[Fix] Slack follow-ups disappear after human side discussions

apps/api/src/handlers/slack/events/fast-agent.ts

@@ -200,13 +200,6 @@ export async function processFastAgentMessage(params: {
         ts: message.ts,
         bot_id: message.bot_id,
       }));
-    const hasOtherHumanParticipant = threadContext.some(
-      (message) =>
-        message.ts !== event.ts &&
-        !message.bot_id &&
-        Boolean(message.user) &&
-        message.user !== event.user,
-    );
 
     const needsCanonicalAdmission =
       !releaseFastAgentLock ||
@@ -219,12 +212,16 @@ export async function processFastAgentMessage(params: {
       currentMessageId: event.ts,
       userId,
       question,
+      threadContext: serializedThreadContext,
       ...(attachments.images.length ? { images: attachments.images } : {}),
       ...(currentMessage?.username
         ? { senderDisplayName: currentMessage.username }
         : {}),
       ...(event.user ? { senderExternalId: event.user } : {}),
-      directedAtRoomote,
+      directedAtRoomote:
+        directedAtRoomote ||
+        event.channel_type === 'im' ||
+        event.channel_type === 'mpim',
     };
     let durableTurn: FastAgentDurableTurn | null = null;
     if (needsCanonicalAdmission) {
@@ -299,7 +296,6 @@ export async function processFastAgentMessage(params: {
       allowSilentAmbientReply:
         event.channel_type !== 'im' &&
         event.channel_type !== 'mpim' &&
-        hasOtherHumanParticipant &&
         !directedAtRoomote,
       ...(roomoteSlackUserId ? { slackRoomoteUserId: roomoteSlackUserId } : {}),
       adapter: {

apps/api/src/handlers/slack/events/message-entry.ts

@@ -143,7 +143,7 @@ type UnmentionedSlackThreadReplyRoutingDecision =
   | { shouldRoute: false }
   | {
       shouldRoute: true;
-      threadMessages: SlackThreadMessage[];
+      threadMessages?: SlackThreadMessage[];
       taskId?: string;
     };
 
@@ -281,11 +281,23 @@ async function markHumanMentionedSlackThread(params: {
   event: SlackEvent;
   slack: SlackNotifier;
   botUserId: string | null | undefined;
+  teamId: string;
 }): Promise<void> {
   if (!mentionsSlackUserOtherThanBot(params.event, params.botUserId)) {
     return;
   }
 
+  if (
+    params.event.thread_ts &&
+    (await hasBoundSlackFastAgentSession({
+      teamId: params.teamId,
+      channelId: params.event.channel,
+      threadId: params.event.thread_ts,
+    }))
+  ) {
+    return;
+  }
+
   await markExplicitMentionRequiredSlackThread(params);
 }
 
@@ -321,6 +333,19 @@ export async function shouldRouteUnmentionedSlackThreadReplyToAgent(params: {
     return { shouldRoute: false };
   }
 
+  // Established Fast conversations observe human discussion too. Fast decides
+  // whether to engage under its quiet-participation rules; legacy task threads
+  // retain their explicit-mention/interjection gate below.
+  if (
+    await hasBoundSlackFastAgentSession({
+      teamId,
+      channelId: event.channel,
+      threadId: event.thread_ts,
+    })
+  ) {
+    return { shouldRoute: true };
+  }
+
   if (
     mentionsSlackUserOtherThanBotWithoutMentioningBot(
       event,
@@ -333,36 +358,25 @@ export async function shouldRouteUnmentionedSlackThreadReplyToAgent(params: {
   let roomoteThreadMatch: Awaited<
     ReturnType<typeof findRoomoteOwnedSlackThread>
   > | null = null;
-  let isFastAgentThread = false;
 
   let eligibilityReason: 'roomote-owned-thread' | null = null;
 
   {
-    isFastAgentThread = await hasBoundSlackFastAgentSession({
+    roomoteThreadMatch = await findRoomoteOwnedSlackThread({
       teamId,
       channelId: event.channel,
-      threadId: event.thread_ts,
+      threadTs: event.thread_ts,
     });
 
-    roomoteThreadMatch = isFastAgentThread
+    const taskThreadRoute = roomoteThreadMatch
       ? null
-      : await findRoomoteOwnedSlackThread({
-          teamId,
+      : await resolveSlackThreadFollowUpRoute({
+          threadId: event.thread_ts,
           channelId: event.channel,
-          threadTs: event.thread_ts,
+          slackTeamId: teamId,
         });
 
-    const taskThreadRoute =
-      isFastAgentThread || roomoteThreadMatch
-        ? null
-        : await resolveSlackThreadFollowUpRoute({
-            threadId: event.thread_ts,
-            channelId: event.channel,
-            slackTeamId: teamId,
-          });
-
     if (
-      isFastAgentThread ||
       roomoteThreadMatch ||
       (taskThreadRoute && taskThreadRoute.kind !== 'fresh')
     ) {
@@ -431,7 +445,6 @@ export async function shouldRouteUnmentionedSlackThreadReplyToAgent(params: {
     isAutomationReportThread: Boolean(
       roomoteThreadMatch?.isAutomationReportThread,
     ),
-    isOpenConversationThread: isFastAgentThread,
     threadMessages: sharedHistory,
     compareMessageIds: compareNumericMessageIds,
   });
@@ -1377,6 +1390,7 @@ export async function handleMessageOrAppMentionEvent(params: {
     event,
     slack: context.slack,
     botUserId: context.slackInstallation.botUserId,
+    teamId: context.teamId,
   });
 
   const mentionedThreadAliasTaskId =