Notify callers when watched children close and cap responses (#3192)

Wake caller agents when watched children close and cap embedded child responses at 4,000 characters.

Co-authored-by: Hanjun(Tony) <54878785+wilgon456@users.noreply.github.com>
Co-authored-by: Thong Van <minhthong@gmail.com>

packages/server/src/server/agent/agent-prompt.test.ts

@@ -52,6 +52,7 @@ interface FinishNotificationScenario {
   resolveChildPermissionWhileIdle(requestId?: string): void;
   finishChild(): void;
   finishChildAndReadParentPrompt(): Promise<string>;
+  closeChildAndReadParentPrompt(): Promise<string>;
   parentPrompts(): string[];
   wasParentPrompted(): boolean;
 }
@@ -211,6 +212,25 @@ function createFinishNotificationScenario(
 
       return parentPrompt;
     },
+    async closeChildAndReadParentPrompt() {
+      const parentPrompt = new Promise<string>((resolve) => {
+        resolveParentPrompt = resolve;
+      });
+
+      childAgent.lifecycle = "running";
+      subscriber?.({
+        type: "agent_state",
+        agent: childAgent,
+      });
+
+      childAgent.lifecycle = "closed";
+      subscriber?.({
+        type: "agent_state",
+        agent: childAgent,
+      });
+
+      return parentPrompt;
+    },
     parentPrompts() {
       return parentPrompts;
     },
@@ -279,6 +299,34 @@ test("finish notifications tell the parent the child's last assistant message",
   );
 });
 
+test("finish notifications truncate oversized child responses", async () => {
+  const included = "x".repeat(4000);
+  const omitted = "TAIL-MARKER".repeat(50);
+  const scenario = createFinishNotificationScenario({
+    childLastAssistantMessage: included + omitted,
+  });
+
+  scenario.startWatchingChild();
+  const parentPrompt = await scenario.finishChildAndReadParentPrompt();
+
+  expect(parentPrompt).toContain(included);
+  expect(parentPrompt).toContain(
+    `[truncated ${omitted.length} chars; use get_agent_activity for the full response]`,
+  );
+  expect(parentPrompt).not.toContain("TAIL-MARKER");
+});
+
+test("closing a watched child notifies the caller", async () => {
+  const scenario = createFinishNotificationScenario();
+
+  scenario.startWatchingChild();
+  const parentPrompt = await scenario.closeChildAndReadParentPrompt();
+
+  expect(parentPrompt).toEqual(
+    formatSystemNotificationPrompt("Agent child-agent (Child Agent) was closed."),
+  );
+});
+
 test("finish notifications survive permission responses", async () => {
   const scenario = createFinishNotificationScenario();
 

packages/server/src/server/agent/agent-prompt.ts

@@ -253,10 +253,14 @@ export interface SetupFinishNotificationParams {
   logger: Logger;
 }
 
+type FinishNotificationReason = "finished" | "errored" | "needs permission" | "was closed";
+
+const FINISH_NOTIFICATION_MESSAGE_LIMIT = 4000;
+
 interface FinishNotificationBodyInput {
   childAgentId: string;
   title: string;
-  reason: "finished" | "errored" | "needs permission";
+  reason: FinishNotificationReason;
   lastAssistantMessage: string | null;
   permissionRequest?: AgentPermissionRequest;
 }
@@ -278,8 +282,12 @@ function formatFinishNotificationBody(params: FinishNotificationBodyInput): stri
       )}\n</permission-request>`,
     );
   }
-  const lastAssistantMessage = params.lastAssistantMessage?.trim();
+  let lastAssistantMessage = params.lastAssistantMessage?.trim();
   if (lastAssistantMessage) {
+    if (lastAssistantMessage.length > FINISH_NOTIFICATION_MESSAGE_LIMIT) {
+      const omitted = lastAssistantMessage.length - FINISH_NOTIFICATION_MESSAGE_LIMIT;
+      lastAssistantMessage = `${lastAssistantMessage.slice(0, FINISH_NOTIFICATION_MESSAGE_LIMIT)}\n[truncated ${omitted} chars; use get_agent_activity for the full response]`;
+    }
     sections.push(`<agent-response>\n${lastAssistantMessage}\n</agent-response>`);
   }
   return sections.join("\n\n");
@@ -312,7 +320,7 @@ export function setupFinishNotification(params: SetupFinishNotificationParams):
   }
 
   async function notify(
-    reason: "finished" | "errored" | "needs permission",
+    reason: FinishNotificationReason,
     permissionRequest?: AgentPermissionRequest,
   ): Promise<void> {
     const callerRecord = await agentStorage.get(callerAgentId);
@@ -344,10 +352,7 @@ export function setupFinishNotification(params: SetupFinishNotificationParams):
     });
   }
 
-  function notifySafely(
-    reason: "finished" | "errored" | "needs permission",
-    options: NotifySafelyOptions = {},
-  ): void {
+  function notifySafely(reason: FinishNotificationReason, options: NotifySafelyOptions = {}): void {
     if (stopped) return;
     if (options.terminal ?? true) stop();
     notificationQueue = notificationQueue
@@ -387,7 +392,7 @@ export function setupFinishNotification(params: SetupFinishNotificationParams):
           return;
         }
         if (event.agent.lifecycle === "closed") {
-          stop();
+          notifySafely("was closed");
           return;
         }
         return;