fix(server): treat agent-authored thread root as implicit mention

When a member posts a plain reply under a thread whose root was authored
by an agent, treat that agent as implicitly mentioned so the reply
re-triggers the agent. Matches the intuition that "reply to X" addresses
X and fixes the case where the thread author is not the issue assignee.

Only applies when the reply has no explicit mentions of its own — an
explicit @mention is a deliberate redirect and is respected. Dedup via
the existing HasPendingTaskForIssueAndAgent check prevents a double task
when the thread root author is also the issue assignee.

server/cmd/server/comment_trigger_integration_test.go

@@ -470,6 +470,31 @@ func TestCommentTriggerThreadInheritedMention(t *testing.T) {
 			t.Errorf("expected 1 pending task (reply mentions agent explicitly), got %d", n)
 		}
 	})
+
+	t.Run("reply to non-assignee agent's thread triggers that agent (implicit author mention)", func(t *testing.T) {
+		clearTasks(t, issueID)
+		// Agent (not the issue's assignee) starts a top-level thread with plain content.
+		threadID := postCommentAsAgent(t, issueID, "Here is what I found.", agentID, nil)
+		// Member replies without any explicit mention — expected: the thread's
+		// author agent is triggered via implicit parent-author mention.
+		postComment(t, issueID, "Thanks, please continue", strPtr(threadID))
+		if n := countPendingTasks(t, issueID); n != 1 {
+			t.Errorf("expected 1 pending task (implicit author mention), got %d", n)
+		}
+	})
+
+	t.Run("reply to non-assignee agent's thread does not inherit when reply mentions others", func(t *testing.T) {
+		clearTasks(t, issueID)
+		// Agent starts a top-level thread.
+		threadID := postCommentAsAgent(t, issueID, "Initial analysis.", agentID, nil)
+		// Member replies mentioning only another member — implicit author mention
+		// is suppressed because the reply already has explicit mentions.
+		reply := fmt.Sprintf("cc [@Someone](mention://member/%s)", testUserID)
+		postComment(t, issueID, reply, strPtr(threadID))
+		if n := countPendingTasks(t, issueID); n != 0 {
+			t.Errorf("expected 0 pending tasks (explicit non-agent mention overrides author mention), got %d", n)
+		}
+	})
 }
 
 // TestCommentTriggerCoalescing verifies that rapid-fire comments don't create

server/internal/handler/comment.go

@@ -375,6 +375,11 @@ func (h *Handler) isReplyToMemberThread(ctx context.Context, parent *db.Comment,
 // re-triggered by subsequent replies in the same thread — unless the reply
 // explicitly @mentions only non-agent entities (members, issues), which
 // signals the user is talking to other people and not the agent.
+// When the parent (thread root) itself was authored by an agent, that agent
+// is treated as an implicit mention so "replying under an agent's comment"
+// re-triggers that agent — matching the intuition that replies address the
+// author being replied to. Same gate: only when the reply has no explicit
+// mentions.
 // Skips self-mentions, 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).
@@ -390,6 +395,23 @@ func (h *Handler) enqueueMentionedAgentTasks(ctx context.Context, issue db.Issue
 	// is making a deliberate choice about who to involve; don't auto-inherit.
 	if parentComment != nil && len(mentions) == 0 {
 		mentions = util.ParseMentions(parentComment.Content)
+		// Implicit mention: if the thread root was posted by an agent, treat
+		// this plain reply as addressing that agent. Dedup against parent's
+		// own mentions so we don't double-count when the parent @-mentioned
+		// itself somehow.
+		if parentComment.AuthorType == "agent" {
+			parentAuthorID := uuidToString(parentComment.AuthorID)
+			already := false
+			for _, m := range mentions {
+				if m.Type == "agent" && m.ID == parentAuthorID {
+					already = true
+					break
+				}
+			}
+			if !already {
+				mentions = append(mentions, util.Mention{Type: "agent", ID: parentAuthorID})
+			}
+		}
 	}
 	for _, m := range mentions {
 		if m.Type != "agent" {