diff --git a/extensions/copilot/src/event-bridge.test.ts b/extensions/copilot/src/event-bridge.test.ts
index f205f8e81cbb..38e48907ec70 100644
--- a/extensions/copilot/src/event-bridge.test.ts
+++ b/extensions/copilot/src/event-bridge.test.ts
@@ -678,6 +678,26 @@ describe("attachEventBridge", () => {
     expect(bridge.hasObservedSessionIdle()).toBe(true);
   });
 
+  it("ignores subagent idle events while waiting for the root session", async () => {
+    const session = createFakeSession();
+    const bridge = attachEventBridge(session, {
+      getSdkSessionId: () => "sdk-session-id",
+      isAborted: () => false,
+    });
+
+    const idle = bridge.awaitSessionIdle();
+    session.emit("session.idle", {
+      ...makeEvent("session.idle", {}),
+      agentId: "subagent-1",
+    });
+    await flushAsync();
+    expect(bridge.hasObservedSessionIdle()).toBe(false);
+
+    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 d4610e591796..22ffb44256db 100644
--- a/extensions/copilot/src/event-bridge.ts
+++ b/extensions/copilot/src/event-bridge.ts
@@ -224,7 +224,10 @@ export function attachEventBridge(
     }
   });
 
-  registerListener(session, unsubscribeFns, "session.idle", () => {
+  registerListener(session, unsubscribeFns, "session.idle", (event) => {
+    if (!isRootCompactionEvent(event)) {
+      return;
+    }
     observedSessionIdle = true;
     resolveSessionIdle?.();
     resolveSessionIdle = undefined;