fix(slack): prefer explicit reply targets over inherited threads (#129069)

extensions/slack/src/message-action-dispatch.test.ts

@@ -731,6 +731,26 @@ describe("handleSlackMessageAction", () => {
   });
 
   it.each([
+    {
+      name: "prefers an explicit reply target over an inherited Slack thread",
+      params: {
+        to: "channel:C1",
+        message: "Reply to the requested message",
+        threadId: "111.222",
+        replyTo: "999.000",
+      },
+      expected: { content: "Reply to the requested message", threadTs: "999.000" },
+    },
+    {
+      name: "falls back to the Slack thread when the reply target is not a timestamp",
+      params: {
+        to: "channel:C1",
+        message: "Reply in the current thread",
+        threadId: "111.222",
+        replyTo: "msg-internal-1",
+      },
+      expected: { content: "Reply in the current thread", threadTs: "111.222" },
+    },
     {
       name: "passes replyBroadcast through for Slack thread sends",
       params: {

extensions/slack/src/message-action-dispatch.ts

@@ -24,6 +24,7 @@ import {
   resolveSlackReplyDeliveryMessages,
   type SlackReplyDeliveryMessage,
 } from "./reply-blocks.js";
+import { resolveSlackThreadTsValue } from "./thread-ts.js";
 import { countSlackTextUtf8Bytes } from "./truncate.js";
 
 type SlackActionInvoke = (
@@ -140,7 +141,7 @@ export async function handleSlackMessageAction(params: {
         mediaUrl: mediaUrl ?? undefined,
         ...(readSlackForceDocument(actionParams) ? { forceDocument: true } : {}),
         accountId,
-        threadTs: threadId ?? replyTo ?? undefined,
+        threadTs: resolveSlackThreadTsValue({ replyToId: replyTo, threadId }),
         ...(topLevel ? { topLevel: true } : {}),
         ...(replyBroadcast ? { replyBroadcast } : {}),
       },