fix(copilot): isolate root compaction events

extensions/copilot/src/event-bridge.test.ts

@@ -662,6 +662,33 @@ describe("attachEventBridge", () => {
     expect(bridge.isCompacting()).toBe(false);
   });
 
+  it("ignores subagent compaction events when tracking the root session", async () => {
+    const session = createFakeSession();
+    const onCompactionStart = vi.fn();
+    const onCompactionComplete = vi.fn();
+    const bridge = attachEventBridge(session, {
+      getSdkSessionId: () => "sdk-session-id",
+      isAborted: () => false,
+      onCompactionStart,
+      onCompactionComplete,
+    });
+
+    session.emit("session.compaction_start", {
+      ...makeEvent("session.compaction_start", {}),
+      agentId: "subagent-1",
+    });
+    session.emit("session.compaction_complete", {
+      ...makeEvent("session.compaction_complete", { success: true }),
+      agentId: "subagent-1",
+    });
+    await bridge.awaitCompactionCompletion();
+
+    expect(bridge.hasObservedCompaction()).toBe(false);
+    expect(bridge.isCompacting()).toBe(false);
+    expect(onCompactionStart).not.toHaveBeenCalled();
+    expect(onCompactionComplete).not.toHaveBeenCalled();
+  });
+
   it("session.error populates streamError with errorCode or errorType only when not aborted", () => {
     const activeSession = createFakeSession();
     const activeBridge = attachEventBridge(activeSession, {

extensions/copilot/src/event-bridge.ts

@@ -183,7 +183,10 @@ export function attachEventBridge(
     }
   });
 
-  registerListener(session, unsubscribeFns, "session.compaction_start", () => {
+  registerListener(session, unsubscribeFns, "session.compaction_start", (event) => {
+    if (!isRootCompactionEvent(event)) {
+      return;
+    }
     observedCompaction = true;
     if (activeCompactionCount === 0) {
       compactionIdle = new Promise<void>((resolve) => {
@@ -195,6 +198,9 @@ export function attachEventBridge(
   });
 
   registerListener(session, unsubscribeFns, "session.compaction_complete", (event) => {
+    if (!isRootCompactionEvent(event)) {
+      return;
+    }
     activeCompactionCount = Math.max(0, activeCompactionCount - 1);
     enqueueCompactionCallback(() =>
       options.onCompactionComplete?.({
@@ -410,6 +416,12 @@ function isAssistantMessageEvent(
   return event?.type === "assistant.message";
 }
 
+function isRootCompactionEvent(event: { agentId?: string }): boolean {
+  // SDK session events include subagent compaction; only root compaction
+  // affects the pooled root session's cleanup and reuse lifecycle.
+  return event.agentId === undefined;
+}
+
 function joinReasoning(order: string[], reasoningById: Map<string, string>): string {
   return order.map((reasoningId) => reasoningById.get(reasoningId) ?? "").join("");
 }