diff --git a/extensions/copilot/src/attempt.test.ts b/extensions/copilot/src/attempt.test.ts
index 29f591b0c981..ca954d350aa6 100644
--- a/extensions/copilot/src/attempt.test.ts
+++ b/extensions/copilot/src/attempt.test.ts
@@ -468,6 +468,7 @@ describe("runCopilotAttempt", () => {
 
     releaseBeforeCompaction.resolve();
     activeSession?.emit("session.compaction_complete", { success: true });
+    activeSession?.emit("session.idle", {});
     await vi.waitFor(() => {
       expect(activeSession?.disconnect).toHaveBeenCalledTimes(1);
     });
@@ -1814,6 +1815,10 @@ describe("runCopilotAttempt", () => {
       }),
       expect.anything(),
     );
+    sdk.sessions[0]?.emit("session.idle", {});
+    await vi.waitFor(() => {
+      expect(sdk.sessions[0]?.disconnect).toHaveBeenCalledTimes(1);
+    });
   });
 
   it("marks a timeout during active SDK compaction", async () => {
@@ -1837,6 +1842,7 @@ describe("runCopilotAttempt", () => {
     expect(sdk.sessions[0]?.disconnect).not.toHaveBeenCalled();
 
     sdk.sessions[0]?.emit("session.compaction_complete", { messagesRemoved: 3, success: true });
+    sdk.sessions[0]?.emit("session.idle", {});
     await vi.waitFor(() => {
       expect(sdk.sessions[0]?.disconnect).toHaveBeenCalledTimes(1);
     });
@@ -1848,12 +1854,54 @@ describe("runCopilotAttempt", () => {
     );
   });
 
+  it("retains a timed-out session until later compaction reaches session.idle", async () => {
+    const afterCompaction = vi.fn();
+    const onDeferredCompaction = vi.fn();
+    initializeGlobalHookRunner(
+      createMockPluginRegistry([{ hookName: "after_compaction", handler: afterCompaction }]),
+    );
+    let activeSession: FakeSession | undefined;
+    const sdk = makeFakeSdk({
+      onCreateSession: (session) => {
+        activeSession = session;
+        session.sendAndWait.mockRejectedValueOnce(
+          new Error("Timeout after 60000ms waiting for session.idle"),
+        );
+      },
+    });
+
+    const result = await runCopilotAttempt(makeParams(), {
+      onDeferredCompaction,
+      pool: makeFakePool(sdk),
+    });
+
+    expect(result.timedOut).toBe(true);
+    expect(result.timedOutDuringCompaction).toBe(false);
+    expect(onDeferredCompaction).toHaveBeenCalledWith(
+      expect.objectContaining({ sdkSessionId: "sess-1" }),
+    );
+    expect(activeSession?.disconnect).not.toHaveBeenCalled();
+
+    activeSession?.emit("session.compaction_start", {});
+    activeSession?.emit("session.compaction_complete", { messagesRemoved: 3, success: true });
+    await vi.waitFor(() => {
+      expect(afterCompaction).toHaveBeenCalledTimes(1);
+    });
+    expect(activeSession?.disconnect).not.toHaveBeenCalled();
+
+    activeSession?.emit("session.idle", {});
+    await vi.waitFor(() => {
+      expect(activeSession?.disconnect).toHaveBeenCalledTimes(1);
+    });
+  });
+
   it("does not mark a timeout after SDK compaction has completed as active compaction", async () => {
     const sdk = makeFakeSdk({
       onCreateSession: (session) => {
         session.sendAndWait.mockImplementationOnce(async () => {
           session.emit("session.compaction_start", {});
           session.emit("session.compaction_complete", { success: true });
+          session.emit("session.idle", {});
           return undefined;
         });
       },
@@ -1935,6 +1983,7 @@ describe("runCopilotAttempt", () => {
       expect(dualWriteMock.dualWriteCopilotTranscriptBestEffort).toHaveBeenCalledTimes(1);
     });
     sdk.sessions[0]?.emit("session.compaction_complete", { success: true });
+    sdk.sessions[0]?.emit("session.idle", {});
     mirror.resolve();
 
     const result = await attempt;
@@ -1978,6 +2027,10 @@ describe("runCopilotAttempt", () => {
     // replay-shim incorrectly treated the attempt as side-effect-safe.
     expect(result.replayMetadata?.hadPotentialSideEffects).toBe(true);
     expect(result.replayMetadata?.replaySafe).toBe(false);
+    sdk.sessions[0]?.emit("session.idle", {});
+    await vi.waitFor(() => {
+      expect(sdk.sessions[0]?.disconnect).toHaveBeenCalledTimes(1);
+    });
   });
 
   it("G1: SDK timeout flushes the in-flight delta chain before snapshot so assistant text is preserved", async () => {
@@ -2019,6 +2072,10 @@ describe("runCopilotAttempt", () => {
     expect(result.timedOut).toBe(true);
     expect(onAssistantDelta).toHaveBeenCalledTimes(1);
     expect(result.assistantTexts?.join("")).toContain("partial-");
+    session.emit("session.idle", {});
+    await vi.waitFor(() => {
+      expect(session.disconnect).toHaveBeenCalledTimes(1);
+    });
   });
 
   it("model translation: unsupported provider", async () => {
diff --git a/extensions/copilot/src/attempt.ts b/extensions/copilot/src/attempt.ts
index 714f32f42c15..fc1af802f341 100644
--- a/extensions/copilot/src/attempt.ts
+++ b/extensions/copilot/src/attempt.ts
@@ -186,34 +186,39 @@ async function finalizeCopilotAttempt(
   return result;
 }
 
-async function awaitCompactionCompletionOrAbort(
-  bridge: ReturnType<typeof attachEventBridge>,
-  abortSignal: AbortSignal | undefined,
-): Promise<"aborted" | "completed"> {
-  if (!abortSignal) {
-    await bridge.awaitCompactionCompletion();
+async function awaitDeferredCleanupCompletionOrAbort(params: {
+  abortSignal: AbortSignal | undefined;
+  awaitSessionIdle: boolean;
+  bridge: ReturnType<typeof attachEventBridge>;
+}): Promise<"aborted" | "completed"> {
+  const awaitCompletion = async () => {
+    if (params.awaitSessionIdle) {
+      await params.bridge.awaitSessionIdle();
+    }
+    await params.bridge.awaitCompactionCompletion();
+  };
+  if (!params.abortSignal) {
+    await awaitCompletion();
     return "completed";
   }
-  if (abortSignal.aborted) {
+  if (params.abortSignal.aborted) {
     return "aborted";
   }
   let resolveAbort: () => void = () => undefined;
   const aborted = new Promise<"aborted">((resolve) => {
     resolveAbort = () => resolve("aborted");
   });
-  abortSignal.addEventListener("abort", resolveAbort, { once: true });
+  params.abortSignal.addEventListener("abort", resolveAbort, { once: true });
   try {
-    return await Promise.race([
-      bridge.awaitCompactionCompletion().then(() => "completed" as const),
-      aborted,
-    ]);
+    return await Promise.race([awaitCompletion().then(() => "completed" as const), aborted]);
   } finally {
-    abortSignal.removeEventListener("abort", resolveAbort);
+    params.abortSignal.removeEventListener("abort", resolveAbort);
   }
 }
 
 function deferBackgroundCompactionCleanup(params: {
   abortSignal: AbortSignal | undefined;
+  awaitSessionIdle: boolean;
   bridge: ReturnType<typeof attachEventBridge>;
   handle: PooledClient;
   pool: CopilotClientPool;
@@ -226,8 +231,9 @@ function deferBackgroundCompactionCleanup(params: {
   return (async () => {
     let outcome: "aborted" | "completed" | "deadline" = "deadline";
     try {
-      outcome = await awaitCompactionCompletionBeforeDeadline({
+      outcome = await awaitDeferredCleanupBeforeDeadline({
         abortSignal: params.abortSignal,
+        awaitSessionIdle: params.awaitSessionIdle,
         bridge: params.bridge,
         timeoutMs: params.timeoutMs,
       });
@@ -284,8 +290,9 @@ async function cancelBackgroundCompactionBeforeTeardown(session: SessionLike): P
   }
 }
 
-async function awaitCompactionCompletionBeforeDeadline(params: {
+async function awaitDeferredCleanupBeforeDeadline(params: {
   abortSignal: AbortSignal | undefined;
+  awaitSessionIdle: boolean;
   bridge: ReturnType<typeof attachEventBridge>;
   timeoutMs: number;
 }): Promise<"aborted" | "completed" | "deadline"> {
@@ -294,10 +301,7 @@ async function awaitCompactionCompletionBeforeDeadline(params: {
     timeoutId = setTimeout(() => resolve("deadline"), params.timeoutMs);
   });
   try {
-    return await Promise.race([
-      awaitCompactionCompletionOrAbort(params.bridge, params.abortSignal),
-      deadline,
-    ]);
+    return await Promise.race([awaitDeferredCleanupCompletionOrAbort(params), deadline]);
   } finally {
     if (timeoutId !== undefined) {
       clearTimeout(timeoutId);
@@ -802,7 +806,9 @@ export async function runCopilotAttempt(
     }
   } finally {
     settled = true;
-    if (bridge?.hasObservedCompaction() && session && handle) {
+    const retainSessionForDeferredCleanup =
+      bridge?.hasObservedCompaction() || (timedOut && bridge?.hasObservedSessionIdle() === false);
+    if (retainSessionForDeferredCleanup && bridge && session && handle) {
       const cleanupAbort = new AbortController();
       const abortCleanup = () => cleanupAbort.abort();
       if (params.abortSignal?.aborted) {
@@ -812,6 +818,7 @@ export async function runCopilotAttempt(
       }
       const cleanup = deferBackgroundCompactionCleanup({
         abortSignal: cleanupAbort.signal,
+        awaitSessionIdle: !bridge.hasObservedSessionIdle(),
         bridge,
         handle,
         pool: deps.pool,
@@ -837,8 +844,9 @@ export async function runCopilotAttempt(
       }
       params.abortSignal?.removeEventListener("abort", onAbort);
     } else {
-      // `sendAndWait` resolves on `session.idle`, which the SDK defines as
-      // no background agents in flight. Only an observed compaction needs retention.
+      // A normal sendAndWait result has observed session.idle, which the SDK
+      // defines as no background agents in flight. Timeouts retain the bridge
+      // until that event so compaction that starts after the timer still completes.
       await bridge?.awaitCompactionChain();
       bridge?.detach();
       params.abortSignal?.removeEventListener("abort", onAbort);
diff --git a/extensions/copilot/src/event-bridge.test.ts b/extensions/copilot/src/event-bridge.test.ts
index 54b4f14f7714..f205f8e81cbb 100644
--- a/extensions/copilot/src/event-bridge.test.ts
+++ b/extensions/copilot/src/event-bridge.test.ts
@@ -17,6 +17,7 @@ const REGISTERED_EVENT_TYPES = [
   "tool.execution_complete",
   "session.compaction_start",
   "session.compaction_complete",
+  "session.idle",
   "session.error",
   "abort",
 ] as const;
@@ -662,6 +663,21 @@ describe("attachEventBridge", () => {
     expect(bridge.isCompacting()).toBe(false);
   });
 
+  it("waits for the SDK terminal idle event", async () => {
+    const session = createFakeSession();
+    const bridge = attachEventBridge(session, {
+      getSdkSessionId: () => "sdk-session-id",
+      isAborted: () => false,
+    });
+
+    const idle = bridge.awaitSessionIdle();
+    await flushAsync();
+    session.emit("session.idle", makeEvent("session.idle", {}));
+    await idle;
+
+    expect(bridge.hasObservedSessionIdle()).toBe(true);
+  });
+
   it("keeps compaction pending after an abort until the SDK reports completion", async () => {
     const session = createFakeSession();
     const bridge = attachEventBridge(session, {
diff --git a/extensions/copilot/src/event-bridge.ts b/extensions/copilot/src/event-bridge.ts
index fb5a20561129..d4610e591796 100644
--- a/extensions/copilot/src/event-bridge.ts
+++ b/extensions/copilot/src/event-bridge.ts
@@ -69,9 +69,11 @@ export interface EventBridgeController {
   recordSendResult(result: SessionEvent | undefined): boolean;
   awaitCompactionChain(): Promise<void>;
   awaitCompactionCompletion(): Promise<void>;
+  awaitSessionIdle(): Promise<void>;
   settleCompactionWait(): void;
   awaitDeltaChain(): Promise<void>;
   hasObservedCompaction(): boolean;
+  hasObservedSessionIdle(): boolean;
   isCompacting(): boolean;
   snapshot(): EventBridgeSnapshot;
   buildAssistantMessage(args: BuildAssistantMessageArgs): AssistantMessage | undefined;
@@ -104,6 +106,11 @@ export function attachEventBridge(
   let compactionChain = Promise.resolve();
   let compactionIdle = Promise.resolve();
   let resolveCompactionIdle: (() => void) | undefined;
+  let observedSessionIdle = false;
+  let resolveSessionIdle: (() => void) | undefined;
+  const sessionIdle = new Promise<void>((resolve) => {
+    resolveSessionIdle = resolve;
+  });
   let firstDeltaError: unknown;
   let detached = false;
   const unsubscribeFns: Array<() => void> = [];
@@ -217,6 +224,12 @@ export function attachEventBridge(
     }
   });
 
+  registerListener(session, unsubscribeFns, "session.idle", () => {
+    observedSessionIdle = true;
+    resolveSessionIdle?.();
+    resolveSessionIdle = undefined;
+  });
+
   registerListener(session, unsubscribeFns, "session.error", (event) => {
     if (!options.isAborted()) {
       streamError = createPromptError(
@@ -254,6 +267,9 @@ export function attachEventBridge(
       }
       await compactionChain;
     },
+    awaitSessionIdle() {
+      return observedSessionIdle ? Promise.resolve() : sessionIdle;
+    },
     settleCompactionWait() {
       activeCompactionCount = 0;
       resolveCompactionIdle?.();
@@ -265,6 +281,9 @@ export function attachEventBridge(
     hasObservedCompaction() {
       return observedCompaction;
     },
+    hasObservedSessionIdle() {
+      return observedSessionIdle;
+    },
     isCompacting() {
       return activeCompactionCount > 0;
     },