122 ++++++++--------
 ...ginationIndexMigrationIntegrationTest.java | 108 ++++++++++++++
 .../RoomControllerPaginationTest.java         |  69 +++++++++
 .../RoomRepositoryImplIntegrationTest.java    |  30 ++++
 .../chatapp/service/RoomCursorCodecTest.java  |  29 ++++
 .../ktb/chatapp/service/RoomServiceTest.java  |  40 +++++-
 .../features/chat/rooms/ChatRoomsView.js      |  78 ++++++++--
 .../features/chat/rooms/RoomsTable.js         |   5 +-
 .../rooms/__tests__/ChatRoomsView.test.js     |  34 ++++-
 .../chat/rooms/__tests__/useRoomList.test.js  |  70 ++++++++-
 .../rooms/__tests__/useRoomsSocket.test.js    |  16 +++
 .../features/chat/rooms/useRoomList.js        |  93 ++++++++++--
 .../features/chat/rooms/useRoomsSocket.js     |   3 +-
 e2e/actions/chat.actions.js                   | 103 ++++++++++++-
 e2e/actions/chat.actions.test.js              |  74 ++++++++++
 e2e/artillery/Makefile                        |  17 ++-
 e2e/artillery/README.md                       |  12 ++
 e2e/artillery/aggregate-observations.js       |  24 +++-
 e2e/artillery/all-scenarios.js                |  71 +--------
 e2e/artillery/artillery-auth-negative.yaml    |  17 +++
 e2e/artillery/artillery-config.yaml           |   3 +-
 e2e/artillery/artillery-media-profile.yaml    |  17 +++
 e2e/artillery/artillery-messaging.yaml        |  17 +++
 e2e/artillery/artillery-room-lifecycle.yaml   |  17 +++
 e2e/artillery/observation.js                  | 136 +++++++++++++++++-
 e2e/artillery/observation.test.js             |  43 ++++++
 e2e/artillery/profile-scenarios.js            |  53 +++++++
 e2e/artillery/report-observations.js          |  20 ++-
 e2e/artillery/scenarios/auth.scenario.js      |  10 +-
 e2e/artillery/scenarios/chat.scenario.js      |  28 ++--
 37 files changed, 1343 insertions(+), 230 deletions(-)
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/config/RoomPaginationIndexMigration.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/exception/InvalidRoomCursorException.java
 create mode 100644 apps/backend/src/main/java/com/ktb/chatapp/service/RoomCursorCodec.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/config/RoomPaginationIndexMigrationIntegrationTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerPaginationTest.java
 create mode 100644 apps/backend/src/test/java/com/ktb/chatapp/service/RoomCursorCodecTest.java
 create mode 100644 e2e/actions/chat.actions.test.js
 create mode 100644 e2e/artillery/artillery-auth-negative.yaml
 create mode 100644 e2e/artillery/artillery-media-profile.yaml
 create mode 100644 e2e/artillery/artillery-messaging.yaml
 create mode 100644 e2e/artillery/artillery-room-lifecycle.yaml
 create mode 100644 e2e/artillery/profile-scenarios.js

diff --git a/apps/backend/src/main/java/com/ktb/chatapp/config/RoomPaginationIndexMigration.java b/apps/backend/src/main/java/com/ktb/chatapp/config/RoomPaginationIndexMigration.java
new file mode 100644
index 00000000..e5463dd6
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/config/RoomPaginationIndexMigration.java
@@ -0,0 +1,82 @@
+package com.ktb.chatapp.config;
+
+import com.mongodb.client.MongoCollection;
+import com.mongodb.client.model.IndexOptions;
+import com.mongodb.client.model.UpdateOneModel;
+import com.mongodb.client.model.UpdateOptions;
+import com.mongodb.client.model.WriteModel;
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+import lombok.RequiredArgsConstructor;
+import org.bson.Document;
+import org.bson.types.ObjectId;
+import org.springframework.boot.ApplicationArguments;
+import org.springframework.boot.ApplicationRunner;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.stereotype.Component;
+
+@Component
+@RequiredArgsConstructor
+public class RoomPaginationIndexMigration implements ApplicationRunner {
+
+    static final String COLLECTION = "rooms";
+    static final String INDEX_NAME = "rooms_created_at_id_desc";
+    private static final Document INDEX_KEY = new Document("createdAt", -1).append("_id", -1);
+    private static final Document MISSING_CREATED_AT = new Document("$or", List.of(
+        new Document("createdAt", new Document("$exists", false)),
+        new Document("createdAt", null)
+    ));
+
+    private final MongoTemplate mongoTemplate;
+
+    @Override
+    public void run(ApplicationArguments args) {
+        MongoCollection<Document> rooms = mongoTemplate.getCollection(COLLECTION);
+        backfillCreatedAt(rooms);
+        ensureIndex(rooms);
+    }
+
+    private void backfillCreatedAt(MongoCollection<Document> rooms) {
+        List<Document> missing = rooms.find(MISSING_CREATED_AT)
+            .projection(new Document("_id", 1))
+            .into(new ArrayList<>());
+        List<WriteModel<Document>> updates = new ArrayList<>();
+        for (Document room : missing) {
+            Object rawId = room.get("_id");
+            ObjectId objectId = objectId(rawId);
+            if (objectId == null) {
+                throw new IllegalStateException(
+                    "Cannot backfill rooms.createdAt: invalid ObjectId=" + rawId
+                );
+            }
+            Date createdAt = Date.from(Instant.ofEpochSecond(objectId.getTimestamp()));
+            updates.add(new UpdateOneModel<>(
+                new Document("_id", rawId).append("$or", MISSING_CREATED_AT.get("$or")),
+                new Document("$set", new Document("createdAt", createdAt)),
+                new UpdateOptions()
+            ));
+        }
+        if (!updates.isEmpty()) rooms.bulkWrite(updates);
+    }
+
+    private void ensureIndex(MongoCollection<Document> rooms) {
+        for (Document index : rooms.listIndexes()) {
+            Document key = index.get("key", Document.class);
+            if (INDEX_KEY.equals(key)) return;
+            if (INDEX_NAME.equals(index.getString("name"))) {
+                throw new IllegalStateException(
+                    "Cannot create room pagination index: conflicting index name=" + INDEX_NAME
+                );
+            }
+        }
+        rooms.createIndex(INDEX_KEY, new IndexOptions().name(INDEX_NAME));
+    }
+
+    private ObjectId objectId(Object rawId) {
+        if (rawId instanceof ObjectId objectId) return objectId;
+        if (rawId instanceof String id && ObjectId.isValid(id)) return new ObjectId(id);
+        return null;
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
index 40b74f76..232fff3b 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/controller/RoomController.java
@@ -7,6 +7,7 @@
 import com.ktb.chatapp.repository.UserRepository;
 import com.ktb.chatapp.service.RecentMessageCounter;
 import com.ktb.chatapp.service.RoomService;
+import com.ktb.chatapp.exception.InvalidRoomCursorException;
 import io.swagger.v3.oas.annotations.Operation;
 import io.swagger.v3.oas.annotations.Parameter;
 import io.swagger.v3.oas.annotations.media.Content;
@@ -86,8 +87,8 @@ public ResponseEntity<HealthResponse> healthCheck() {
         }
     }
 
-    // 전체 채팅방 목록 조회
-    @Operation(summary = "채팅방 목록 조회", description = "전체 채팅방 목록을 최신순으로 조회합니다. Rate Limit이 적용됩니다.")
+    // 채팅방 목록 조회
+    @Operation(summary = "채팅방 목록 조회", description = "채팅방 목록을 최신순 커서 페이지로 조회합니다. Rate Limit이 적용됩니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "200", description = "채팅방 목록 조회 성공",
             content = @Content(schema = @Schema(implementation = RoomsResponse.class))),
@@ -101,10 +102,22 @@ public ResponseEntity<HealthResponse> healthCheck() {
     })
     @GetMapping
     @RateLimit
-    public ResponseEntity<?> getAllRooms(Principal principal) {
+    public ResponseEntity<?> getAllRooms(
+            Principal principal,
+            @RequestParam(defaultValue = "30") String size,
+            @RequestParam(required = false) String cursor) {
 
         try {
-            RoomsResponse response = roomService.getAllRooms(principal.getName());
+            int pageSize;
+            try {
+                pageSize = Integer.parseInt(size);
+            } catch (NumberFormatException exception) {
+                return invalidPageRequest("INVALID_PAGE_SIZE", "size는 1에서 100 사이의 정수여야 합니다.");
+            }
+            if (pageSize < 1 || pageSize > 100) {
+                return invalidPageRequest("INVALID_PAGE_SIZE", "size는 1에서 100 사이의 정수여야 합니다.");
+            }
+            RoomsResponse response = roomService.getAllRooms(principal.getName(), pageSize, cursor);
 
             // 캐시 설정
             return ResponseEntity.ok()
@@ -112,6 +125,8 @@ public ResponseEntity<?> getAllRooms(Principal principal) {
                 .header("Last-Modified", java.time.Instant.now().toString())
                 .body(response);
 
+        } catch (InvalidRoomCursorException exception) {
+            return invalidPageRequest("INVALID_CURSOR", "유효하지 않은 채팅방 페이지 커서입니다.");
         } catch (Exception e) {
             log.error("방 목록 조회 에러", e);
 
@@ -134,6 +149,14 @@ public ResponseEntity<?> getAllRooms(Principal principal) {
         }
     }
 
+    private ResponseEntity<StandardResponse<Object>> invalidPageRequest(String code, String message) {
+        return ResponseEntity.badRequest().body(StandardResponse.builder()
+            .success(false)
+            .code(code)
+            .message(message)
+            .build());
+    }
+
     @Operation(summary = "채팅방 생성", description = "새로운 채팅방을 생성합니다. 비밀번호를 설정하여 비공개 방을 만들 수 있습니다.")
     @ApiResponses({
         @ApiResponse(responseCode = "201", description = "채팅방 생성 성공",
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/dto/PageMetadata.java b/apps/backend/src/main/java/com/ktb/chatapp/dto/PageMetadata.java
index 7b62fd92..ccd6ef23 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/dto/PageMetadata.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/dto/PageMetadata.java
@@ -1,5 +1,6 @@
 package com.ktb.chatapp.dto;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import lombok.AllArgsConstructor;
 import lombok.Builder;
 import lombok.Data;
@@ -9,21 +10,10 @@
 @Builder
 @NoArgsConstructor
 @AllArgsConstructor
+@JsonInclude(JsonInclude.Include.NON_NULL)
 public class PageMetadata {
-    private long total;
-    private int page;
     private int pageSize;
-    private long totalPages;
     private boolean hasMore;
     private int currentCount;
-    private SortInfo sort;
-
-    @Data
-    @Builder
-    @NoArgsConstructor
-    @AllArgsConstructor
-    public static class SortInfo {
-        private String field;
-        private String order;
-    }
+    private String nextCursor;
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/exception/InvalidRoomCursorException.java b/apps/backend/src/main/java/com/ktb/chatapp/exception/InvalidRoomCursorException.java
new file mode 100644
index 00000000..af7b908e
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/exception/InvalidRoomCursorException.java
@@ -0,0 +1,12 @@
+package com.ktb.chatapp.exception;
+
+public class InvalidRoomCursorException extends RuntimeException {
+
+    public InvalidRoomCursorException() {
+        super("INVALID_CURSOR");
+    }
+
+    public InvalidRoomCursorException(Throwable cause) {
+        super("INVALID_CURSOR", cause);
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java
index 67653f69..a34fbb45 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryCustom.java
@@ -1,8 +1,12 @@
 package com.ktb.chatapp.repository;
 
 import com.ktb.chatapp.model.Room;
+import java.time.LocalDateTime;
+import java.util.List;
 
 public interface RoomRepositoryCustom {
 
     Room addParticipantAndReturn(String roomId, String userId);
+
+    List<Room> findPage(LocalDateTime cursorCreatedAt, String cursorId, int limit);
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java
index dbf4ac63..4ad1424c 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/repository/RoomRepositoryImpl.java
@@ -1,12 +1,16 @@
 package com.ktb.chatapp.repository;
 
 import com.ktb.chatapp.model.Room;
+import java.time.LocalDateTime;
+import java.util.List;
 import lombok.RequiredArgsConstructor;
 import org.springframework.data.mongodb.core.FindAndModifyOptions;
 import org.springframework.data.mongodb.core.MongoTemplate;
 import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.Update;
+import org.springframework.data.domain.Sort;
+import org.bson.types.ObjectId;
 
 @RequiredArgsConstructor
 public class RoomRepositoryImpl implements RoomRepositoryCustom {
@@ -25,4 +29,22 @@ public Room addParticipantAndReturn(String roomId, String userId) {
             Room.class
         );
     }
+
+    @Override
+    public List<Room> findPage(LocalDateTime cursorCreatedAt, String cursorId, int limit) {
+        Query query = new Query().limit(limit).with(Sort.by(
+            Sort.Order.desc("createdAt"),
+            Sort.Order.desc("_id")
+        ));
+        if (cursorCreatedAt != null && cursorId != null) {
+            query.addCriteria(new Criteria().orOperator(
+                Criteria.where("createdAt").lt(cursorCreatedAt),
+                new Criteria().andOperator(
+                    Criteria.where("createdAt").is(cursorCreatedAt),
+                    Criteria.where("_id").lt(new ObjectId(cursorId))
+                )
+            ));
+        }
+        return mongoTemplate.find(query, Room.class);
+    }
 }
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomCursorCodec.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomCursorCodec.java
new file mode 100644
index 00000000..49feeb4b
--- /dev/null
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomCursorCodec.java
@@ -0,0 +1,47 @@
+package com.ktb.chatapp.service;
+
+import com.ktb.chatapp.exception.InvalidRoomCursorException;
+import java.nio.charset.StandardCharsets;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeParseException;
+import java.util.Base64;
+import org.bson.types.ObjectId;
+
+final class RoomCursorCodec {
+
+    private static final String VERSION = "v1";
+
+    private RoomCursorCodec() {
+    }
+
+    static String encode(LocalDateTime createdAt, String id) {
+        if (createdAt == null || !ObjectId.isValid(id)) {
+            throw new InvalidRoomCursorException();
+        }
+        String payload = String.join("|", VERSION, createdAt.toString(), id);
+        return Base64.getUrlEncoder().withoutPadding()
+            .encodeToString(payload.getBytes(StandardCharsets.UTF_8));
+    }
+
+    static Cursor decode(String cursor) {
+        if (cursor == null || cursor.isBlank()) return null;
+        try {
+            String payload = new String(
+                Base64.getUrlDecoder().decode(cursor),
+                StandardCharsets.UTF_8
+            );
+            String[] parts = payload.split("\\|", -1);
+            if (parts.length != 3 || !VERSION.equals(parts[0]) || !ObjectId.isValid(parts[2])) {
+                throw new InvalidRoomCursorException();
+            }
+            return new Cursor(LocalDateTime.parse(parts[1]), parts[2]);
+        } catch (InvalidRoomCursorException exception) {
+            throw exception;
+        } catch (IllegalArgumentException | DateTimeParseException exception) {
+            throw new InvalidRoomCursorException(exception);
+        }
+    }
+
+    record Cursor(LocalDateTime createdAt, String id) {
+    }
+}
diff --git a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
index b1ab821e..dcb44978 100644
--- a/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
+++ b/apps/backend/src/main/java/com/ktb/chatapp/service/RoomService.java
@@ -8,7 +8,6 @@
 import com.ktb.chatapp.repository.RoomRepository;
 import com.ktb.chatapp.repository.UserRepository;
 import java.time.LocalDateTime;
-import java.util.Comparator;
 import java.util.HashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -34,69 +33,66 @@ public class RoomService {
     private final PasswordEncoder passwordEncoder;
     private final ApplicationEventPublisher eventPublisher;
 
-    public RoomsResponse getAllRooms(String name) {
-
-        try {
-            List<Room> rooms = roomRepository.findAll();
-            Set<String> userIds = rooms.stream()
-                .flatMap(room -> {
-                    Set<String> ids = new LinkedHashSet<>();
-                    if (room.getCreator() != null) {
-                        ids.add(room.getCreator());
-                    }
-                    if (room.getParticipantIds() != null) {
-                        ids.addAll(room.getParticipantIds());
-                    }
-                    return ids.stream();
-                })
-                .collect(Collectors.toSet());
-            Map<String, User> usersById = userIds.isEmpty()
-                ? Map.of()
-                : userRepository.findSummariesByIdIn(userIds).stream()
-                    .filter(user -> user != null && user.getId() != null)
-                    .collect(Collectors.toMap(User::getId, Function.identity()));
-            Set<String> roomIds = rooms.stream()
-                .map(Room::getId)
-                .filter(java.util.Objects::nonNull)
-                .collect(Collectors.toSet());
-            Map<String, Integer> recentMessageCounts = roomIds.isEmpty()
-                ? Map.of()
-                : recentMessageCounter.countRecentMessages(roomIds);
-
-            // 전체 방을 조회해 최신순으로 정렬한다
-            List<RoomResponse> roomResponses = rooms.stream()
-                .map(room -> mapToRoomResponse(
-                    room,
-                    name,
-                    usersById,
-                    recentMessageCounts.getOrDefault(room.getId(), 0)))
-                .sorted(Comparator.comparing(
-                    RoomResponse::getCreatedAtDateTime,
-                    Comparator.nullsLast(Comparator.reverseOrder())))
-                .collect(Collectors.toList());
-
-            PageMetadata metadata = PageMetadata.builder()
-                .total(roomResponses.size())
-                .page(0)
-                .pageSize(roomResponses.size())
-                .totalPages(1)
-                .hasMore(false)
-                .currentCount(roomResponses.size())
-                .build();
-
-            return RoomsResponse.builder()
-                .success(true)
-                .data(roomResponses)
-                .metadata(metadata)
-                .build();
+    public RoomsResponse getAllRooms(String name, int pageSize, String encodedCursor) {
+        RoomCursorCodec.Cursor cursor = RoomCursorCodec.decode(encodedCursor);
+        List<Room> page = roomRepository.findPage(
+            cursor == null ? null : cursor.createdAt(),
+            cursor == null ? null : cursor.id(),
+            pageSize + 1
+        );
+        boolean hasMore = page.size() > pageSize;
+        List<Room> rooms = hasMore ? page.subList(0, pageSize) : page;
+        Set<String> userIds = rooms.stream()
+            .flatMap(room -> {
+                Set<String> ids = new LinkedHashSet<>();
+                if (room.getCreator() != null) {
+                    ids.add(room.getCreator());
+                }
+                if (room.getParticipantIds() != null) {
+                    ids.addAll(room.getParticipantIds());
+                }
+                return ids.stream();
+            })
+            .collect(Collectors.toSet());
+        Map<String, User> usersById = userIds.isEmpty()
+            ? Map.of()
+            : userRepository.findSummariesByIdIn(userIds).stream()
+                .filter(user -> user != null && user.getId() != null)
+                .collect(Collectors.toMap(User::getId, Function.identity()));
+        Set<String> roomIds = rooms.stream()
+            .map(Room::getId)
+            .filter(java.util.Objects::nonNull)
+            .collect(Collectors.toSet());
+        Map<String, Integer> recentMessageCounts = roomIds.isEmpty()
+            ? Map.of()
+            : recentMessageCounter.countRecentMessages(roomIds);
+
+        List<RoomResponse> roomResponses = rooms.stream()
+            .map(room -> mapToRoomResponse(
+                room,
+                name,
+                usersById,
+                recentMessageCounts.getOrDefault(room.getId(), 0)))
+            .collect(Collectors.toList());
+
+        String nextCursor = hasMore && !rooms.isEmpty()
+            ? RoomCursorCodec.encode(
+                rooms.get(rooms.size() - 1).getCreatedAt(),
+                rooms.get(rooms.size() - 1).getId())
+            : null;
+
+        PageMetadata metadata = PageMetadata.builder()
+            .pageSize(pageSize)
+            .hasMore(hasMore)
+            .currentCount(roomResponses.size())
+            .nextCursor(nextCursor)
+            .build();
 
-        } catch (Exception e) {
-            log.error("방 목록 조회 에러", e);
-            return RoomsResponse.builder()
-                .success(false)
-                .data(List.of())
-                .build();
-        }
+        return RoomsResponse.builder()
+            .success(true)
+            .data(roomResponses)
+            .metadata(metadata)
+            .build();
     }
 
     public HealthResponse getHealthStatus() {
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/config/RoomPaginationIndexMigrationIntegrationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/config/RoomPaginationIndexMigrationIntegrationTest.java
new file mode 100644
index 00000000..8a3a84c4
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/config/RoomPaginationIndexMigrationIntegrationTest.java
@@ -0,0 +1,108 @@
+package com.ktb.chatapp.config;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.mongodb.client.MongoClient;
+import com.mongodb.client.MongoClients;
+import com.mongodb.client.MongoCollection;
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+import org.bson.Document;
+import org.bson.types.ObjectId;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+import org.testcontainers.mongodb.MongoDBContainer;
+
+@Testcontainers(disabledWithoutDocker = true)
+class RoomPaginationIndexMigrationIntegrationTest {
+
+    @Container
+    private static final MongoDBContainer MONGO = new MongoDBContainer("mongo:8.3.4");
+
+    private static MongoClient mongoClient;
+    private static MongoTemplate mongoTemplate;
+
+    @BeforeAll
+    static void setUpClient() {
+        mongoClient = MongoClients.create(MONGO.getConnectionString());
+        mongoTemplate = new MongoTemplate(mongoClient, "room-pagination-migration-test");
+    }
+
+    @AfterAll
+    static void closeClient() {
+        mongoClient.close();
+    }
+
+    @BeforeEach
+    void clearRooms() {
+        mongoTemplate.dropCollection(RoomPaginationIndexMigration.COLLECTION);
+    }
+
+    @Test
+    void backfillsCreatedAtAndCreatesIndexIdempotently() {
+        ObjectId id = new ObjectId();
+        rooms().insertOne(new Document("_id", id).append("name", "room"));
+        RoomPaginationIndexMigration migration = new RoomPaginationIndexMigration(mongoTemplate);
+
+        migration.run(null);
+        assertDoesNotThrow(() -> migration.run(null));
+
+        assertNotNull(rooms().find(new Document("_id", id)).first().getDate("createdAt"));
+        assertEquals(RoomPaginationIndexMigration.INDEX_NAME, paginationIndex().getString("name"));
+    }
+
+    @Test
+    void rejectsInvalidIdsBeforeChangingAnyRoom() {
+        rooms().insertMany(List.of(
+            new Document("_id", new ObjectId()).append("name", "valid"),
+            new Document("_id", "invalid-id").append("name", "invalid")
+        ));
+
+        assertThrows(IllegalStateException.class,
+            () -> new RoomPaginationIndexMigration(mongoTemplate).run(null));
+
+        assertEquals(0, rooms().countDocuments(new Document("createdAt", new Document("$type", "date"))));
+    }
+
+    @Test
+    void paginationSortUsesTheCompoundIndex() {
+        List<Document> roomDocuments = new ArrayList<>();
+        for (int index = 0; index < 100; index++) {
+            roomDocuments.add(new Document("_id", new ObjectId()).append("createdAt", new Date()));
+        }
+        rooms().insertMany(roomDocuments);
+        new RoomPaginationIndexMigration(mongoTemplate).run(null);
+
+        Document find = new Document("find", RoomPaginationIndexMigration.COLLECTION)
+            .append("filter", new Document())
+            .append("sort", new Document("createdAt", -1).append("_id", -1))
+            .append("limit", 31);
+        Document explain = mongoTemplate.getDb().runCommand(
+            new Document("explain", find).append("verbosity", "executionStats")
+        );
+
+        assertTrue(explain.toJson().contains("IXSCAN"));
+        assertTrue(explain.get("executionStats", Document.class).getInteger("totalDocsExamined") <= 31);
+    }
+
+    private static MongoCollection<Document> rooms() {
+        return mongoTemplate.getCollection(RoomPaginationIndexMigration.COLLECTION);
+    }
+
+    private static Document paginationIndex() {
+        return rooms().listIndexes().into(new ArrayList<>()).stream()
+            .filter(index -> RoomPaginationIndexMigration.INDEX_NAME.equals(index.getString("name")))
+            .findFirst()
+            .orElseThrow();
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerPaginationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerPaginationTest.java
new file mode 100644
index 00000000..a498f693
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/controller/RoomControllerPaginationTest.java
@@ -0,0 +1,69 @@
+package com.ktb.chatapp.controller;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.ktb.chatapp.dto.RoomsResponse;
+import com.ktb.chatapp.dto.StandardResponse;
+import com.ktb.chatapp.repository.UserRepository;
+import com.ktb.chatapp.service.RecentMessageCounter;
+import com.ktb.chatapp.service.RoomService;
+import com.ktb.chatapp.exception.InvalidRoomCursorException;
+import java.security.Principal;
+import java.util.List;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.http.ResponseEntity;
+
+@ExtendWith(MockitoExtension.class)
+class RoomControllerPaginationTest {
+
+    @Mock private UserRepository userRepository;
+    @Mock private RecentMessageCounter recentMessageCounter;
+    @Mock private RoomService roomService;
+
+    private RoomController controller;
+
+    @BeforeEach
+    void setUp() {
+        controller = new RoomController(userRepository, recentMessageCounter, roomService);
+    }
+
+    @Test
+    void forwardsValidatedPageRequest() {
+        RoomsResponse rooms = RoomsResponse.builder().success(true).data(List.of()).build();
+        when(roomService.getAllRooms("viewer@test.com", 30, "cursor")).thenReturn(rooms);
+
+        ResponseEntity<?> response = controller.getAllRooms(principal(), "30", "cursor");
+
+        assertEquals(200, response.getStatusCode().value());
+        verify(roomService).getAllRooms("viewer@test.com", 30, "cursor");
+    }
+
+    @Test
+    void rejectsOutOfRangePageSize() {
+        ResponseEntity<?> response = controller.getAllRooms(principal(), "101", null);
+
+        assertEquals(400, response.getStatusCode().value());
+        assertEquals("INVALID_PAGE_SIZE", ((StandardResponse<?>) response.getBody()).getCode());
+    }
+
+    @Test
+    void mapsMalformedCursorToBadRequest() {
+        when(roomService.getAllRooms("viewer@test.com", 30, "broken"))
+            .thenThrow(new InvalidRoomCursorException());
+
+        ResponseEntity<?> response = controller.getAllRooms(principal(), "30", "broken");
+
+        assertEquals(400, response.getStatusCode().value());
+        assertEquals("INVALID_CURSOR", ((StandardResponse<?>) response.getBody()).getCode());
+    }
+
+    private static Principal principal() {
+        return () -> "viewer@test.com";
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java b/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java
index fe9e1cab..29954b47 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/repository/RoomRepositoryImplIntegrationTest.java
@@ -12,6 +12,7 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
+import java.time.LocalDateTime;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -107,4 +108,33 @@ void concurrentJoinsDoNotLoseOrDuplicateParticipants() throws Exception {
             assertTrue(stored.getParticipantIds().contains("user-" + index));
         }
     }
+
+    @Test
+    void findPageUsesCreatedAtAndIdAsStableCursor() {
+        mongoTemplate.dropCollection(Room.class);
+        LocalDateTime timestamp = LocalDateTime.of(2026, 8, 12, 10, 0);
+        Room newest = room("507f1f77bcf86cd799439013", timestamp.plusMinutes(1));
+        Room tiedHigherId = room("507f1f77bcf86cd799439012", timestamp);
+        Room tiedLowerId = room("507f1f77bcf86cd799439011", timestamp);
+        Room oldest = room("507f1f77bcf86cd799439010", timestamp.minusMinutes(1));
+        List.of(oldest, tiedLowerId, newest, tiedHigherId).forEach(mongoTemplate::save);
+
+        List<Room> firstPage = repository.findPage(null, null, 2);
+        List<Room> secondPage = repository.findPage(
+            firstPage.get(1).getCreatedAt(), firstPage.get(1).getId(), 2);
+
+        assertEquals(List.of(newest.getId(), tiedHigherId.getId()),
+            firstPage.stream().map(Room::getId).toList());
+        assertEquals(List.of(tiedLowerId.getId(), oldest.getId()),
+            secondPage.stream().map(Room::getId).toList());
+    }
+
+    private static Room room(String id, LocalDateTime createdAt) {
+        return Room.builder()
+            .id(id)
+            .creator("creator-1")
+            .createdAt(createdAt)
+            .participantIds(Set.of("creator-1"))
+            .build();
+    }
 }
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomCursorCodecTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomCursorCodecTest.java
new file mode 100644
index 00000000..9dc8ecf2
--- /dev/null
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomCursorCodecTest.java
@@ -0,0 +1,29 @@
+package com.ktb.chatapp.service;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.time.LocalDateTime;
+import com.ktb.chatapp.exception.InvalidRoomCursorException;
+import org.junit.jupiter.api.Test;
+
+class RoomCursorCodecTest {
+
+    @Test
+    void roundTripsVersionedCursor() {
+        LocalDateTime createdAt = LocalDateTime.of(2026, 8, 12, 10, 30, 15, 123_000_000);
+        String id = "507f1f77bcf86cd799439011";
+
+        RoomCursorCodec.Cursor decoded = RoomCursorCodec.decode(RoomCursorCodec.encode(createdAt, id));
+
+        assertEquals(createdAt, decoded.createdAt());
+        assertEquals(id, decoded.id());
+    }
+
+    @Test
+    void rejectsInvalidCursorPayload() {
+        assertThrows(InvalidRoomCursorException.class, () -> RoomCursorCodec.decode("broken"));
+        assertThrows(InvalidRoomCursorException.class,
+            () -> RoomCursorCodec.encode(LocalDateTime.now(), "not-an-object-id"));
+    }
+}
diff --git a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
index 07eb8aef..e7007b12 100644
--- a/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
+++ b/apps/backend/src/test/java/com/ktb/chatapp/service/RoomServiceTest.java
@@ -4,6 +4,7 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anySet;
@@ -16,6 +17,7 @@
 import com.ktb.chatapp.dto.RoomResponse;
 import com.ktb.chatapp.dto.RoomsResponse;
 import com.ktb.chatapp.event.RoomUpdatedEvent;
+import com.ktb.chatapp.exception.InvalidRoomCursorException;
 import com.ktb.chatapp.model.Room;
 import com.ktb.chatapp.model.User;
 import com.ktb.chatapp.repository.RoomRepository;
@@ -58,9 +60,9 @@ void setUp() {
 
     @Test
     void getAllRooms_emptyList_avoidsBatchLookups() {
-        when(roomRepository.findAll()).thenReturn(List.of());
+        when(roomRepository.findPage(null, null, 31)).thenReturn(List.of());
 
-        RoomsResponse response = roomService.getAllRooms("viewer@test.com");
+        RoomsResponse response = roomService.getAllRooms("viewer@test.com", 30, null);
 
         assertTrue(response.isSuccess());
         assertTrue(response.getData().isEmpty());
@@ -79,12 +81,12 @@ void getAllRooms_batchesUsersAndMessageCountsOnceAndKeepsContract() throws Excep
         User creator = user("creator-1", "Creator", "creator@test.com");
         User participant = user("participant-1", null, null);
 
-        when(roomRepository.findAll()).thenReturn(List.of(oldRoom, newRoom));
+        when(roomRepository.findPage(null, null, 31)).thenReturn(List.of(newRoom, oldRoom));
         when(userRepository.findSummariesByIdIn(anySet())).thenReturn(List.of(creator, participant));
         when(recentMessageCounter.countRecentMessages(anySet()))
             .thenReturn(Map.of("room-old", 7));
 
-        RoomsResponse response = roomService.getAllRooms("viewer@test.com");
+        RoomsResponse response = roomService.getAllRooms("viewer@test.com", 30, null);
 
         assertTrue(response.isSuccess());
         assertEquals(List.of("room-new", "room-old"), response.getData().stream()
@@ -115,6 +117,36 @@ void getAllRooms_batchesUsersAndMessageCountsOnceAndKeepsContract() throws Excep
         assertFalse(json.has("createdAtDateTime"));
     }
 
+    @Test
+    void getAllRooms_returnsOpaqueCursorFromLookaheadItem() {
+        LocalDateTime newest = LocalDateTime.of(2026, 8, 12, 10, 0);
+        Room first = room("507f1f77bcf86cd799439011", "First", null, newest, Set.of());
+        Room second = room("507f1f77bcf86cd799439010", "Second", null, newest.minusMinutes(1), Set.of());
+        when(roomRepository.findPage(null, null, 2)).thenReturn(List.of(first, second));
+
+        RoomsResponse response = roomService.getAllRooms("viewer@test.com", 1, null);
+
+        assertEquals(List.of(first.getId()), response.getData().stream().map(RoomResponse::getId).toList());
+        assertTrue(response.getMetadata().isHasMore());
+        assertEquals(1, response.getMetadata().getCurrentCount());
+        assertTrue(response.getMetadata().getNextCursor() != null);
+    }
+
+    @Test
+    void getAllRooms_rejectsMalformedCursorBeforeQueryingRepository() {
+        assertThrows(InvalidRoomCursorException.class,
+            () -> roomService.getAllRooms("viewer@test.com", 30, "not-a-cursor"));
+        verify(roomRepository, never()).findPage(any(), any(), org.mockito.ArgumentMatchers.anyInt());
+    }
+
+    @Test
+    void getAllRooms_propagatesRepositoryFailure() {
+        when(roomRepository.findPage(null, null, 31)).thenThrow(new IllegalStateException("mongo unavailable"));
+
+        assertThrows(IllegalStateException.class,
+            () -> roomService.getAllRooms("viewer@test.com", 30, null));
+    }
+
     @Test
     void joinRoom_usesUserIdAtomicUpdateAndSharesOneResponseWithEvent() {
         Room before = room("room-1", "Room", "creator-1", LocalDateTime.now(),
diff --git a/apps/frontend/features/chat/rooms/ChatRoomsView.js b/apps/frontend/features/chat/rooms/ChatRoomsView.js
index c7f9416a..45ecb902 100644
--- a/apps/frontend/features/chat/rooms/ChatRoomsView.js
+++ b/apps/frontend/features/chat/rooms/ChatRoomsView.js
@@ -7,7 +7,7 @@ import {
   useServerConnection,
   CONNECTION_STATUS,
 } from './useServerConnection';
-import { useRoomList } from './useRoomList';
+import { ROOM_LIST_STATUS, useRoomList } from './useRoomList';
 import RoomsTable from './RoomsTable';
 import ConnectionErrorBanner from '@/components/ConnectionErrorBanner';
 
@@ -45,9 +45,16 @@ export default function ChatRoomsView({ router }) {
     error,
     loading,
     refreshing,
-    joiningRoom,
+    joiningRoomId,
+    navigationTarget,
+    listStatus,
+    hasMore,
+    loadingMore,
+    hasNewRooms,
+    setHasNewRooms,
     fetchRooms,
     refreshRooms,
+    loadMoreRooms,
     handleJoinRoom,
   } = useRoomList({
     currentUser,
@@ -134,7 +141,12 @@ export default function ChatRoomsView({ router }) {
     };
   }, [currentUserKey, connectionStatus]);
 
-  useRoomsSocket({ currentUser, setConnectionStatus, setRooms });
+  useRoomsSocket({
+    currentUser,
+    setConnectionStatus,
+    setRooms,
+    onRoomCreated: () => setHasNewRooms(true),
+  });
 
   return (
     <Box
@@ -170,7 +182,9 @@ export default function ChatRoomsView({ router }) {
                 <Button
                   variant="outline"
                   size="sm"
-                  onClick={() => fetchRooms()}
+                  onClick={() => connectionStatus === CONNECTION_STATUS.ERROR
+                    ? attemptConnection().catch(() => {})
+                    : fetchRooms()}
                   disabled={isRetrying}
                 >
                   <RefreshOutlineIcon size={16} />
@@ -222,19 +236,57 @@ export default function ChatRoomsView({ router }) {
           </Callout.Root>
         )}
 
-        <Box data-testid="rooms-content-slot" $css={{ minHeight: '430px', width: '100%' }}>
-        {connectionStatus === CONNECTION_STATUS.ERROR ? (
-          <ConnectionErrorBanner message="채팅 서버와 연결할 수 없습니다. 잠시 후 다시 시도해주세요." />
-        ) : loading ? (
+        {connectionStatus === CONNECTION_STATUS.ERROR && listStatus === ROOM_LIST_STATUS.READY && (
+          <Box data-testid="rooms-socket-error">
+            <ConnectionErrorBanner message="실시간 연결이 끊겼습니다. 현재 조회된 목록은 계속 볼 수 있습니다." />
+          </Box>
+        )}
+
+        {hasNewRooms && (
+          <Callout.Root colorPalette="primary" data-testid="rooms-new-available">
+            <HStack $css={{ gap: '$200', alignItems: 'center', justifyContent: 'space-between' }}>
+              <Text typography="body2">새 채팅방이 있습니다.</Text>
+              <Button size="sm" variant="outline" onClick={() => refreshRooms()}>
+                목록 갱신
+              </Button>
+            </HStack>
+          </Callout.Root>
+        )}
+
+        <Box
+          data-testid="rooms-content-slot"
+          data-state={listStatus}
+          data-joining-room-id={joiningRoomId || undefined}
+          data-navigation-target={navigationTarget || undefined}
+          $css={{ minHeight: '430px', width: '100%' }}
+        >
+        {listStatus === ROOM_LIST_STATUS.LOADING || loading ? (
           <Box $css={{ minHeight: '430px', display: 'flex', alignItems: 'center', justifyContent: 'center' }}>
             <LoadingIndicator text="채팅방 목록을 불러오는 중..." />
           </Box>
+        ) : listStatus === ROOM_LIST_STATUS.ERROR ? (
+          <Box data-testid="rooms-load-error" $css={{ minHeight: '430px' }}>
+            <ConnectionErrorBanner message="채팅방 목록을 불러오지 못했습니다. 다시 시도해주세요." />
+          </Box>
         ) : rooms.length > 0 ? (
-          <RoomsTable
-            rooms={rooms}
-            connectionStatus={connectionStatus}
-            onJoinRoom={handleJoinRoom}
-          />
+          <VStack $css={{ gap: '$200', width: '100%' }}>
+            <RoomsTable
+              rooms={rooms}
+              connectionStatus={connectionStatus}
+              joiningRoomId={joiningRoomId}
+              onJoinRoom={handleJoinRoom}
+            />
+            {hasMore && (
+              <Button
+                variant="outline"
+                onClick={loadMoreRooms}
+                disabled={loadingMore}
+                data-testid="load-more-rooms-button"
+              >
+                {loadingMore ? '불러오는 중' : '더 보기'}
+              </Button>
+            )}
+          </VStack>
         ) : !error && (
           <VStack
             $css={{ gap: '$300', alignItems: 'center', justifyContent: 'center', minHeight: '430px', padding: '$400' }}
diff --git a/apps/frontend/features/chat/rooms/RoomsTable.js b/apps/frontend/features/chat/rooms/RoomsTable.js
index 94bb1422..17b0bc35 100644
--- a/apps/frontend/features/chat/rooms/RoomsTable.js
+++ b/apps/frontend/features/chat/rooms/RoomsTable.js
@@ -4,7 +4,7 @@ import { Button, Text, VStack, HStack } from '@vapor-ui/core';
 import * as Table from '@/components/Table';
 import { CONNECTION_STATUS } from './useServerConnection';
 
-const RoomsTable = ({ rooms, connectionStatus, onJoinRoom }) => {
+const RoomsTable = ({ rooms, connectionStatus, joiningRoomId, onJoinRoom }) => {
   if (!rooms || rooms.length === 0) return null;
 
   return (
@@ -81,8 +81,9 @@ const RoomsTable = ({ rooms, connectionStatus, onJoinRoom }) => {
                   colorPalette="primary"
                   size="md"
                   onClick={() => onJoinRoom(room._id)}
-                  disabled={connectionStatus !== CONNECTION_STATUS.CONNECTED}
+                  disabled={connectionStatus !== CONNECTION_STATUS.CONNECTED || Boolean(joiningRoomId)}
                   data-testid="join-chat-room-button"
+                  data-room-id={room._id}
                 >
                   입장
                 </Button>
diff --git a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
index 0d1d71ef..bb1eee8b 100644
--- a/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/ChatRoomsView.test.js
@@ -7,6 +7,8 @@ import { CONNECTION_STATUS } from '../useServerConnection';
 const mocks = vi.hoisted(() => ({
   connectionStatus: 'checking',
   error: null,
+  listStatus: 'ready',
+  rooms: [],
   fetchRooms: vi.fn(() => Promise.resolve()),
   refreshRooms: vi.fn(() => Promise.resolve(true)),
   attemptConnection: vi.fn(() => Promise.resolve(true)),
@@ -39,15 +41,24 @@ vi.mock('../useServerConnection', async () => {
 });
 
 vi.mock('../useRoomList', () => ({
+  ROOM_LIST_STATUS: { LOADING: 'loading', READY: 'ready', ERROR: 'error' },
   useRoomList: () => ({
-    rooms: [],
+    rooms: mocks.rooms,
     setRooms: vi.fn(),
     error: mocks.error,
     loading: false,
     refreshing: false,
     joiningRoom: false,
+    joiningRoomId: null,
+    navigationTarget: null,
+    listStatus: mocks.listStatus,
+    hasMore: false,
+    loadingMore: false,
+    hasNewRooms: false,
+    setHasNewRooms: vi.fn(),
     fetchRooms: mocks.fetchRooms,
     refreshRooms: mocks.refreshRooms,
+    loadMoreRooms: vi.fn(),
     handleJoinRoom: vi.fn(),
   }),
 }));
@@ -60,6 +71,8 @@ describe('ChatRoomsView', () => {
   beforeEach(() => {
     mocks.connectionStatus = CONNECTION_STATUS.CHECKING;
     mocks.error = null;
+    mocks.listStatus = 'ready';
+    mocks.rooms = [];
     mocks.fetchRooms.mockClear();
     mocks.refreshRooms.mockClear();
     mocks.attemptConnection.mockClear();
@@ -148,5 +161,24 @@ describe('ChatRoomsView', () => {
     render(<ChatRoomsView router={{ push: vi.fn() }} />);
 
     expect(screen.getByTestId('rooms-content-slot')).toBeInTheDocument();
+    expect(screen.getByTestId('rooms-content-slot')).toHaveAttribute('data-state', 'ready');
+  });
+
+  it('keeps the REST room list visible when the socket connection fails', () => {
+    mocks.connectionStatus = CONNECTION_STATUS.ERROR;
+    mocks.rooms = [{ _id: 'room-1', name: '방 1', createdAt: '2026-08-12T10:00:00' }];
+
+    render(<ChatRoomsView router={{ push: vi.fn() }} />);
+
+    expect(screen.getByTestId('rooms-socket-error')).toBeInTheDocument();
+    expect(screen.getByTestId('join-chat-room-button')).toHaveAttribute('data-room-id', 'room-1');
+  });
+
+  it('exposes a dedicated room-list load error state', () => {
+    mocks.listStatus = 'error';
+
+    render(<ChatRoomsView router={{ push: vi.fn() }} />);
+
+    expect(screen.getByTestId('rooms-load-error')).toBeInTheDocument();
   });
 });
diff --git a/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js b/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
index d3e29169..07ce46a1 100644
--- a/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/useRoomList.test.js
@@ -1,7 +1,7 @@
 import { act, renderHook } from '@testing-library/react';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import axiosInstance from '@/services/axios';
-import { useRoomList } from '../useRoomList';
+import { ROOM_LIST_STATUS, useRoomList } from '../useRoomList';
 import { CONNECTION_STATUS } from '../useServerConnection';
 
 vi.mock('@/services/axios', () => ({
@@ -11,7 +11,9 @@ vi.mock('@/services/axios', () => ({
   },
 }));
 
-const roomsResponse = (rooms) => ({ data: { data: rooms } });
+const roomsResponse = (rooms, metadata = {}) => ({
+  data: { success: true, data: rooms, metadata },
+});
 
 const renderRoomList = (overrides = {}) =>
   renderHook(() => {
@@ -48,8 +50,12 @@ describe('useRoomList', () => {
     });
 
     expect(attemptConnection).not.toHaveBeenCalled();
-    expect(axiosInstance.get).toHaveBeenCalledWith('/api/rooms');
-    expect(setConnectionStatus).toHaveBeenCalledWith(CONNECTION_STATUS.CONNECTED);
+    expect(axiosInstance.get).toHaveBeenCalledWith('/api/rooms', {
+      params: { size: 30 },
+      maxRetries: 0,
+    });
+    expect(setConnectionStatus).not.toHaveBeenCalled();
+    expect(result.current.listStatus).toBe(ROOM_LIST_STATUS.READY);
   });
 
   it('replaces the list on refresh without leaving the refreshing flag on', async () => {
@@ -121,4 +127,60 @@ describe('useRoomList', () => {
     expect(result.current.rooms).toEqual([{ _id: 'room-1' }]);
   });
 
+  it('loads the next cursor page and removes duplicate room ids', async () => {
+    axiosInstance.get
+      .mockResolvedValueOnce(roomsResponse(
+        [{ _id: 'room-1' }, { _id: 'room-2' }],
+        { hasMore: true, nextCursor: 'cursor-1' }
+      ))
+      .mockResolvedValueOnce(roomsResponse(
+        [{ _id: 'room-2' }, { _id: 'room-3' }],
+        { hasMore: false, nextCursor: null }
+      ));
+    const { result } = renderRoomList();
+
+    await act(async () => result.current.fetchRooms());
+    await act(async () => result.current.loadMoreRooms());
+
+    expect(result.current.rooms.map((room) => room._id)).toEqual(['room-1', 'room-2', 'room-3']);
+    expect(axiosInstance.get).toHaveBeenLastCalledWith('/api/rooms', {
+      params: { size: 30, cursor: 'cursor-1' },
+      maxRetries: 0,
+    });
+    expect(result.current.hasMore).toBe(false);
+  });
+
+  it('rejects a success HTTP response with an unsuccessful API body', async () => {
+    axiosInstance.get.mockResolvedValue({ data: { success: false, data: [] } });
+    const { result } = renderRoomList();
+
+    await act(async () => result.current.fetchRooms());
+
+    expect(result.current.listStatus).toBe(ROOM_LIST_STATUS.ERROR);
+    expect(result.current.error.title).toBe('채팅방 목록 로드 실패');
+  });
+
+  it('never renders more than one server page from an oversized response', async () => {
+    axiosInstance.get.mockResolvedValue(roomsResponse(
+      Array.from({ length: 900 }, (_, index) => ({ _id: `room-${index}` }))
+    ));
+    const { result } = renderRoomList();
+
+    await act(async () => result.current.fetchRooms());
+
+    expect(result.current.rooms).toHaveLength(30);
+  });
+
+  it('routes exactly once after a successful room join', async () => {
+    const router = { push: vi.fn() };
+    axiosInstance.post.mockResolvedValue({ data: { success: true } });
+    const { result } = renderRoomList({ router });
+
+    await act(async () => result.current.handleJoinRoom('room-1'));
+
+    expect(router.push).toHaveBeenCalledTimes(1);
+    expect(router.push).toHaveBeenCalledWith('/chat/room-1');
+    expect(result.current.navigationTarget).toBe('room-1');
+  });
+
 });
diff --git a/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js b/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js
index a8aeabd3..d8f89fd8 100644
--- a/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js
+++ b/apps/frontend/features/chat/rooms/__tests__/useRoomsSocket.test.js
@@ -116,4 +116,20 @@ describe('useRoomsSocket', () => {
 
     expect(setRooms).not.toHaveBeenCalled();
   });
+
+  it('marks a new room as available without growing the loaded room array', async () => {
+    const socket = createSocket();
+    const setRooms = vi.fn();
+    const onRoomCreated = vi.fn();
+
+    renderRoomsSocket(socket, { setRooms, onRoomCreated });
+    await waitFor(() => {
+      expect(socket.on).toHaveBeenCalledWith('roomCreated', expect.any(Function));
+    });
+
+    handlerFor(socket, 'roomCreated')({ _id: 'new-room' });
+
+    expect(onRoomCreated).toHaveBeenCalledWith({ _id: 'new-room' });
+    expect(setRooms).not.toHaveBeenCalled();
+  });
 });
diff --git a/apps/frontend/features/chat/rooms/useRoomList.js b/apps/frontend/features/chat/rooms/useRoomList.js
index fce4d5f1..82bb3f4d 100644
--- a/apps/frontend/features/chat/rooms/useRoomList.js
+++ b/apps/frontend/features/chat/rooms/useRoomList.js
@@ -2,13 +2,19 @@ import { useState, useCallback, useRef } from 'react';
 import axiosInstance from '@/services/axios';
 import { CONNECTION_STATUS } from './useServerConnection';
 
+export const ROOM_LIST_STATUS = {
+  LOADING: 'loading',
+  READY: 'ready',
+  ERROR: 'error',
+};
+
+const ROOM_PAGE_SIZE = 30;
+
 export const useRoomList = ({
   currentUser,
   router,
   connectionStatus,
-  setConnectionStatus,
   isRetrying,
-  attemptConnection,
 }) => {
   const [rooms, setRooms] = useState([]);
   const [error, setError] = useState(null);
@@ -16,6 +22,13 @@ export const useRoomList = ({
   const [refreshing, setRefreshing] = useState(false);
   const [isInitialLoad, setIsInitialLoad] = useState(true);
   const [joiningRoom, setJoiningRoom] = useState(false);
+  const [joiningRoomId, setJoiningRoomId] = useState(null);
+  const [navigationTarget, setNavigationTarget] = useState(null);
+  const [listStatus, setListStatus] = useState(ROOM_LIST_STATUS.LOADING);
+  const [nextCursor, setNextCursor] = useState(null);
+  const [hasMore, setHasMore] = useState(false);
+  const [loadingMore, setLoadingMore] = useState(false);
+  const [hasNewRooms, setHasNewRooms] = useState(false);
 
   const isLoadingRef = useRef(false);
 
@@ -24,7 +37,7 @@ export const useRoomList = ({
     let errorType = 'danger';
     let showRetry = !isRetrying;
 
-    if (error.message === 'AUTH_EXPIRED') {
+    if (error.code === 'AUTH_EXPIRED' || error.message === 'AUTH_EXPIRED') {
       errorMessage = '인증이 만료되었습니다. 다시 로그인해주세요.';
       errorType = 'danger';
       showRetry = false;
@@ -36,7 +49,7 @@ export const useRoomList = ({
         showRetry,
       });
 
-      setConnectionStatus(CONNECTION_STATUS.ERROR);
+      setListStatus(ROOM_LIST_STATUS.ERROR);
       return;
     }
 
@@ -53,19 +66,32 @@ export const useRoomList = ({
       showRetry,
     });
 
-    setConnectionStatus(CONNECTION_STATUS.ERROR);
-  }, [isRetrying, setConnectionStatus]);
+    setListStatus(ROOM_LIST_STATUS.ERROR);
+  }, [isRetrying]);
 
-  const loadRooms = useCallback(async () => {
-    const response = await axiosInstance.get('/api/rooms');
+  const loadRooms = useCallback(async ({ cursor = null, append = false } = {}) => {
+    const response = await axiosInstance.get('/api/rooms', {
+      params: { size: ROOM_PAGE_SIZE, ...(cursor ? { cursor } : {}) },
+      maxRetries: 0,
+    });
 
-    if (!response?.data?.data) {
+    if (response?.data?.success !== true || !Array.isArray(response?.data?.data)) {
       throw new Error('INVALID_RESPONSE');
     }
+    const receivedRooms = response.data.data.slice(0, ROOM_PAGE_SIZE);
 
-    setRooms(response.data.data);
-    setConnectionStatus(CONNECTION_STATUS.CONNECTED);
-  }, [setConnectionStatus]);
+    if (append) {
+      setRooms((currentRooms) => {
+        const knownIds = new Set(currentRooms.map((room) => room._id));
+        return [...currentRooms, ...receivedRooms.filter((room) => !knownIds.has(room._id))];
+      });
+    } else {
+      setRooms(receivedRooms);
+    }
+    setNextCursor(response.data.metadata?.nextCursor || null);
+    setHasMore(Boolean(response.data.metadata?.hasMore));
+    setListStatus(ROOM_LIST_STATUS.READY);
+  }, []);
 
   const fetchRooms = useCallback(async () => {
     if (!currentUser?.token || isLoadingRef.current) {
@@ -76,9 +102,11 @@ export const useRoomList = ({
       isLoadingRef.current = true;
 
       setLoading(true);
+      setListStatus(ROOM_LIST_STATUS.LOADING);
       setError(null);
 
       await loadRooms();
+      setHasNewRooms(false);
 
       if (isInitialLoad) {
         setIsInitialLoad(false);
@@ -105,6 +133,7 @@ export const useRoomList = ({
       setRefreshing(true);
 
       await loadRooms();
+      setHasNewRooms(false);
       setError(null);
 
       return true;
@@ -125,6 +154,25 @@ export const useRoomList = ({
     }
   }, [currentUser, loadRooms]);
 
+  const loadMoreRooms = useCallback(async () => {
+    if (!currentUser?.token || !hasMore || !nextCursor || loadingMore) return false;
+    try {
+      setLoadingMore(true);
+      await loadRooms({ cursor: nextCursor, append: true });
+      return true;
+    } catch (error) {
+      setError({
+        title: '채팅방 추가 로드 실패',
+        message: '다음 채팅방 목록을 불러오지 못했습니다.',
+        type: 'warning',
+        showRetry: false,
+      });
+      return false;
+    } finally {
+      setLoadingMore(false);
+    }
+  }, [currentUser, hasMore, nextCursor, loadingMore, loadRooms]);
+
   const handleJoinRoom = useCallback(async (roomId) => {
     if (connectionStatus !== CONNECTION_STATUS.CONNECTED) {
       setError({
@@ -136,28 +184,35 @@ export const useRoomList = ({
     }
 
     setJoiningRoom(true);
+    setJoiningRoomId(roomId);
+    setNavigationTarget(null);
 
     try {
       const response = await axiosInstance.post(`/api/rooms/${roomId}/join`, {});
 
       if (response.data.success) {
+        setNavigationTarget(roomId);
         router.push(`/chat/${roomId}`);
+      } else {
+        throw new Error('INVALID_RESPONSE');
       }
     } catch (error) {
       let errorMessage = '입장에 실패했습니다.';
-      if (error.response?.status === 404) {
+      const status = error.status ?? error.response?.status;
+      if (status === 404) {
         errorMessage = '채팅방을 찾을 수 없습니다.';
-      } else if (error.response?.status === 403) {
+      } else if (status === 403) {
         errorMessage = '채팅방 입장 권한이 없습니다.';
       }
 
       setError({
         title: '채팅방 입장 실패',
-        message: error.response?.data?.message || errorMessage,
+        message: error.data?.message || error.response?.data?.message || errorMessage,
         type: 'danger',
       });
     } finally {
       setJoiningRoom(false);
+      setJoiningRoomId(null);
     }
   }, [connectionStatus, router]);
 
@@ -169,8 +224,16 @@ export const useRoomList = ({
     loading,
     refreshing,
     joiningRoom,
+    joiningRoomId,
+    navigationTarget,
+    listStatus,
+    hasMore,
+    loadingMore,
+    hasNewRooms,
+    setHasNewRooms,
     fetchRooms,
     refreshRooms,
+    loadMoreRooms,
     handleJoinRoom,
   };
 };
diff --git a/apps/frontend/features/chat/rooms/useRoomsSocket.js b/apps/frontend/features/chat/rooms/useRoomsSocket.js
index 1cf1a997..183d5b4c 100644
--- a/apps/frontend/features/chat/rooms/useRoomsSocket.js
+++ b/apps/frontend/features/chat/rooms/useRoomsSocket.js
@@ -11,6 +11,7 @@ export const useRoomsSocket = ({
   currentUser,
   setConnectionStatus,
   setRooms,
+  onRoomCreated,
 }) => {
   const socketRef = useRef(null);
 
@@ -47,7 +48,7 @@ export const useRoomsSocket = ({
             setConnectionStatus(CONNECTION_STATUS.ERROR);
           },
           roomCreated: (newRoom) => {
-            setRooms((prev) => [newRoom, ...prev]);
+            onRoomCreated?.(newRoom);
           },
           roomUpdated: (updatedRoom) => {
             setRooms((prev) =>
diff --git a/e2e/actions/chat.actions.js b/e2e/actions/chat.actions.js
index 52411350..77550cbd 100644
--- a/e2e/actions/chat.actions.js
+++ b/e2e/actions/chat.actions.js
@@ -1,6 +1,62 @@
 const { bannedWordSafeToken } = require('../utils/bannedWordSafeText');
 
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
+const ROOM_READY_TIMEOUT = Number(process.env.ROOM_READY_TIMEOUT || 5000);
+
+function scenarioError(code, message, cause) {
+  const error = new Error(`${code}: ${message}`, cause ? { cause } : undefined);
+  error.code = code;
+  return error;
+}
+
+async function waitForRoomListReady(page, { timeout = ROOM_READY_TIMEOUT } = {}) {
+  const content = page.getByTestId('rooms-content-slot');
+  try {
+    await content.waitFor({ state: 'visible', timeout });
+    await page.waitForFunction(
+      () => ['ready', 'error'].includes(
+        document.querySelector('[data-testid="rooms-content-slot"]')?.dataset.state
+      ),
+      undefined,
+      { timeout }
+    );
+  } catch (error) {
+    throw scenarioError(
+      'ROOM_LIST_READY_TIMEOUT',
+      `채팅방 목록이 ${timeout}ms 안에 준비되지 않았습니다. Current URL: ${page.url()}`,
+      error
+    );
+  }
+
+  const state = await content.getAttribute('data-state');
+  if (state === 'error') {
+    throw scenarioError('ROOM_LIST_LOAD_FAILED', `채팅방 목록 API가 실패했습니다. Current URL: ${page.url()}`);
+  }
+
+  const buttons = page.getByTestId('join-chat-room-button');
+  const hasRooms = await buttons.count() > 0;
+  const isEmpty = await page.getByTestId('rooms-empty').isVisible().catch(() => false);
+  if (!hasRooms && !isEmpty) {
+    throw scenarioError(
+      'ROOM_LIST_RENDER_INCONSISTENT',
+      `목록 상태는 ready지만 방 버튼과 빈 상태가 모두 없습니다. Current URL: ${page.url()}`
+    );
+  }
+
+  return { buttons, hasRooms, isEmpty };
+}
+
+async function waitForChatRoomReady(page, { timeout = ROOM_READY_TIMEOUT } = {}) {
+  try {
+    await page.getByTestId('chat-message-input').waitFor({ state: 'visible', timeout });
+  } catch (error) {
+    throw scenarioError(
+      'CHAT_ROOM_RENDER_TIMEOUT',
+      `채팅 입력창이 ${timeout}ms 안에 표시되지 않았습니다. Current URL: ${page.url()}`,
+      error
+    );
+  }
+}
 
 /**
  * 첫 번째 채팅방 입장 액션
@@ -18,14 +74,51 @@ async function joinFirstChatRoomAction(page) {
 async function joinRandomChatRoomAction(page) {
   await page.goto(`${BASE_URL}/chat`);
 
-  // 채팅방 버튼이 최소 하나 이상 로드될 때까지 대기
-  await page.getByTestId('join-chat-room-button').first().waitFor({ state: 'visible' });
+  const { buttons: chatRoomButtons, hasRooms } = await waitForRoomListReady(page);
+  if (!hasRooms) {
+    throw scenarioError('ROOM_LIST_EMPTY', '입장할 채팅방이 없습니다.');
+  }
 
-  const chatRoomButtons = page.getByTestId('join-chat-room-button');
   const count = await chatRoomButtons.count();
 
   const randomIndex = Math.floor(Math.random() * count);
-  await chatRoomButtons.nth(randomIndex).click();
+  const button = chatRoomButtons.nth(randomIndex);
+  const roomId = await button.getAttribute('data-room-id');
+  if (!roomId) {
+    throw scenarioError('ROOM_LIST_RENDER_INCONSISTENT', '입장 버튼에 room ID가 없습니다.');
+  }
+
+  const joinResponsePromise = page.waitForResponse((response) =>
+    response.request().method() === 'POST' &&
+    new URL(response.url()).pathname === `/api/rooms/${roomId}/join`,
+  { timeout: ROOM_READY_TIMEOUT });
+
+  let response;
+  try {
+    [response] = await Promise.all([joinResponsePromise, button.click()]);
+  } catch (error) {
+    throw scenarioError('JOIN_HTTP_FAILED', `채팅방 ${roomId} 입장 요청이 완료되지 않았습니다.`, error);
+  }
+
+  if (!response.ok()) {
+    throw scenarioError('JOIN_HTTP_FAILED', `채팅방 ${roomId} 입장 요청이 HTTP ${response.status()}로 실패했습니다.`);
+  }
+  const body = await response.json().catch(() => null);
+  if (body?.success !== true) {
+    throw scenarioError('JOIN_RESPONSE_INVALID', `채팅방 ${roomId} 입장 응답이 success=true가 아닙니다.`);
+  }
+
+  try {
+    await page.waitForURL(`${BASE_URL}/chat/${roomId}`, { timeout: ROOM_READY_TIMEOUT });
+  } catch (error) {
+    throw scenarioError(
+      'JOIN_NAVIGATION_TIMEOUT',
+      `입장 API는 200이지만 ${ROOM_READY_TIMEOUT}ms 안에 이동하지 못했습니다. Room ID: ${roomId}, Current URL: ${page.url()}`,
+      error
+    );
+  }
+  await waitForChatRoomReady(page);
+  return roomId;
 }
 
 /**
@@ -122,4 +215,6 @@ module.exports = {
   uploadFileAction,
   scrollChatToTopAction,
   addEmojiReactionAction,
+  waitForRoomListReady,
+  waitForChatRoomReady,
 };
diff --git a/e2e/actions/chat.actions.test.js b/e2e/actions/chat.actions.test.js
new file mode 100644
index 00000000..45590a8f
--- /dev/null
+++ b/e2e/actions/chat.actions.test.js
@@ -0,0 +1,74 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { joinRandomChatRoomAction, waitForRoomListReady } = require('./chat.actions');
+
+function roomListPage({ state = 'ready', buttonCount = 1, emptyVisible = false } = {}) {
+  const content = {
+    waitFor: async () => {},
+    getAttribute: async (name) => name === 'data-state' ? state : null,
+  };
+  const button = {
+    getAttribute: async () => '507f1f77bcf86cd799439011',
+    click: async () => {},
+  };
+  const buttons = {
+    count: async () => buttonCount,
+    nth: () => button,
+  };
+  return {
+    content,
+    button,
+    buttons,
+    page: {
+      url: () => 'http://localhost:3000/chat',
+      waitForFunction: async () => {},
+      getByTestId: (testId) => {
+        if (testId === 'rooms-content-slot') return content;
+        if (testId === 'join-chat-room-button') return buttons;
+        if (testId === 'rooms-empty') return { isVisible: async () => emptyVisible };
+        throw new Error(`unexpected test id: ${testId}`);
+      },
+    },
+  };
+}
+
+test('accepts both a populated and an empty ready room list', async () => {
+  const populated = roomListPage({ buttonCount: 2 });
+  assert.equal((await waitForRoomListReady(populated.page)).hasRooms, true);
+
+  const empty = roomListPage({ buttonCount: 0, emptyVisible: true });
+  assert.equal((await waitForRoomListReady(empty.page)).isEmpty, true);
+});
+
+test('reports an explicit room-list load failure', async () => {
+  const { page } = roomListPage({ state: 'error', buttonCount: 0 });
+  await assert.rejects(waitForRoomListReady(page), { code: 'ROOM_LIST_LOAD_FAILED' });
+});
+
+test('joins by the room id carried by the selected button and waits for that URL', async () => {
+  const roomId = '507f1f77bcf86cd799439011';
+  const fixture = roomListPage();
+  const waitedUrls = [];
+  fixture.page.goto = async () => {};
+  fixture.page.waitForResponse = async (predicate) => {
+    const response = {
+      url: () => `http://localhost:3000/api/rooms/${roomId}/join`,
+      request: () => ({ method: () => 'POST' }),
+      ok: () => true,
+      status: () => 200,
+      json: async () => ({ success: true }),
+    };
+    assert.equal(predicate(response), true);
+    return response;
+  };
+  fixture.page.waitForURL = async (url) => { waitedUrls.push(url); };
+  const originalGetByTestId = fixture.page.getByTestId;
+  fixture.page.getByTestId = (testId) => testId === 'chat-message-input'
+    ? { waitFor: async () => {} }
+    : originalGetByTestId(testId);
+
+  const selected = await joinRandomChatRoomAction(fixture.page);
+
+  assert.equal(selected, roomId);
+  assert.deepEqual(waitedUrls, [`http://localhost:3000/chat/${roomId}`]);
+});
diff --git a/e2e/artillery/Makefile b/e2e/artillery/Makefile
index 8f161b5c..34a88fee 100644
--- a/e2e/artillery/Makefile
+++ b/e2e/artillery/Makefile
@@ -19,7 +19,7 @@ export FORBIDDEN_WORDS ?= "b3sig78jv,9c0hej6x,lbl276sz"
 export PHASE1_DURATION ?= 5 # 해당 시간 내에
 export PHASE1_ARRIVAL_COUNT ?= 1 # 몇명의 가상 유저를 생성할건지?
 
-.PHONY: verify-env artillery
+.PHONY: verify-env artillery artillery-auth-negative artillery-room-lifecycle artillery-messaging artillery-media-profile
 
 verify-env:
 	# Check Node.js installed
@@ -36,5 +36,16 @@ verify-env:
 	@pnpm --filter e2e exec playwright install chromium
 
 artillery: verify-env
-	@echo "🚀 Running basic head test ($(PHASE1_ARRIVAL_COUNT) users for $(PHASE1_DURATION)s)..."
-	@pnpm --filter e2e exec artillery run artillery/artillery-config.yaml
\ No newline at end of file
+	@$(MAKE) artillery-room-lifecycle
+
+artillery-auth-negative: verify-env
+	@pnpm --filter e2e exec artillery run artillery/artillery-auth-negative.yaml
+
+artillery-room-lifecycle: verify-env
+	@pnpm --filter e2e exec artillery run artillery/artillery-room-lifecycle.yaml
+
+artillery-messaging: verify-env
+	@pnpm --filter e2e exec artillery run artillery/artillery-messaging.yaml
+
+artillery-media-profile: verify-env
+	@pnpm --filter e2e exec artillery run artillery/artillery-media-profile.yaml
diff --git a/e2e/artillery/README.md b/e2e/artillery/README.md
index 533f71bf..05a2af1f 100644
--- a/e2e/artillery/README.md
+++ b/e2e/artillery/README.md
@@ -18,6 +18,18 @@
   PHASE1_ARRIVAL_COUNT=10 PHASE1_DURATION=30 make artillery
   ```
 
+  기본 실행은 방 생성·랜덤 입장만 검증합니다. 기능별 부하는 다음처럼 분리해서 실행합니다.
+
+  ```bash
+  make artillery-auth-negative  # VU당 의도된 로그인 401 한 건
+  make artillery-room-lifecycle # 목록 조회, 방 생성, 랜덤 입장
+  make artillery-messaging      # 일반/대량/금칙어 메시지
+  make artillery-media-profile  # 파일 업로드와 프로필 변경
+  ```
+
+  `auth-negative`의 `POST /api/auth/login` 401만 `expected_failure`이며, 다른 프로필의
+  401과 모든 404는 `unexpected_failure`로 집계됩니다.
+
 ## Stage 0 관측 실행
 
 브라우저 흐름을 부하 테스트로 올리기 전에 1 VU로 실제 호출 경로를 진단합니다. 이 실행은
diff --git a/e2e/artillery/aggregate-observations.js b/e2e/artillery/aggregate-observations.js
index 193e0fb0..2abc21d6 100644
--- a/e2e/artillery/aggregate-observations.js
+++ b/e2e/artillery/aggregate-observations.js
@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 const fs = require('fs');
 const path = require('path');
-const { summarizeSamples } = require('./observation');
+const { enrichHttpSample, summarizeHttpFailures, summarizeSamples } = require('./observation');
 
 const runDirectories = process.argv.slice(2).map((value) => path.resolve(value));
 if (runDirectories.length === 0) {
@@ -22,14 +22,14 @@ for (const report of reports) {
     samples.runtime.push(...(report.samples.runtime || []));
     samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
+samples.http = samples.http.map(enrichHttpSample);
 
 const timestamps = Object.values(samples).flat()
     .flatMap((sample) => [sample.startedAt, sample.endedAt])
     .filter(Boolean)
     .sort();
 const rankedHttp = summarizeSamples(samples.http.filter((sample) =>
-    sample.name !== 'GET /api/health' &&
-    !(sample.name === 'POST /api/auth/login' && sample.status === 401)));
+    sample.name !== 'GET /api/health' && sample.outcome !== 'expected_failure'));
 const expectedTotal = Number(process.env.EXPECTED_TOTAL_VUS || 0);
 const expectedPerRun = Number(process.env.EXPECTED_VUS || process.env.PHASE1_ARRIVAL_COUNT || 0);
 const expectedVirtualUsers = expectedTotal > 0
@@ -37,6 +37,13 @@ const expectedVirtualUsers = expectedTotal > 0
     : expectedPerRun > 0 ? expectedPerRun * runDirectories.length : null;
 const completedVirtualUsers = reports.filter((report) => report.samples.actions.length > 0 &&
     report.samples.actions.every((sample) => sample.success)).length;
+const profiles = [...new Set(reports.map((report) => report.metadata?.workload?.profile).filter(Boolean))];
+const expected401Samples = samples.http.filter((sample) => sample.outcome === 'expected_failure' && sample.status === 401);
+const unexpected401Samples = samples.http.filter((sample) => sample.outcome === 'unexpected_failure' && sample.status === 401);
+const expected401PerVuSatisfied = profiles.length === 1 && profiles[0] === 'auth-negative'
+    ? reports.every((report) => report.samples.http.map(enrichHttpSample)
+        .filter((sample) => sample.outcome === 'expected_failure' && sample.status === 401).length === 1)
+    : expected401Samples.length === 0;
 
 const aggregate = {
     schemaVersion: 1,
@@ -55,6 +62,12 @@ const aggregate = {
         targetReached: expectedVirtualUsers === null ? null : reports.length === expectedVirtualUsers,
         adoptable: expectedVirtualUsers === null ? false :
             reports.length === expectedVirtualUsers && completedVirtualUsers / expectedVirtualUsers >= 0.99,
+        httpExpectations: {
+            profiles,
+            expected401: expected401Samples.length,
+            unexpected401: unexpected401Samples.length,
+            expected401PerVuSatisfied,
+        },
     },
     summary: {
         actions: summarizeSamples(samples.actions),
@@ -62,6 +75,11 @@ const aggregate = {
         resources: summarizeSamples(samples.resources),
         httpDiagnostic: summarizeSamples(samples.http),
         httpRanked: rankedHttp,
+        httpFailures: summarizeHttpFailures(samples.http),
+        expectedHttpFailures: summarizeHttpFailures(samples.http.filter((sample) =>
+            sample.outcome === 'expected_failure')),
+        unexpectedHttpFailures: summarizeHttpFailures(samples.http.filter((sample) =>
+            sample.outcome === 'unexpected_failure')),
         socket: summarizeSamples(samples.socket),
         runtime: summarizeSamples(samples.runtime),
         cls: {
diff --git a/e2e/artillery/all-scenarios.js b/e2e/artillery/all-scenarios.js
index 961c8b3e..41db7748 100644
--- a/e2e/artillery/all-scenarios.js
+++ b/e2e/artillery/all-scenarios.js
@@ -1,75 +1,14 @@
-const { loginScenario, failedLoginScenario } = require('./scenarios/auth.scenario.js');
-const {
-    chatRoomCreationScenario,
-    massMessageScenario,
-    fileUploadScenario,
-    forbiddenWordScenario,
-} = require('./scenarios/chat.scenario.js');
-const {
-    fullProfileUpdateScenario,
-} = require('./scenarios/profile.scenario.js');
-const { createObservation } = require('./observation.js');
-
-function generateUserSchema() {
-    const timestamp = Date.now();
-    const randomId = Math.random().toString(36).substring(2, 8);
-
-    // 고유한 테스트 사용자 생성
-    const testUser = {
-        email: `loadtest_${timestamp}_${randomId}@example.com`,
-        password: 'Password123!',
-        passwordConfirm: 'Password123!',
-        name: `Load Test User ${randomId}`,
-    };
-    return testUser;
-}
-
-const allScenariosFlat = [
-    // auth Scenarios
-    failedLoginScenario,
-    loginScenario,
-
-    // chat Scenarios
-    chatRoomCreationScenario,
-    massMessageScenario,
-    fileUploadScenario,
-    forbiddenWordScenario,
-
-    // profile Scenarios
-    fullProfileUpdateScenario,
-];
-
+const { roomLifecycleProfile } = require('./profile-scenarios.js');
 
 /**
- * 통합 시나리오 순차 실행
- * 모든 개별 시나리오를 순서대로 실행
- *
- * 각 단계를 try/catch로 감싸 에러 메시지 앞에 실패한 시나리오 이름을 붙여
- * 다시 던진다 — Artillery는 uncaught error를 "errors.<메시지>" 카운터로
- * 자동 집계하는데, Playwright 자체 에러 메시지(예: "locator.click: Timeout
- * 30000ms exceeded.")만으로는 여러 시나리오가 같은 종류의 액션을 쓰는 경우
- * 부하테스트 리포트에서 어느 시나리오가 실패했는지 구분할 수 없었다. 실제
- * 에러 타입/스택/재시도 동작은 그대로 두고 메시지 텍스트만 태그를 붙인다.
+ * 기존 기본 진입점은 방 생명주기 프로필만 실행한다.
+ * 인증 실패, 메시징, 미디어/프로필 부하는 각 전용 설정으로 실행한다.
  */
 async function allScenarios(page, vuContext) {
-    const testUser = generateUserSchema();
-    vuContext.vars.testUser = testUser;
-    vuContext.vars.observation = createObservation(page, vuContext);
-
-    try {
-        for (const scenario of allScenariosFlat) {
-            try {
-                await scenario(page, vuContext);
-            } catch (err) {
-                err.message = `[${scenario.name}] ${err.message}`;
-                throw err;
-            }
-        }
-    } finally {
-        await vuContext.vars.observation.finish();
-    }
+    return roomLifecycleProfile(page, vuContext);
 }
 
 module.exports = {
     allScenarios,
+    roomLifecycleProfile,
 };
diff --git a/e2e/artillery/artillery-auth-negative.yaml b/e2e/artillery/artillery-auth-negative.yaml
new file mode 100644
index 00000000..82474803
--- /dev/null
+++ b/e2e/artillery/artillery-auth-negative.yaml
@@ -0,0 +1,17 @@
+config:
+  target: "{{ $env.BASE_URL }}"
+  phases:
+    - name: "auth negative"
+      duration: "{{ $env.PHASE1_DURATION }}"
+      arrivalCount: "{{ $env.PHASE1_ARRIVAL_COUNT }}"
+  processor: "./profile-scenarios.js"
+  engines:
+    playwright:
+      launchOptions:
+        headless: true
+      contextOptions:
+        viewport: null
+
+scenarios:
+  - engine: playwright
+    flowFunction: authNegativeProfile
diff --git a/e2e/artillery/artillery-config.yaml b/e2e/artillery/artillery-config.yaml
index 94af6719..20f75fbe 100644
--- a/e2e/artillery/artillery-config.yaml
+++ b/e2e/artillery/artillery-config.yaml
@@ -9,10 +9,9 @@ config:
     playwright:
       launchOptions:
         headless: true
-        slowMo: 100
       contextOptions:
         viewport: null
 
 scenarios:
   - engine: playwright
-    flowFunction: allScenarios
+    flowFunction: roomLifecycleProfile
diff --git a/e2e/artillery/artillery-media-profile.yaml b/e2e/artillery/artillery-media-profile.yaml
new file mode 100644
index 00000000..d4526fa1
--- /dev/null
+++ b/e2e/artillery/artillery-media-profile.yaml
@@ -0,0 +1,17 @@
+config:
+  target: "{{ $env.BASE_URL }}"
+  phases:
+    - name: "media and profile"
+      duration: "{{ $env.PHASE1_DURATION }}"
+      arrivalCount: "{{ $env.PHASE1_ARRIVAL_COUNT }}"
+  processor: "./profile-scenarios.js"
+  engines:
+    playwright:
+      launchOptions:
+        headless: true
+      contextOptions:
+        viewport: null
+
+scenarios:
+  - engine: playwright
+    flowFunction: mediaProfile
diff --git a/e2e/artillery/artillery-messaging.yaml b/e2e/artillery/artillery-messaging.yaml
new file mode 100644
index 00000000..0b066ad6
--- /dev/null
+++ b/e2e/artillery/artillery-messaging.yaml
@@ -0,0 +1,17 @@
+config:
+  target: "{{ $env.BASE_URL }}"
+  phases:
+    - name: "messaging"
+      duration: "{{ $env.PHASE1_DURATION }}"
+      arrivalCount: "{{ $env.PHASE1_ARRIVAL_COUNT }}"
+  processor: "./profile-scenarios.js"
+  engines:
+    playwright:
+      launchOptions:
+        headless: true
+      contextOptions:
+        viewport: null
+
+scenarios:
+  - engine: playwright
+    flowFunction: messagingProfile
diff --git a/e2e/artillery/artillery-room-lifecycle.yaml b/e2e/artillery/artillery-room-lifecycle.yaml
new file mode 100644
index 00000000..9da2eae1
--- /dev/null
+++ b/e2e/artillery/artillery-room-lifecycle.yaml
@@ -0,0 +1,17 @@
+config:
+  target: "{{ $env.BASE_URL }}"
+  phases:
+    - name: "room lifecycle"
+      duration: "{{ $env.PHASE1_DURATION }}"
+      arrivalCount: "{{ $env.PHASE1_ARRIVAL_COUNT }}"
+  processor: "./profile-scenarios.js"
+  engines:
+    playwright:
+      launchOptions:
+        headless: true
+      contextOptions:
+        viewport: null
+
+scenarios:
+  - engine: playwright
+    flowFunction: roomLifecycleProfile
diff --git a/e2e/artillery/observation.js b/e2e/artillery/observation.js
index aa8d3e83..12a11b40 100644
--- a/e2e/artillery/observation.js
+++ b/e2e/artillery/observation.js
@@ -37,6 +37,7 @@ function createRunMetadata(environment = process.env) {
             loadGenerator: environment.LOAD_IMAGE_DIGEST || null,
         },
         workload: {
+            profile: environment.LOAD_PROFILE || null,
             durationSeconds: numberFromEnvironment(environment.PHASE1_DURATION),
             arrivalCount: numberFromEnvironment(environment.PHASE1_ARRIVAL_COUNT),
             virtualUsersPerPod: numberFromEnvironment(environment.VUS_PER_POD),
@@ -147,6 +148,58 @@ function normalizeDocumentPath(rawUrl) {
     return url.pathname;
 }
 
+function classifyHttpOutcome({ action, method, normalizedPath, status, requestSucceeded = true }) {
+    const expectedFailure = action === 'failed_login' &&
+        method === 'POST' && normalizedPath === '/api/auth/login' && status === 401;
+    if (expectedFailure) return 'expected_failure';
+    if (!requestSucceeded || status === null || status >= 400) return 'unexpected_failure';
+    return 'success';
+}
+
+function enrichHttpSample(sample) {
+    if (sample.outcome) return sample;
+    const separator = sample.name?.indexOf(' ') ?? -1;
+    const method = sample.method || (separator > 0 ? sample.name.slice(0, separator) : null);
+    const normalizedPath = sample.normalizedPath || (separator > 0 ? sample.name.slice(separator + 1) : null);
+    const outcome = classifyHttpOutcome({
+        action: sample.action,
+        method,
+        normalizedPath,
+        status: sample.status ?? null,
+        requestSucceeded: sample.status !== null && sample.status !== undefined,
+    });
+    return { ...sample, method, normalizedPath, outcome, success: outcome !== 'unexpected_failure' };
+}
+
+function summarizeHttpFailures(samples) {
+    const groups = new Map();
+    for (const original of samples.filter((item) => item.status >= 400)) {
+        const sample = enrichHttpSample(original);
+        const key = [sample.status, sample.method, sample.normalizedPath, sample.action].join('|');
+        const group = groups.get(key) || {
+            status: sample.status,
+            method: sample.method,
+            path: sample.normalizedPath,
+            action: sample.action || null,
+            outcome: sample.outcome,
+            count: 0,
+            urls: new Set(),
+            pageUrls: new Set(),
+        };
+        group.count += 1;
+        if (sample.url) group.urls.add(sample.url);
+        if (sample.pageUrl) group.pageUrls.add(sample.pageUrl);
+        groups.set(key, group);
+    }
+    return [...groups.values()]
+        .map((group) => ({
+            ...group,
+            urls: [...group.urls].slice(0, 10),
+            pageUrls: [...group.pageUrls].slice(0, 10),
+        }))
+        .sort((left, right) => right.count - left.count);
+}
+
 function parseSocketEvent(payload) {
     const text = Buffer.isBuffer(payload) ? payload.toString('utf8') : String(payload);
     const arrayStart = text.indexOf('[');
@@ -230,6 +283,7 @@ function createObservation(page, vuContext) {
     const vuId = vuContext?._uid || vuContext?.vars?.$uuid || randomUUID();
     const artifactPrefix = `${process.pid}-${sanitizeArtifactSegment(vuId)}-${randomUUID()}`;
     const runMetadata = createRunMetadata();
+    runMetadata.workload.profile = vuContext?.vars?.loadProfile || runMetadata.workload.profile;
     let currentAction = null;
     let socketSequence = 0;
 
@@ -274,26 +328,47 @@ function createObservation(page, vuContext) {
         });
     };
 
-    const recordHttp = async (request, success) => {
+    const recordHttp = async (request, requestSucceeded) => {
         const started = startedRequests.get(request);
         startedRequests.delete(request);
         if (!started) return;
         const response = await request.response().catch(() => null);
+        const status = response?.status() ?? null;
+        const outcome = classifyHttpOutcome({
+            action: started.action,
+            method: started.method,
+            normalizedPath: started.path,
+            status,
+            requestSucceeded,
+        });
         samples.http.push({
-            name: `${request.method()} ${started.path}`,
+            name: `${started.method} ${started.path}`,
+            method: started.method,
+            normalizedPath: started.path,
+            url: started.url,
             action: started.action,
+            pageUrl: started.pageUrl,
+            resourceType: started.resourceType,
             durationMs: performance.now() - started.at,
             startedAt: started.wallStartedAt,
             endedAt: new Date().toISOString(),
-            success: success && response !== null && response.status() < 400,
-            status: response?.status() ?? null,
+            success: outcome !== 'unexpected_failure',
+            outcome,
+            status,
         });
     };
 
     const onRequest = (request) => {
         const apiPath = normalizeApiPath(request.url());
         if (apiPath) startedRequests.set(request, {
-            at: performance.now(), wallStartedAt: new Date().toISOString(), path: apiPath, action: currentAction,
+            at: performance.now(),
+            wallStartedAt: new Date().toISOString(),
+            path: apiPath,
+            url: request.url(),
+            method: request.method(),
+            resourceType: request.resourceType(),
+            pageUrl: page.url(),
+            action: currentAction,
         });
     };
     const onRequestFinished = (request) => { void recordHttp(request, true); };
@@ -301,6 +376,29 @@ function createObservation(page, vuContext) {
     const onResponse = (response) => {
         const request = response.request();
         const resourceType = request.resourceType();
+        const apiPath = normalizeApiPath(response.url());
+        if (response.status() >= 400 && !apiPath) {
+            const responseUrl = new URL(response.url(), page.url());
+            const targetUrl = new URL(process.env.BASE_URL || page.url());
+            if (responseUrl.origin === targetUrl.origin) {
+                const normalizedPath = normalizeDocumentPath(response.url());
+                samples.http.push({
+                    name: `${request.method()} ${normalizedPath}`,
+                    method: request.method(),
+                    normalizedPath,
+                    url: response.url(),
+                    action: currentAction,
+                    pageUrl: page.url(),
+                    resourceType,
+                    durationMs: Math.max(request.timing()?.responseStart || 0, 0),
+                    startedAt: new Date(Date.now() - Math.max(request.timing()?.responseStart || 0, 0)).toISOString(),
+                    endedAt: new Date().toISOString(),
+                    success: false,
+                    outcome: 'unexpected_failure',
+                    status: response.status(),
+                });
+            }
+        }
         if (!RESOURCE_TYPES.has(resourceType)) return;
         const timing = request.timing();
         const ttfbMs = timing?.responseStart;
@@ -442,6 +540,9 @@ function createObservation(page, vuContext) {
             '[data-testid="login-submit-button"]',
             '[data-testid="chat-room-name-input"]', '[data-testid="chat-message-input"]',
             '[data-testid="message-submission-status"]', '[data-testid="file-message-container"]',
+            '[data-testid="rooms-content-slot"]', '[data-testid="join-chat-room-button"]',
+            '[data-testid="rooms-empty"]', '[data-testid="rooms-load-error"]',
+            '[data-testid="rooms-socket-error"]',
         ];
         const ui = {};
         for (const selector of selectors) {
@@ -453,17 +554,34 @@ function createObservation(page, vuContext) {
             visibilityState: document.visibilityState,
             url: window.location.href,
         })).catch(() => null);
+        const roomList = await page.locator('[data-testid="rooms-content-slot"]').first().evaluate((element) => ({
+            state: element.dataset.state || null,
+            joiningRoomId: element.dataset.joiningRoomId || null,
+            navigationTarget: element.dataset.navigationTarget || null,
+        })).catch(() => null);
         return {
             url: page.url(),
             document: documentState,
+            roomList,
             ui,
-            recentHttp: samples.http.slice(-10).map(({ name, status, success, action, startedAt, endedAt }) =>
-                ({ name, status, success, action, startedAt, endedAt })),
+            recentHttp: samples.http.slice(-10).map(({
+                name, method, normalizedPath, url, status, success, outcome, action, pageUrl, startedAt, endedAt,
+            }) => ({
+                name, method, normalizedPath, url, status, success, outcome, action, pageUrl, startedAt, endedAt,
+            })),
             recentDocuments: samples.documents.slice(-10),
             recentResources: samples.resources.slice(-20),
             recentBrowserErrors: samples.browser.slice(-10),
             recentEventLoopLag: samples.runtime.slice(-10),
             pendingSocketEvents: [...pendingSocketEvents.keys()].map(String),
+            pendingRequests: [...startedRequests.values()].map((request) => ({
+                method: request.method,
+                path: request.path,
+                url: request.url,
+                action: request.action,
+                pageUrl: request.pageUrl,
+                startedAt: request.wallStartedAt,
+            })),
         };
     }
 
@@ -570,6 +688,7 @@ function createObservation(page, vuContext) {
                 documents: summarizeSamples(samples.documents),
                 resources: summarizeSamples(samples.resources),
                 http: summarizeSamples(samples.http),
+                httpFailures: summarizeHttpFailures(samples.http),
                 socket: summarizeSamples(samples.socket),
                 runtime: summarizeSamples(samples.runtime),
                 cls: {
@@ -612,6 +731,9 @@ module.exports = {
     inspectLocator,
     normalizeApiPath,
     normalizeDocumentPath,
+    classifyHttpOutcome,
+    enrichHttpSample,
+    summarizeHttpFailures,
     parseSocketEvent,
     serializeError,
     summarizeSamples,
diff --git a/e2e/artillery/observation.test.js b/e2e/artillery/observation.test.js
index 0d0dfa31..b0fc90d3 100644
--- a/e2e/artillery/observation.test.js
+++ b/e2e/artillery/observation.test.js
@@ -14,6 +14,9 @@ const {
     serializeError,
     summarizeSamples,
     classifyRoomCreateFailure,
+    classifyHttpOutcome,
+    summarizeHttpFailures,
+    enrichHttpSample,
 } = require('./observation');
 
 test('normalizes dynamic API identifiers without changing static routes', () => {
@@ -50,6 +53,45 @@ test('summarizes and ranks samples by cumulative duration', () => {
     });
 });
 
+test('classifies only the failed-login 401 as an expected HTTP failure', () => {
+    assert.equal(classifyHttpOutcome({
+        action: 'failed_login', method: 'POST', normalizedPath: '/api/auth/login', status: 401,
+    }), 'expected_failure');
+    assert.equal(classifyHttpOutcome({
+        action: 'room_list_display', method: 'GET', normalizedPath: '/api/rooms', status: 401,
+    }), 'unexpected_failure');
+});
+
+test('backfills outcome fields when reading a legacy HTTP sample', () => {
+    assert.deepEqual(enrichHttpSample({
+        name: 'POST /api/auth/login', action: 'failed_login', status: 401, success: false,
+    }), {
+        name: 'POST /api/auth/login', action: 'failed_login', status: 401,
+        method: 'POST', normalizedPath: '/api/auth/login', outcome: 'expected_failure', success: true,
+    });
+});
+
+test('groups HTTP failures by status, method, path, and action with evidence URLs', () => {
+    const result = summarizeHttpFailures([
+        {
+            status: 404, method: 'GET', normalizedPath: '/api/files/view/{filename}',
+            action: 'file_upload', outcome: 'unexpected_failure',
+            url: 'https://chat.example.com/api/files/view/a.jpg', pageUrl: 'https://chat.example.com/chat/1',
+        },
+        {
+            status: 404, method: 'GET', normalizedPath: '/api/files/view/{filename}',
+            action: 'file_upload', outcome: 'unexpected_failure',
+            url: 'https://chat.example.com/api/files/view/b.jpg', pageUrl: 'https://chat.example.com/chat/2',
+        },
+    ]);
+    assert.equal(result[0].count, 2);
+    assert.equal(result[0].path, '/api/files/view/{filename}');
+    assert.deepEqual(result[0].urls, [
+        'https://chat.example.com/api/files/view/a.jpg',
+        'https://chat.example.com/api/files/view/b.jpg',
+    ]);
+});
+
 test('records run identity and workload metadata from the execution environment', () => {
     const metadata = createRunMetadata({
         BASE_URL: 'https://chat.example.com',
@@ -74,6 +116,7 @@ test('records run identity and workload metadata from the execution environment'
         loadGenerator: 'sha256:load',
     });
     assert.deepEqual(metadata.workload, {
+        profile: null,
         durationSeconds: 60,
         arrivalCount: 500,
         virtualUsersPerPod: 4,
diff --git a/e2e/artillery/profile-scenarios.js b/e2e/artillery/profile-scenarios.js
new file mode 100644
index 00000000..f294e53a
--- /dev/null
+++ b/e2e/artillery/profile-scenarios.js
@@ -0,0 +1,53 @@
+const { failedLoginScenario, loginScenario } = require('./scenarios/auth.scenario.js');
+const {
+    chatRoomCreationScenario,
+    randomRoomJoinScenario,
+    massMessageScenario,
+    fileUploadScenario,
+    forbiddenWordScenario,
+} = require('./scenarios/chat.scenario.js');
+const { fullProfileUpdateScenario } = require('./scenarios/profile.scenario.js');
+const { createObservation } = require('./observation.js');
+const { randomUUID } = require('crypto');
+
+function generateUserSchema() {
+    const id = randomUUID();
+    return {
+        email: `loadtest_${id}@example.com`,
+        password: 'Password123!',
+        passwordConfirm: 'Password123!',
+        name: `Load Test User ${id.slice(0, 8)}`,
+    };
+}
+
+async function runProfile(page, vuContext, profileName, scenarios) {
+    vuContext.vars.testUser = generateUserSchema();
+    vuContext.vars.loadProfile = profileName;
+    vuContext.vars.observation = createObservation(page, vuContext);
+    try {
+        for (const scenario of scenarios) {
+            try {
+                await scenario(page, vuContext);
+            } catch (error) {
+                error.message = `[${scenario.name}] ${error.message}`;
+                throw error;
+            }
+        }
+    } finally {
+        await vuContext.vars.observation.finish();
+    }
+}
+
+const authNegativeProfile = (page, vuContext) =>
+    runProfile(page, vuContext, 'auth-negative', [failedLoginScenario]);
+
+const roomLifecycleProfile = (page, vuContext) =>
+    runProfile(page, vuContext, 'room-lifecycle', [loginScenario, chatRoomCreationScenario, randomRoomJoinScenario]);
+
+const messagingProfile = (page, vuContext) =>
+    runProfile(page, vuContext, 'messaging', [loginScenario, chatRoomCreationScenario, massMessageScenario, forbiddenWordScenario]);
+
+const mediaProfile = (page, vuContext) =>
+    runProfile(page, vuContext, 'media-profile', [loginScenario, chatRoomCreationScenario, fileUploadScenario, fullProfileUpdateScenario]);
+
+module.exports = { authNegativeProfile, roomLifecycleProfile, messagingProfile, mediaProfile };
diff --git a/e2e/artillery/report-observations.js b/e2e/artillery/report-observations.js
index 66a5717f..4367e224 100644
--- a/e2e/artillery/report-observations.js
+++ b/e2e/artillery/report-observations.js
@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 const fs = require('fs');
 const path = require('path');
-const { summarizeSamples } = require('./observation');
+const { enrichHttpSample, summarizeHttpFailures, summarizeSamples } = require('./observation');
 
 const runDirectory = path.resolve(process.argv[2] || 'artillery/results');
 const files = fs.readdirSync(runDirectory).filter((file) => file.startsWith('vu-') && file.endsWith('.json'));
@@ -18,17 +18,35 @@ for (const report of reports) {
     samples.runtime.push(...(report.samples.runtime || []));
     samples.layoutShifts.push(...(report.samples.layoutShifts || []));
 }
+samples.http = samples.http.map(enrichHttpSample);
+const profiles = [...new Set(reports.map((report) => report.metadata?.workload?.profile).filter(Boolean))];
+const expected401Samples = samples.http.filter((sample) => sample.outcome === 'expected_failure' && sample.status === 401);
+const unexpected401Samples = samples.http.filter((sample) => sample.outcome === 'unexpected_failure' && sample.status === 401);
 
 const aggregate = {
     schemaVersion: 1,
     runId: reports[0].runId,
     generatedAt: new Date().toISOString(),
     virtualUsers: reports.length,
+    httpExpectations: {
+        profiles,
+        expected401: expected401Samples.length,
+        unexpected401: unexpected401Samples.length,
+        expected401PerVuSatisfied: profiles.length === 1 && profiles[0] === 'auth-negative'
+            ? reports.every((report) => report.samples.http.map(enrichHttpSample)
+                .filter((sample) => sample.outcome === 'expected_failure' && sample.status === 401).length === 1)
+            : expected401Samples.length === 0,
+    },
     summary: {
         actions: summarizeSamples(samples.actions),
         documents: summarizeSamples(samples.documents),
         resources: summarizeSamples(samples.resources),
         http: summarizeSamples(samples.http),
+        httpFailures: summarizeHttpFailures(samples.http),
+        expectedHttpFailures: summarizeHttpFailures(samples.http.filter((sample) =>
+            sample.outcome === 'expected_failure')),
+        unexpectedHttpFailures: summarizeHttpFailures(samples.http.filter((sample) =>
+            sample.outcome === 'unexpected_failure')),
         socket: summarizeSamples(samples.socket),
         runtime: summarizeSamples(samples.runtime),
         cls: {
diff --git a/e2e/artillery/scenarios/auth.scenario.js b/e2e/artillery/scenarios/auth.scenario.js
index e7a70bfd..7b42c822 100644
--- a/e2e/artillery/scenarios/auth.scenario.js
+++ b/e2e/artillery/scenarios/auth.scenario.js
@@ -1,5 +1,6 @@
 const { loginAction, openLoginAction, registerAction, logoutAction } = require('../../actions/auth.actions');
 const { expect } = require('@playwright/test');
+const { waitForRoomListReady } = require('../../actions/chat.actions');
 
 const BASE_URL = process.env.BASE_URL || 'http://localhost:3000';
 
@@ -27,10 +28,7 @@ async function loginScenario(page, vuContext) {
 
         // URL 만으로는 완주를 판정할 수 없다. 방 목록이 실제로 그려져야
         // 로그인이 쓸모 있는 상태로 끝난 것이다(목록이 비어 있는 경우 포함).
-        await observe('room_list_display', () => expect(
-                page.getByTestId('join-chat-room-button').first()
-                    .or(page.getByTestId('rooms-empty'))
-            ).toBeVisible());
+        await observe('room_list_display', () => waitForRoomListReady(page));
 
         // 컨텍스트에 사용자 정보 저장 (다른 시나리오에서 사용 가능)
         vuContext.vars.testUser = testUser;
@@ -54,8 +52,8 @@ async function failedLoginScenario(page, vuContext) {
         }, false, false));
 
         await page.waitForTimeout(500);
-        const errorElement = page.getByTestId('login-error-message');
-        await expect(errorElement).toBeVisible();
+        await observe('failed_login_error_display', () =>
+            expect(page.getByTestId('login-error-message')).toBeVisible());
     } catch (error) {
         console.error('Failed login scenario failed:', error.message);
         throw error;
diff --git a/e2e/artillery/scenarios/chat.scenario.js b/e2e/artillery/scenarios/chat.scenario.js
index d8021931..a0c17a93 100644
--- a/e2e/artillery/scenarios/chat.scenario.js
+++ b/e2e/artillery/scenarios/chat.scenario.js
@@ -4,6 +4,7 @@ const {
     sendMessageAction,
     sendMultipleMessagesAction,
     uploadFileAction,
+    waitForChatRoomReady,
 } = require('../../actions/chat.actions');
 const { bannedWordSafeText } = require('../../utils/bannedWordSafeText');
 const { expect } = require('@playwright/test');
@@ -58,12 +59,7 @@ async function chatRoomCreationScenario(page, vuContext) {
 async function massMessageScenario(page, vuContext) {
     const observe = vuContext.vars.observation.action;
     try {
-        // 1. 랜덤 채팅방 입장
-        await observe('room_join', async () => {
-            await joinRandomChatRoomAction(page);
-            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
-            await expect(page.getByTestId('chat-message-input')).toBeVisible();
-        });
+        await waitForChatRoomReady(page);
 
         // 2. 여러 메시지 연속 전송 (10개)
         console.log(`Sending ${MASS_MESSAGE_COUNT} messages...`);
@@ -83,12 +79,7 @@ async function massMessageScenario(page, vuContext) {
 async function fileUploadScenario(page, vuContext) {
     const observe = vuContext.vars.observation.action;
     try {
-        // 1. 랜덤 채팅방 입장
-        await observe('room_join_for_file', async () => {
-            await joinRandomChatRoomAction(page);
-            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
-            await expect(page.getByTestId('chat-message-input')).toBeVisible();
-        });
+        await waitForChatRoomReady(page);
 
         // 2. 이미지 파일 업로드
         const filePath = path.resolve(__dirname, '../../fixtures/images/profile.jpg');
@@ -126,12 +117,7 @@ async function forbiddenWordScenario(page, vuContext) {
         : ['b3sig78jv', '9c0hej6x', 'lbl276sz'];
 
     try {
-        // 1. 랜덤 채팅방 입장
-        await observe('room_join_for_forbidden_word', async () => {
-            await joinRandomChatRoomAction(page);
-            await expect(page).toHaveURL(new RegExp(`${BASE_URL}/chat/\\w+`));
-            await expect(page.getByTestId('chat-message-input')).toBeVisible();
-        });
+        await waitForChatRoomReady(page);
 
         // 2. 금칙어 메시지 전송 시도
         const forbiddenWord = FORBIDDEN_WORDS[Math.floor(Math.random() * FORBIDDEN_WORDS.length)];
@@ -150,10 +136,16 @@ async function forbiddenWordScenario(page, vuContext) {
     }
 }
 
+async function randomRoomJoinScenario(page, vuContext) {
+    const observe = vuContext.vars.observation.action;
+    await observe('room_join', () => joinRandomChatRoomAction(page));
+}
+
 module.exports = {
     gotoChatPage,
     chatRoomCreationScenario,
     massMessageScenario,
     fileUploadScenario,
     forbiddenWordScenario,
+    randomRoomJoinScenario,
 };