fix: keep Slack thread replies in thread

src/slack/monitor.tool-result.test.ts

@@ -122,4 +122,38 @@ describe("monitorSlackProvider tool results", () => {
     expect(sendMock.mock.calls[0][1]).toBe("PFX tool update");
     expect(sendMock.mock.calls[1][1]).toBe("PFX final reply");
   });
+
+  it("threads replies when incoming message is in a thread", async () => {
+    replyMock.mockResolvedValue({ text: "thread reply" });
+
+    const controller = new AbortController();
+    const run = monitorSlackProvider({
+      botToken: "bot-token",
+      appToken: "app-token",
+      abortSignal: controller.signal,
+    });
+
+    await waitForEvent("message");
+    const handler = getSlackHandlers()?.get("message");
+    if (!handler) throw new Error("Slack message handler not registered");
+
+    await handler({
+      event: {
+        type: "message",
+        user: "U1",
+        text: "hello",
+        ts: "123",
+        thread_ts: "456",
+        channel: "C1",
+        channel_type: "im",
+      },
+    });
+
+    await flush();
+    controller.abort();
+    await run;
+
+    expect(sendMock).toHaveBeenCalledTimes(1);
+    expect(sendMock.mock.calls[0][2]).toMatchObject({ threadTs: "456" });
+  });
 });

src/slack/monitor.ts

@@ -700,6 +700,9 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
       );
     }
 
+    // Only thread replies if the incoming message was in a thread.
+    const incomingThreadTs = message.thread_ts;
+
     const dispatcher = createReplyDispatcher({
       responsePrefix: cfg.messages?.responsePrefix,
       deliver: async (payload) => {
@@ -709,6 +712,7 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
           token: botToken,
           runtime,
           textLimit,
+          threadTs: incomingThreadTs,
         });
       },
       onError: (err, info) => {
@@ -1379,6 +1383,7 @@ async function deliverReplies(params: {
   token: string;
   runtime: RuntimeEnv;
   textLimit: number;
+  threadTs?: string;
 }) {
   const chunkLimit = Math.min(params.textLimit, 4000);
   for (const payload of params.replies) {
@@ -1389,12 +1394,11 @@ async function deliverReplies(params: {
 
     if (mediaList.length === 0) {
       for (const chunk of chunkText(text, chunkLimit)) {
-        const threadTs = undefined;
         const trimmed = chunk.trim();
         if (!trimmed || trimmed === SILENT_REPLY_TOKEN) continue;
         await sendMessageSlack(params.target, trimmed, {
           token: params.token,
-          threadTs,
+          threadTs: params.threadTs,
         });
       }
     } else {
@@ -1402,11 +1406,10 @@ async function deliverReplies(params: {
       for (const mediaUrl of mediaList) {
         const caption = first ? text : "";
         first = false;
-        const threadTs = undefined;
         await sendMessageSlack(params.target, caption, {
           token: params.token,
           mediaUrl,
-          threadTs,
+          threadTs: params.threadTs,
         });
       }
     }