diff --git a/src/gateway/storage/layout.test.ts b/src/gateway/storage/layout.test.ts
index 719a0d5b..6adbb07c 100644
--- a/src/gateway/storage/layout.test.ts
+++ b/src/gateway/storage/layout.test.ts
@@ -54,6 +54,18 @@ describe("encodeStorageKey / decodeStorageKey", () => {
   it("trims whitespace before encoding", () => {
     expect(encodeStorageKey("  key  ")).toBe(encodeStorageKey("key"));
   });
+
+  it.each([".", ".."])("encodes the reserved dot segment %s", (key) => {
+    const encoded = encodeStorageKey(key);
+    expect(encoded).not.toBe(key);
+    expect(decodeStorageKey(encoded)).toBe(key);
+    expect(encoded).not.toBe(encodeStorageKey(encoded));
+  });
+
+  it("preserves existing encodings for ordinary IDs containing dots", () => {
+    expect(encodeStorageKey("release.v1")).toBe("release.v1");
+    expect(encodeStorageKey(".hidden")).toBe(".hidden");
+  });
 });
 
 describe("resolveStorageLayout", () => {
@@ -81,6 +93,23 @@ describe("getSessionDirectory", () => {
     expect(dir).toContain("sessions");
     expect(dir).toContain("my-session");
   });
+
+  it.each([".", "..", " . ", " .. ", "../other", "a/b", "a\\b", "%2E%2E"])(
+    "keeps %s in its own direct child directory",
+    (sessionId) => {
+      const layout = createLayout("/root");
+      const dir = getSessionDirectory(layout, sessionId);
+      expect(path.dirname(dir)).toBe(getSessionsRootDirectory(layout));
+      expect(dir).not.toBe(getSessionsRootDirectory(layout));
+      expect(dir).not.toBe(layout.rootDir);
+    },
+  );
+
+  it.each(["", "   "])("rejects an empty storage key %j", (sessionId) => {
+    expect(() => getSessionDirectory(createLayout("/root"), sessionId)).toThrow(
+      "Session id must not be empty",
+    );
+  });
 });
 
 describe("getSessionEventsFilePath", () => {
diff --git a/src/gateway/storage/layout.ts b/src/gateway/storage/layout.ts
index cbf84b84..d11d24d6 100644
--- a/src/gateway/storage/layout.ts
+++ b/src/gateway/storage/layout.ts
@@ -20,7 +20,12 @@ export interface StepCliResolvedStorageLayout {
 }
 
 export function encodeStorageKey(value: string): string {
-  return encodeURIComponent(value.trim());
+  const encoded = encodeURIComponent(value.trim());
+  // encodeURIComponent leaves dot segments unchanged, but path.join treats
+  // them as the current/parent directory instead of a session's own name.
+  return encoded === "." || encoded === ".."
+    ? encoded.replaceAll(".", "%2E")
+    : encoded;
 }
 
 export function decodeStorageKey(value: string): string {
@@ -41,10 +46,11 @@ export function getSessionDirectory(
   storage: StepCliResolvedStorageLayout,
   sessionId: string,
 ): string {
-  return path.join(
-    getSessionsRootDirectory(storage),
-    encodeStorageKey(sessionId),
-  );
+  const key = encodeStorageKey(sessionId);
+  if (!key) {
+    throw new Error("Session id must not be empty");
+  }
+  return path.join(getSessionsRootDirectory(storage), key);
 }
 
 export function getSessionEventsFilePath(
diff --git a/tests/integration/session-purge.test.ts b/tests/integration/session-purge.test.ts
new file mode 100644
index 00000000..475378f3
--- /dev/null
+++ b/tests/integration/session-purge.test.ts
@@ -0,0 +1,128 @@
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import fs from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import {
+  BUILTIN_CLI_DEFAULTS,
+  BUILTIN_STORAGE_LAYOUT_DEFAULTS,
+} from "../../src/bootstrap/config/defaults.js";
+import { StepCliSessionService } from "../../src/gateway/service/session-service.js";
+import {
+  startStepCliHttpServer,
+  type StepCliHttpServerHandle,
+} from "../../src/gateway/service/http-server.js";
+import {
+  getSessionDirectory,
+  resolveStorageLayout,
+  type StepCliResolvedStorageLayout,
+} from "../../src/gateway/storage/layout.js";
+import type { StepCliConfig } from "../../src/gateway/runtime.js";
+
+describe("HTTP session purge storage isolation", () => {
+  let tempDir: string;
+  let layout: StepCliResolvedStorageLayout;
+  let server: StepCliHttpServerHandle | undefined;
+  let sessions: StepCliSessionService;
+  let rootSentinel: string;
+  let sessionSentinel: string;
+
+  beforeEach(async () => {
+    tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "step-session-purge-"));
+    layout = resolveStorageLayout(
+      path.join(tempDir, "storage"),
+      BUILTIN_STORAGE_LAYOUT_DEFAULTS,
+    );
+    rootSentinel = path.join(layout.rootDir, "keep-root.txt");
+    sessionSentinel = path.join(
+      getSessionDirectory(layout, "innocent"),
+      "keep-session.txt",
+    );
+    await fs.mkdir(path.dirname(sessionSentinel), { recursive: true });
+    await fs.writeFile(rootSentinel, "root data");
+    await fs.writeFile(sessionSentinel, "unrelated session data");
+
+    const config: StepCliConfig = {
+      ...BUILTIN_CLI_DEFAULTS,
+      model: "test-model",
+      baseUrl: "https://example.invalid/v1",
+      apiKey: "",
+      workspaceRoot: tempDir,
+      storageRootDir: layout.rootDir,
+      storageLayout: layout,
+      skillsDirectoryName: "skills",
+      maxToolResultCharsInContext: 10_000,
+      interactionProfile: { surface: "service", canAskUser: false },
+      resumeSession: false,
+      autoSaveSession: false,
+      sessionTraceHeaderInjectionBaseUrls: [],
+      useAlternateScreen: false,
+      verbose: false,
+    };
+    sessions = new StepCliSessionService(config, {
+      storageRootDir: layout.rootDir,
+    });
+    await sessions.waitUntilReady();
+    server = await startStepCliHttpServer({
+      host: "127.0.0.1",
+      port: 0,
+      token: "test-service-token",
+      sessions,
+    });
+  });
+
+  afterEach(async () => {
+    if (server) {
+      await server.close();
+      server = undefined;
+    } else {
+      await sessions?.close();
+    }
+    await fs.rm(tempDir, { recursive: true, force: true });
+  });
+
+  async function purge(sessionId: string): Promise<unknown> {
+    // Padding keeps URL dot-segment normalization from hiding the bug before
+    // the service decodes and trims the ID.
+    const encodedId = encodeURIComponent(` ${sessionId} `);
+    const response = await fetch(
+      `${server!.origin}/v1/sessions/${encodedId}?purge=true`,
+      {
+        method: "DELETE",
+        headers: { authorization: "Bearer test-service-token" },
+      },
+    );
+    expect(response.status).toBe(200);
+    return response.json();
+  }
+
+  async function expectOtherDataIntact(): Promise<void> {
+    expect(await fs.readFile(rootSentinel, "utf8")).toBe("root data");
+    expect(await fs.readFile(sessionSentinel, "utf8")).toBe(
+      "unrelated session data",
+    );
+  }
+
+  it.each([".", ".."])(
+    "purging unknown %s preserves other data",
+    async (id) => {
+      await purge(id);
+      await expectOtherDataIntact();
+    },
+  );
+
+  it.each([".", "..", "normal-session"])(
+    "purges only the directory belonging to %s",
+    async (id) => {
+      const target = getSessionDirectory(layout, id);
+      await fs.mkdir(target, { recursive: true });
+      await fs.writeFile(path.join(target, "delete-me.txt"), "target data");
+      expect(await purge(id)).toEqual({
+        ok: true,
+        existed: false,
+        purged: true,
+      });
+      await expect(fs.access(target)).rejects.toMatchObject({ code: "ENOENT" });
+      await expectOtherDataIntact();
+    },
+  );
+});