diff --git a/apps/web/features/issues/components/issue-detail.tsx b/apps/web/features/issues/components/issue-detail.tsx
index 7ca9c12e..d1df85f5 100644
--- a/apps/web/features/issues/components/issue-detail.tsx
+++ b/apps/web/features/issues/components/issue-detail.tsx
@@ -315,7 +315,13 @@ export function IssueDetail({ issueId, onDelete }: IssueDetailProps) {
   const handleDeleteComment = async (commentId: string) => {
     try {
       await api.deleteComment(commentId);
-      setTimeline((prev) => prev.filter((e) => e.id !== commentId));
+      setTimeline((prev) =>
+        prev
+          // Promote replies of deleted comment to top-level
+          .map((e) => e.parent_id === commentId ? { ...e, parent_id: null } : e)
+          // Remove the deleted comment
+          .filter((e) => e.id !== commentId)
+      );
     } catch {
       toast.error("Failed to delete comment");
     }
@@ -379,7 +385,11 @@ export function IssueDetail({ issueId, onDelete }: IssueDetailProps) {
     useCallback((payload: unknown) => {
       const { comment_id, issue_id } = payload as CommentDeletedPayload;
       if (issue_id === id) {
-        setTimeline((prev) => prev.filter((e) => e.id !== comment_id));
+        setTimeline((prev) =>
+          prev
+            .map((e) => e.parent_id === comment_id ? { ...e, parent_id: null } : e)
+            .filter((e) => e.id !== comment_id)
+        );
       }
     }, [id]),
   );
diff --git a/server/cmd/server/activity_listeners.go b/server/cmd/server/activity_listeners.go
index 8a05da40..2a639a79 100644
--- a/server/cmd/server/activity_listeners.go
+++ b/server/cmd/server/activity_listeners.go
@@ -186,19 +186,26 @@ func handleTaskActivity(ctx context.Context, bus *events.Bus, queries *db.Querie
 }
 
 // publishActivityEvent sends an activity:created event for WS broadcasting.
+// Payload matches frontend ActivityCreatedPayload: { issue_id, entry: TimelineEntry }
 func publishActivityEvent(bus *events.Bus, original events.Event, activity db.ActivityLog) {
+	actorType := ""
+	if activity.ActorType.Valid {
+		actorType = activity.ActorType.String
+	}
+	action := activity.Action
 	bus.Publish(events.Event{
 		Type:        protocol.EventActivityCreated,
 		WorkspaceID: original.WorkspaceID,
 		ActorType:   original.ActorType,
 		ActorID:     original.ActorID,
 		Payload: map[string]any{
-			"activity": map[string]any{
+			"issue_id": util.UUIDToString(activity.IssueID),
+			"entry": map[string]any{
+				"type":       "activity",
 				"id":         util.UUIDToString(activity.ID),
-				"issue_id":   util.UUIDToString(activity.IssueID),
-				"actor_type": util.TextToPtr(activity.ActorType),
+				"actor_type": actorType,
 				"actor_id":   util.UUIDToString(activity.ActorID),
-				"action":     activity.Action,
+				"action":     &action,
 				"details":    json.RawMessage(activity.Details),
 				"created_at": util.TimestampToString(activity.CreatedAt),
 			},
diff --git a/server/internal/handler/comment.go b/server/internal/handler/comment.go
index 99d45113..d4f4fc99 100644
--- a/server/internal/handler/comment.go
+++ b/server/internal/handler/comment.go
@@ -94,6 +94,16 @@ func (h *Handler) CreateComment(w http.ResponseWriter, r *http.Request) {
 	var parentID pgtype.UUID
 	if req.ParentID != nil {
 		parentID = parseUUID(*req.ParentID)
+		// Only allow one level of nesting: parent must be a top-level comment
+		parent, err := h.Queries.GetComment(r.Context(), parentID)
+		if err != nil {
+			writeError(w, http.StatusBadRequest, "parent comment not found")
+			return
+		}
+		if parent.ParentID.Valid {
+			writeError(w, http.StatusBadRequest, "replies to replies are not supported")
+			return
+		}
 	}
 
 	comment, err := h.Queries.CreateComment(r.Context(), db.CreateCommentParams{