diff --git a/actions/setup/js/copilot_harness.cjs b/actions/setup/js/copilot_harness.cjs
index c4ef4aff81e..be007fc611a 100644
--- a/actions/setup/js/copilot_harness.cjs
+++ b/actions/setup/js/copilot_harness.cjs
@@ -76,6 +76,7 @@ const PROMPT_FILE_INLINE_THRESHOLD_LABEL = "100KB";
 const MAX_ENV_VAR_PREVIEW_LENGTH = 120;
 const OUTPUT_TAIL_MAX_CHARS = 600;
 const OUTPUT_TAIL_MAX_LINES = 12;
+const POST_RESULT_WATCHDOG_IDLE_TIMEOUT_MS = 20 * 1000;
 const COPILOT_REQUESTS_PROXY_AUTH_403_TEMPLATE_NAME = "copilot_requests_proxy_auth_403.md";
 // Pattern to detect transient CAPIError 400 in copilot output
 const CAPI_ERROR_400_PATTERN = /CAPIError:\s*400/;
@@ -146,6 +147,39 @@ function log(message) {
   process.stderr.write(`[copilot-harness] ${message}\n`);
 }
 
+const NON_TERMINAL_SAFE_OUTPUT_TYPES = new Set(["missing_tool", "missing_data", "report_incomplete"]);
+
+/**
+ * Detect whether safe-outputs already contain a terminal agent result.
+ * Terminal outputs include noop and any non-diagnostic task output types.
+ * @param {string} safeOutputsPath
+ * @returns {boolean}
+ */
+function hasTerminalSafeOutput(safeOutputsPath) {
+  if (!safeOutputsPath) return false;
+  let content = "";
+  try {
+    content = fs.readFileSync(safeOutputsPath, "utf8");
+  } catch {
+    return false;
+  }
+  for (const line of content.split("\n")) {
+    const trimmed = line.trim();
+    if (!trimmed) continue;
+    try {
+      const parsed = JSON.parse(trimmed);
+      const type = parsed && typeof parsed.type === "string" ? parsed.type : "";
+      if (!type) continue;
+      if (type === "noop" || !NON_TERMINAL_SAFE_OUTPUT_TYPES.has(type)) {
+        return true;
+      }
+    } catch {
+      // Ignore malformed lines.
+    }
+  }
+  return false;
+}
+
 /**
  * @param {NodeJS.ProcessEnv} [env]
  * @param {(message: string) => void} [logger]
@@ -911,7 +945,20 @@ async function main() {
         const safeArgs = currentArgs.map((arg, i) => (currentArgs[i - 1] === "--prompt" || currentArgs[i - 1] === "-p" ? "<redacted>" : arg));
         // Driver mode: run copilot_sdk_driver.cjs as a normal subprocess. The harness has
         // already started the sidecar; the driver only opens an SDK client connection.
-        const result = await runProcess({ command, args: currentArgs, attempt, log, logArgs: safeArgs, env: childEnv });
+        const result = await runProcess({
+          command,
+          args: currentArgs,
+          attempt,
+          log,
+          logArgs: safeArgs,
+          env: childEnv,
+          postResultWatchdog: safeOutputsPath
+            ? {
+                shouldArm: () => hasTerminalSafeOutput(safeOutputsPath),
+                inactivityTimeoutMs: POST_RESULT_WATCHDOG_IDLE_TIMEOUT_MS,
+              }
+            : undefined,
+        });
         lastExitCode = result.exitCode;
         const attemptDetections = detectCopilotErrors(result.output);
         detectedCopilotErrors.inferenceAccessError ||= attemptDetections.inferenceAccessError;
@@ -1219,6 +1266,7 @@ if (typeof module !== "undefined" && module.exports) {
     resolveRetryConfig,
     parseCopilotSDKServerArgsFromEnv,
     isCAPIQuotaExceededError,
+    hasTerminalSafeOutput,
   };
 }
 
diff --git a/actions/setup/js/copilot_harness.test.cjs b/actions/setup/js/copilot_harness.test.cjs
index 8def07a256b..739c9e358e2 100644
--- a/actions/setup/js/copilot_harness.test.cjs
+++ b/actions/setup/js/copilot_harness.test.cjs
@@ -28,6 +28,7 @@ const {
   extractDeniedCommands,
   hasNumerousPermissionDeniedIssues,
   hasNoopInSafeOutputs,
+  hasTerminalSafeOutput,
   hasExpectedSafeOutputs,
   INFERENCE_ACCESS_ERROR_PATTERN,
   AGENTIC_ENGINE_TIMEOUT_PATTERN,
@@ -229,6 +230,52 @@ describe("copilot_harness.cjs", () => {
     });
   });
 
+  describe("hasTerminalSafeOutput", () => {
+    it("returns true for noop entries", () => {
+      const tempDir = makeHarnessTempDir("terminal-safeoutput-noop-");
+      const filePath = path.join(tempDir, "safe-outputs.jsonl");
+      try {
+        fs.writeFileSync(filePath, JSON.stringify({ type: "noop", reason: "nothing to do" }) + "\n", "utf8");
+        expect(hasTerminalSafeOutput(filePath)).toBe(true);
+      } finally {
+        fs.rmSync(tempDir, { recursive: true, force: true });
+      }
+    });
+
+    it("returns true for non-diagnostic task outputs", () => {
+      const tempDir = makeHarnessTempDir("terminal-safeoutput-task-");
+      const filePath = path.join(tempDir, "safe-outputs.jsonl");
+      try {
+        fs.writeFileSync(filePath, JSON.stringify({ type: "comment_issue", body: "done" }) + "\n", "utf8");
+        expect(hasTerminalSafeOutput(filePath)).toBe(true);
+      } finally {
+        fs.rmSync(tempDir, { recursive: true, force: true });
+      }
+    });
+
+    it("returns false for diagnostic-only output", () => {
+      const tempDir = makeHarnessTempDir("terminal-safeoutput-diagnostic-");
+      const filePath = path.join(tempDir, "safe-outputs.jsonl");
+      try {
+        fs.writeFileSync(filePath, JSON.stringify({ type: "missing_tool", reason: "missing permission" }) + "\n", "utf8");
+        expect(hasTerminalSafeOutput(filePath)).toBe(false);
+      } finally {
+        fs.rmSync(tempDir, { recursive: true, force: true });
+      }
+    });
+
+    it("returns false for missing_data diagnostic output", () => {
+      const tempDir = makeHarnessTempDir("terminal-safeoutput-missing-data-");
+      const filePath = path.join(tempDir, "safe-outputs.jsonl");
+      try {
+        fs.writeFileSync(filePath, JSON.stringify({ type: "missing_data", reason: "metadata only" }) + "\n", "utf8");
+        expect(hasTerminalSafeOutput(filePath)).toBe(false);
+      } finally {
+        fs.rmSync(tempDir, { recursive: true, force: true });
+      }
+    });
+  });
+
   describe("retry policy: continue on partial execution", () => {
     // Inline the same retry-eligibility logic as the driver for unit testing.
     // The driver retries whenever the session produced output (hasOutput), regardless
diff --git a/actions/setup/js/process_runner.cjs b/actions/setup/js/process_runner.cjs
index 6505d6ec63e..74db7a29301 100644
--- a/actions/setup/js/process_runner.cjs
+++ b/actions/setup/js/process_runner.cjs
@@ -55,7 +55,13 @@ function sleep(ms) {
  *   attempt: number,
  *   log: (message: string) => void,
  *   logArgs?: string[],
- *   env?: NodeJS.ProcessEnv
+ *   env?: NodeJS.ProcessEnv,
+ *   postResultWatchdog?: {
+ *     shouldArm: () => boolean,
+ *     inactivityTimeoutMs: number,
+ *     pollIntervalMs?: number,
+ *     termGraceMs?: number
+ *   }
  * }} options
  *   - command   - The executable to run
  *   - args      - Arguments to pass to the command
@@ -65,7 +71,7 @@ function sleep(ms) {
  *                 Pass a redacted copy to avoid leaking sensitive values.
  * @returns {Promise<{exitCode: number, output: string, hasOutput: boolean, durationMs: number}>}
  */
-function runProcess({ command, args, attempt, log, logArgs, env }) {
+function runProcess({ command, args, attempt, log, logArgs, env, postResultWatchdog }) {
   return new Promise(resolve => {
     const startTime = Date.now();
     // Guard against the promise being settled more than once.  On some systems Node
@@ -76,6 +82,7 @@ function runProcess({ command, args, attempt, log, logArgs, env }) {
     function settle(result) {
       if (settled) return;
       settled = true;
+      if (postResultWatchdogTimer) clearInterval(postResultWatchdogTimer);
       resolve(result);
     }
 
@@ -94,6 +101,16 @@ function runProcess({ command, args, attempt, log, logArgs, env }) {
     let hasOutput = false;
     let stdoutBytes = 0;
     let stderrBytes = 0;
+    let lastActivityAt = Date.now();
+    let watchdogArmed = false;
+    let sentSigtermAt = 0;
+    let sentSigkillAt = 0;
+    const watchdogPollIntervalMs = Math.max(50, Number(postResultWatchdog?.pollIntervalMs) || 1000);
+    const watchdogTermGraceMs = Math.max(50, Number(postResultWatchdog?.termGraceMs) || 5000);
+    const rawInactivityTimeout = Number(postResultWatchdog?.inactivityTimeoutMs);
+    const watchdogInactivityTimeoutMs = Number.isFinite(rawInactivityTimeout) && rawInactivityTimeout > 0 ? Math.max(50, rawInactivityTimeout) : 0;
+    /** @type {NodeJS.Timeout | null} */
+    let postResultWatchdogTimer = null;
 
     child.stdout.on(
       "data",
@@ -101,6 +118,7 @@ function runProcess({ command, args, attempt, log, logArgs, env }) {
         hasOutput = true;
         stdoutBytes += data.length;
         collectedOutput += data.toString();
+        lastActivityAt = Date.now();
         process.stdout.write(data);
       }
     );
@@ -111,10 +129,41 @@ function runProcess({ command, args, attempt, log, logArgs, env }) {
         hasOutput = true;
         stderrBytes += data.length;
         collectedOutput += data.toString();
+        lastActivityAt = Date.now();
         process.stderr.write(data);
       }
     );
 
+    if (postResultWatchdog && watchdogInactivityTimeoutMs > 0) {
+      postResultWatchdogTimer = setInterval(() => {
+        if (settled) return;
+        if (!watchdogArmed) {
+          try {
+            watchdogArmed = postResultWatchdog.shouldArm();
+          } catch {
+            watchdogArmed = false;
+          }
+          if (watchdogArmed) {
+            lastActivityAt = Date.now();
+            log(`attempt ${attempt + 1}: post-result watchdog armed inactivityTimeout=${watchdogInactivityTimeoutMs}ms`);
+          }
+        }
+        if (!watchdogArmed) return;
+        const idleMs = Date.now() - lastActivityAt;
+        if (sentSigtermAt === 0 && idleMs >= watchdogInactivityTimeoutMs) {
+          sentSigtermAt = Date.now();
+          log(`attempt ${attempt + 1}: post-result watchdog terminating idle process after ${idleMs}ms (SIGTERM)`);
+          child.kill("SIGTERM");
+          return;
+        }
+        if (sentSigtermAt > 0 && sentSigkillAt === 0 && Date.now() - sentSigtermAt >= watchdogTermGraceMs) {
+          sentSigkillAt = Date.now();
+          log(`attempt ${attempt + 1}: post-result watchdog forcing process exit after ${watchdogTermGraceMs}ms grace (SIGKILL)`);
+          child.kill("SIGKILL");
+        }
+      }, watchdogPollIntervalMs);
+    }
+
     child.on("exit", (code, signal) => {
       log(`attempt ${attempt + 1}: process exit event` + ` exitCode=${code ?? 1}` + (signal ? ` signal=${signal}` : ""));
     });
diff --git a/actions/setup/js/process_runner.test.cjs b/actions/setup/js/process_runner.test.cjs
index af15b72efe1..95aef30d0c3 100644
--- a/actions/setup/js/process_runner.test.cjs
+++ b/actions/setup/js/process_runner.test.cjs
@@ -181,6 +181,60 @@ describe("process_runner.cjs", () => {
       expect(result.durationMs).toBeGreaterThanOrEqual(0);
     });
 
+    it("terminates a hung process after terminal-result inactivity", async () => {
+      const logs = [];
+      const result = await runProcess({
+        command: process.execPath,
+        args: ["-e", 'process.stdout.write("done"); setInterval(() => {}, 1000);'],
+        attempt: 0,
+        log: msg => logs.push(msg),
+        postResultWatchdog: {
+          shouldArm: () => true,
+          inactivityTimeoutMs: 100,
+          pollIntervalMs: 25,
+          termGraceMs: 200,
+        },
+      });
+      expect(result.exitCode).not.toBe(0);
+      expect(result.durationMs).toBeLessThan(5000);
+      expect(logs.some(line => line.includes("post-result watchdog armed"))).toBe(true);
+      expect(logs.some(line => line.includes("post-result watchdog terminating idle process"))).toBe(true);
+    });
+
+    it("does not terminate processes when watchdog is not armed", async () => {
+      const logs = [];
+      const result = await runProcess({
+        command: process.execPath,
+        args: ["-e", "setTimeout(() => process.exit(0), 250);"],
+        attempt: 0,
+        log: msg => logs.push(msg),
+        postResultWatchdog: {
+          shouldArm: () => false,
+          inactivityTimeoutMs: 50,
+          pollIntervalMs: 25,
+          termGraceMs: 100,
+        },
+      });
+      expect(result.exitCode).toBe(0);
+      expect(logs.some(line => line.includes("post-result watchdog terminating idle process"))).toBe(false);
+    });
+
+    it("does not enable watchdog when inactivityTimeoutMs is missing or invalid", async () => {
+      const logs = [];
+      const result = await runProcess({
+        command: process.execPath,
+        args: ["-e", "setTimeout(() => process.exit(0), 100);"],
+        attempt: 0,
+        log: msg => logs.push(msg),
+        postResultWatchdog: {
+          shouldArm: () => true,
+          // intentionally missing inactivityTimeoutMs
+        },
+      });
+      expect(result.exitCode).toBe(0);
+      expect(logs.some(line => line.includes("post-result watchdog armed"))).toBe(false);
+    });
+
     it("truncates logArgs to 200 chars in spawn log", async () => {
       const logs = [];
       const longArg = "x".repeat(300);