diff --git a/crates/worker/src/handlers.rs b/crates/worker/src/handlers.rs
index af61628..37e04af 100644
--- a/crates/worker/src/handlers.rs
+++ b/crates/worker/src/handlers.rs
@@ -61,6 +61,26 @@ impl NotificationHandler {
                     created_at: Utc::now(),
                 }).await
             }
+            DomainEvent::ThoughtCreated { thought_id, user_id, in_reply_to_id } => {
+                let reply_to_id = match in_reply_to_id {
+                    Some(id) => id,
+                    None => return Ok(()), // not a reply
+                };
+                let original = match self.thoughts.find_by_id(reply_to_id).await? {
+                    Some(t) => t,
+                    None => return Ok(()), // original deleted
+                };
+                if original.user_id == *user_id { return Ok(()); } // no self-notifications
+                self.notifications.save(&Notification {
+                    id: NotificationId::new(),
+                    user_id: original.user_id,
+                    notification_type: NotificationType::Reply,
+                    from_user_id: Some(user_id.clone()),
+                    thought_id: Some(thought_id.clone()),
+                    read: false,
+                    created_at: Utc::now(),
+                }).await
+            }
             // All other events: no notification needed in Plan 3
             _ => Ok(()),
         }
@@ -175,4 +195,81 @@ mod tests {
         assert_eq!(notifs[0].user_id, alice.id);
         assert!(matches!(notifs[0].notification_type, NotificationType::Follow));
     }
+
+    #[tokio::test]
+    async fn reply_creates_notification_for_original_author() {
+        let store = TestStore::default();
+        let alice = alice();
+        let bob_id = UserId::new();
+
+        let original = Thought::new_local(
+            ThoughtId::new(), alice.id.clone(),
+            Content::new_local("original thought").unwrap(),
+            None, Visibility::Public, None, false,
+        );
+        store.users.lock().unwrap().push(alice.clone());
+        store.thoughts.lock().unwrap().push(original.clone());
+
+        let handler = NotificationHandler {
+            thoughts:      Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+
+        handler.handle(&DomainEvent::ThoughtCreated {
+            thought_id: ThoughtId::new(),
+            user_id: bob_id.clone(),
+            in_reply_to_id: Some(original.id.clone()),
+        }).await.unwrap();
+
+        let notifs = store.notifications.lock().unwrap();
+        assert_eq!(notifs.len(), 1);
+        assert_eq!(notifs[0].user_id, alice.id);
+        assert!(matches!(notifs[0].notification_type, NotificationType::Reply));
+    }
+
+    #[tokio::test]
+    async fn self_reply_does_not_create_notification() {
+        let store = TestStore::default();
+        let alice = alice();
+        let original = Thought::new_local(
+            ThoughtId::new(), alice.id.clone(),
+            Content::new_local("original").unwrap(),
+            None, Visibility::Public, None, false,
+        );
+        store.users.lock().unwrap().push(alice.clone());
+        store.thoughts.lock().unwrap().push(original.clone());
+
+        let handler = NotificationHandler {
+            thoughts:      Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+
+        handler.handle(&DomainEvent::ThoughtCreated {
+            thought_id: ThoughtId::new(),
+            user_id: alice.id.clone(),
+            in_reply_to_id: Some(original.id.clone()),
+        }).await.unwrap();
+
+        assert!(store.notifications.lock().unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn thought_without_reply_to_creates_no_notification() {
+        let store = TestStore::default();
+        let alice = alice();
+        store.users.lock().unwrap().push(alice.clone());
+
+        let handler = NotificationHandler {
+            thoughts:      Arc::new(store.clone()),
+            notifications: Arc::new(store.clone()),
+        };
+
+        handler.handle(&DomainEvent::ThoughtCreated {
+            thought_id: ThoughtId::new(),
+            user_id: alice.id.clone(),
+            in_reply_to_id: None,
+        }).await.unwrap();
+
+        assert!(store.notifications.lock().unwrap().is_empty());
+    }
 }