fix(slack): scope debounce key by message timestamp to prevent cross-thread collisions

Top-level channel messages from the same sender shared a bare channel
debounce key, causing concurrent messages in different threads to merge
into a single reply on the wrong thread. Now the debounce key includes
the message timestamp for top-level messages, matching how the downstream
session layer already scopes by canonicalThreadId.

Extracted buildSlackDebounceKey() for testability.

Closes #31935

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

src/slack/monitor/message-handler.debounce-key.test.ts

@@ -0,0 +1,62 @@
+import { describe, expect, it } from "vitest";
+import type { SlackMessageEvent } from "../types.js";
+import { buildSlackDebounceKey } from "./message-handler.js";
+
+function makeMessage(overrides: Partial<SlackMessageEvent> = {}): SlackMessageEvent {
+  return {
+    type: "message",
+    channel: "C123",
+    user: "U456",
+    ts: "1709000000.000100",
+    text: "hello",
+    ...overrides,
+  } as SlackMessageEvent;
+}
+
+describe("buildSlackDebounceKey", () => {
+  const accountId = "default";
+
+  it("returns null when message has no sender", () => {
+    const msg = makeMessage({ user: undefined, bot_id: undefined });
+    expect(buildSlackDebounceKey(msg, accountId)).toBeNull();
+  });
+
+  it("scopes thread replies by thread_ts", () => {
+    const msg = makeMessage({ thread_ts: "1709000000.000001" });
+    expect(buildSlackDebounceKey(msg, accountId)).toBe("slack:default:C123:1709000000.000001:U456");
+  });
+
+  it("isolates unresolved thread replies with maybe-thread prefix", () => {
+    const msg = makeMessage({
+      parent_user_id: "U789",
+      thread_ts: undefined,
+      ts: "1709000000.000200",
+    });
+    expect(buildSlackDebounceKey(msg, accountId)).toBe(
+      "slack:default:C123:maybe-thread:1709000000.000200:U456",
+    );
+  });
+
+  it("scopes top-level messages by their own timestamp to prevent cross-thread collisions", () => {
+    const msgA = makeMessage({ ts: "1709000000.000100" });
+    const msgB = makeMessage({ ts: "1709000000.000200" });
+
+    const keyA = buildSlackDebounceKey(msgA, accountId);
+    const keyB = buildSlackDebounceKey(msgB, accountId);
+
+    // Different timestamps => different debounce keys
+    expect(keyA).not.toBe(keyB);
+    expect(keyA).toBe("slack:default:C123:1709000000.000100:U456");
+    expect(keyB).toBe("slack:default:C123:1709000000.000200:U456");
+  });
+
+  it("falls back to bare channel when no timestamp is available", () => {
+    const msg = makeMessage({ ts: undefined, event_ts: undefined });
+    expect(buildSlackDebounceKey(msg, accountId)).toBe("slack:default:C123:U456");
+  });
+
+  it("uses bot_id as sender fallback", () => {
+    const msg = makeMessage({ user: undefined, bot_id: "B999" });
+    expect(buildSlackDebounceKey(msg, accountId)).toBe("slack:default:C123:1709000000.000100:B999");
+  });
+});

src/slack/monitor/message-handler.ts

@@ -16,6 +16,31 @@ export type SlackMessageHandler = (
   opts: { source: "message" | "app_mention"; wasMentioned?: boolean },
 ) => Promise<void>;
 
+/**
+ * Build a debounce key that isolates messages by thread (or by message timestamp
+ * for top-level channel messages). Without per-message scoping, concurrent
+ * top-level messages from the same sender would share a key and get merged
+ * into a single reply on the wrong thread.
+ */
+export function buildSlackDebounceKey(
+  message: SlackMessageEvent,
+  accountId: string,
+): string | null {
+  const senderId = message.user ?? message.bot_id;
+  if (!senderId) {
+    return null;
+  }
+  const messageTs = message.ts ?? message.event_ts;
+  const threadKey = message.thread_ts
+    ? `${message.channel}:${message.thread_ts}`
+    : message.parent_user_id && messageTs
+      ? `${message.channel}:maybe-thread:${messageTs}`
+      : messageTs
+        ? `${message.channel}:${messageTs}`
+        : message.channel;
+  return `slack:${accountId}:${threadKey}:${senderId}`;
+}
+
 export function createSlackMessageHandler(params: {
   ctx: SlackMonitorContext;
   account: ResolvedSlackAccount;
@@ -31,20 +56,7 @@ export function createSlackMessageHandler(params: {
     opts: { source: "message" | "app_mention"; wasMentioned?: boolean };
   }>({
     debounceMs,
-    buildKey: (entry) => {
-      const senderId = entry.message.user ?? entry.message.bot_id;
-      if (!senderId) {
-        return null;
-      }
-      const messageTs = entry.message.ts ?? entry.message.event_ts;
-      // If Slack flags a thread reply but omits thread_ts, isolate it from root debouncing.
-      const threadKey = entry.message.thread_ts
-        ? `${entry.message.channel}:${entry.message.thread_ts}`
-        : entry.message.parent_user_id && messageTs
-          ? `${entry.message.channel}:maybe-thread:${messageTs}`
-          : entry.message.channel;
-      return `slack:${ctx.accountId}:${threadKey}:${senderId}`;
-    },
+    buildKey: (entry) => buildSlackDebounceKey(entry.message, ctx.accountId),
     shouldDebounce: (entry) => {
       const text = entry.message.text ?? "";
       if (!text.trim()) {