feat(subagent): update announce flow for group continuation

When all runs in a group complete, deliver combined findings plus the
`next` continuation prompt to the parent agent via writeInternal. The
parent can then act on the collected data (summarize, write files, etc).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/agent/subagent/announce.test.ts

@@ -188,7 +188,7 @@ describe("formatCoalescedAnnouncementMessage", () => {
 
     const msg = formatCoalescedAnnouncementMessage(records);
 
-    expect(msg).toContain("All 2 background tasks have completed");
+    expect(msg).toContain("All 2 background task(s) have completed");
     expect(msg).toContain('Task 1: "Task A"');
     expect(msg).toContain("Found issue A");
     expect(msg).toContain('Task 2: "Task B"');
@@ -251,4 +251,44 @@ describe("formatCoalescedAnnouncementMessage", () => {
     expect(msg).toContain("上海：多云，9°C");
     expect(msg).toContain("MUST include findings from every task item above");
   });
+
+  it("includes continuation prompt when next is provided", () => {
+    const records = [
+      makeRecord({ runId: "run-1", label: "AAPL data", findings: "AAPL revenue: $100B" }),
+      makeRecord({ runId: "run-2", label: "MSFT data", findings: "MSFT revenue: $200B" }),
+    ];
+
+    const msg = formatCoalescedAnnouncementMessage(records, "Summarize all data and write a PDF investment report");
+
+    expect(msg).toContain("CONTINUATION TASK");
+    expect(msg).toContain("Summarize all data and write a PDF investment report");
+    expect(msg).toContain("AAPL revenue: $100B");
+    expect(msg).toContain("MSFT revenue: $200B");
+    // Should NOT contain the default summarize instruction
+    expect(msg).not.toContain("Summarize these results naturally for the user");
+  });
+
+  it("uses continuation prompt even for single record when next is provided", () => {
+    const records = [
+      makeRecord({ runId: "run-1", label: "Data collection", findings: "All data collected" }),
+    ];
+
+    const msg = formatCoalescedAnnouncementMessage(records, "Generate the final report");
+
+    expect(msg).toContain("CONTINUATION TASK");
+    expect(msg).toContain("Generate the final report");
+    expect(msg).toContain("All data collected");
+  });
+
+  it("uses default summarize instruction when next is not provided", () => {
+    const records = [
+      makeRecord({ runId: "run-1" }),
+      makeRecord({ runId: "run-2" }),
+    ];
+
+    const msg = formatCoalescedAnnouncementMessage(records);
+
+    expect(msg).not.toContain("CONTINUATION TASK");
+    expect(msg).toContain("Summarize these results naturally for the user");
+  });
 });

packages/core/src/agent/subagent/announce.ts

@@ -193,12 +193,17 @@ export function formatAnnouncementMessage(params: FormatAnnouncementParams): str
 /**
  * Format a coalesced announcement message from multiple completed subagent runs.
  * When only one record is provided, delegates to formatAnnouncementMessage.
+ *
+ * @param next — Optional continuation prompt from a SubagentGroup. When present,
+ *   the parent agent is instructed to execute the continuation using the combined
+ *   findings, rather than just summarizing.
  */
 export function formatCoalescedAnnouncementMessage(
   records: SubagentRunRecord[],
+  next?: string,
 ): string {
-  // Single record: delegate to existing format for backward-compatible behavior
-  if (records.length === 1) {
+  // Single record without continuation: delegate to existing format
+  if (records.length === 1 && !next) {
     const r = records[0]!;
     return formatAnnouncementMessage({
       runId: r.runId,
@@ -214,10 +219,9 @@ export function formatCoalescedAnnouncementMessage(
     });
   }
 
-  // Multiple records: build combined message.
-  // Include a strict raw-findings section so parent can reliably cover every task result.
+  // Multiple records (or single with continuation): build combined message.
   const parts: string[] = [
-    `All ${records.length} background tasks have completed. Here are the combined results:`,
+    `All ${records.length} background task(s) have completed. Here are the combined results:`,
     "",
   ];
 
@@ -262,14 +266,30 @@ export function formatCoalescedAnnouncementMessage(
     );
   }
 
-  parts.push(
-    "",
-    "Summarize these results naturally for the user.",
-    "You MUST include findings from every task item above, without omission.",
-    "Keep it concise, but preserve concrete findings from each task.",
-    "Do not mention technical details like session IDs or that these were background tasks.",
-    "You can respond with NO_REPLY if no announcement is needed.",
-  );
+  // Continuation vs. summarization
+  if (next) {
+    parts.push(
+      "",
+      "---",
+      "",
+      "CONTINUATION TASK: The user's original request requires further work using the findings above.",
+      "Execute the following task now, using ALL the collected data:",
+      "",
+      next,
+      "",
+      "Use the raw findings above as your data source. Call tools as needed to complete this task.",
+      "Do not mention technical details like session IDs or that these were background tasks.",
+    );
+  } else {
+    parts.push(
+      "",
+      "Summarize these results naturally for the user.",
+      "You MUST include findings from every task item above, without omission.",
+      "Keep it concise, but preserve concrete findings from each task.",
+      "Do not mention technical details like session IDs or that these were background tasks.",
+      "You can respond with NO_REPLY if no announcement is needed.",
+    );
+  }
 
   return parts.join("\n");
 }
@@ -289,8 +309,9 @@ export function formatCoalescedAnnouncementMessage(
 export function runCoalescedAnnounceFlow(
   requesterSessionId: string,
   records: SubagentRunRecord[],
+  next?: string,
 ): boolean {
-  const message = formatCoalescedAnnouncementMessage(records);
+  const message = formatCoalescedAnnouncementMessage(records, next);
 
   try {
     const hub = getHub();