feat(trigger): inherit thread-root mentions for reply-triggered agent tasks (#375)

When a top-level comment @mentions an agent (non-assignee), subsequent
replies in the same thread now also trigger that agent via on_mention.
Previously only the current comment's mentions were checked, so replies
without an explicit re-mention would silently skip the agent.

Extends enqueueMentionedAgentTasks to accept the parent comment and
merge its parsed mentions (deduplicated) into the trigger set, reusing
all existing guards (self-trigger, assignee skip, visibility, dedup).

Closes MUL-177

server/cmd/server/comment_trigger_integration_test.go

@@ -323,6 +323,52 @@ func TestCommentTriggerOnMentionNoStatusGate(t *testing.T) {
 	}
 }
 
+// TestCommentTriggerThreadInheritedMention verifies that when a top-level
+// comment @mentions an agent (not the assignee), replies in that thread
+// also trigger the mentioned agent — even without explicitly re-mentioning it.
+func TestCommentTriggerThreadInheritedMention(t *testing.T) {
+	agentID := getAgentID(t)
+
+	// Create an issue NOT assigned to the agent, so on_comment won't fire.
+	issueID := createIssue(t, "Thread-inherited mention test")
+	t.Cleanup(func() {
+		clearTasks(t, issueID)
+		resp := authRequest(t, "DELETE", "/api/issues/"+issueID, nil)
+		resp.Body.Close()
+	})
+
+	t.Run("reply in thread inherits parent mention", func(t *testing.T) {
+		clearTasks(t, issueID)
+		// Top-level comment @mentions the agent.
+		content := fmt.Sprintf("[@Agent](mention://agent/%s) can you review this?", agentID)
+		threadID := postComment(t, issueID, content, nil)
+		if n := countPendingTasks(t, issueID); n != 1 {
+			t.Fatalf("expected 1 pending task after initial mention, got %d", n)
+		}
+		// Clear the task so we can test the reply independently.
+		clearTasks(t, issueID)
+		// Reply in the thread WITHOUT mentioning the agent.
+		postComment(t, issueID, "Here is more context for you", strPtr(threadID))
+		if n := countPendingTasks(t, issueID); n != 1 {
+			t.Errorf("expected 1 pending task from thread-inherited mention, got %d", n)
+		}
+	})
+
+	t.Run("reply does not double-trigger when re-mentioning same agent", func(t *testing.T) {
+		clearTasks(t, issueID)
+		// Top-level comment @mentions the agent.
+		content := fmt.Sprintf("[@Agent](mention://agent/%s) help", agentID)
+		threadID := postComment(t, issueID, content, nil)
+		clearTasks(t, issueID)
+		// Reply also @mentions the same agent — should still be just 1 task.
+		reply := fmt.Sprintf("[@Agent](mention://agent/%s) any update?", agentID)
+		postComment(t, issueID, reply, strPtr(threadID))
+		if n := countPendingTasks(t, issueID); n != 1 {
+			t.Errorf("expected 1 pending task (no duplicate), got %d", n)
+		}
+	})
+}
+
 // TestCommentTriggerCoalescing verifies that rapid-fire comments don't create
 // duplicate tasks (coalescing dedup).
 func TestCommentTriggerCoalescing(t *testing.T) {

server/internal/handler/comment.go

@@ -188,7 +188,8 @@ func (h *Handler) CreateComment(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Trigger @mentioned agents: parse agent mentions and enqueue tasks for each.
-	h.enqueueMentionedAgentTasks(r.Context(), issue, comment, authorType, authorID)
+	// Pass parentComment so that replies inherit mentions from the thread root.
+	h.enqueueMentionedAgentTasks(r.Context(), issue, comment, parentComment, authorType, authorID)
 
 	writeJSON(w, http.StatusCreated, resp)
 }
@@ -248,16 +249,34 @@ func (h *Handler) isReplyToMemberThread(parent *db.Comment, content string, issu
 }
 
 // enqueueMentionedAgentTasks parses @agent mentions from comment content and
-// enqueues a task for each mentioned agent. Skips self-mentions, agents that
-// are already the issue's assignee (handled by on_comment), agents with
-// on_mention trigger disabled, and private agents mentioned by non-owner
-// members (only the agent owner or workspace admin/owner can mention a
-// private agent).
+// enqueues a task for each mentioned agent. When parentComment is non-nil
+// (i.e. the comment is a reply), mentions from the parent (thread root) are
+// also included so that agents mentioned in the top-level comment are
+// re-triggered by subsequent replies in the same thread.
+// Skips self-mentions, agents that are already the issue's assignee (handled
+// by on_comment), agents with on_mention trigger disabled, and private agents
+// mentioned by non-owner members (only the agent owner or workspace
+// admin/owner can mention a private agent).
 // Note: no status gate here — @mention is an explicit action and should work
 // even on done/cancelled issues (the agent can reopen the issue if needed).
-func (h *Handler) enqueueMentionedAgentTasks(ctx context.Context, issue db.Issue, comment db.Comment, authorType, authorID string) {
+func (h *Handler) enqueueMentionedAgentTasks(ctx context.Context, issue db.Issue, comment db.Comment, parentComment *db.Comment, authorType, authorID string) {
 	wsID := uuidToString(issue.WorkspaceID)
 	mentions := util.ParseMentions(comment.Content)
+	// When replying in a thread, also include mentions from the parent comment
+	// so that agents mentioned in the thread root are triggered by replies.
+	if parentComment != nil {
+		parentMentions := util.ParseMentions(parentComment.Content)
+		seen := make(map[string]bool, len(mentions))
+		for _, m := range mentions {
+			seen[m.Type+":"+m.ID] = true
+		}
+		for _, m := range parentMentions {
+			if !seen[m.Type+":"+m.ID] {
+				mentions = append(mentions, m)
+				seen[m.Type+":"+m.ID] = true
+			}
+		}
+	}
 	for _, m := range mentions {
 		if m.Type != "agent" {
 			continue