diff --git a/crates/application/src/services/federation_event.rs b/crates/application/src/services/federation_event.rs
new file mode 100644
index 0000000..8254083
--- /dev/null
+++ b/crates/application/src/services/federation_event.rs
@@ -0,0 +1,281 @@
+use std::sync::Arc;
+use domain::{
+    errors::DomainError,
+    events::DomainEvent,
+    ports::{OutboundFederationPort, ThoughtRepository, UserRepository},
+};
+
+pub struct FederationEventService {
+    pub thoughts:  Arc<dyn ThoughtRepository>,
+    pub users:     Arc<dyn UserRepository>,
+    pub ap:        Arc<dyn OutboundFederationPort>,
+    pub base_url:  String,
+}
+
+impl FederationEventService {
+    pub async fn process(&self, event: &DomainEvent) -> Result<(), DomainError> {
+        match event {
+            DomainEvent::ThoughtCreated { thought_id, user_id, .. } => {
+                let thought = match self.thoughts.find_by_id(thought_id).await? {
+                    Some(t) if t.local => t,
+                    _ => return Ok(()),
+                };
+                let user = match self.users.find_by_id(user_id).await? {
+                    Some(u) => u,
+                    None => return Ok(()),
+                };
+                self.ap.broadcast_create(user_id, &thought, user.username.as_str()).await
+            }
+
+            DomainEvent::ThoughtDeleted { thought_id, user_id } => {
+                let ap_id = format!("{}/thoughts/{}", self.base_url, thought_id);
+                self.ap.broadcast_delete(user_id, &ap_id).await
+            }
+
+            DomainEvent::ThoughtUpdated { thought_id, user_id } => {
+                let thought = match self.thoughts.find_by_id(thought_id).await? {
+                    Some(t) if t.local => t,
+                    _ => return Ok(()),
+                };
+                let user = match self.users.find_by_id(user_id).await? {
+                    Some(u) => u,
+                    None => return Ok(()),
+                };
+                self.ap.broadcast_update(user_id, &thought, user.username.as_str()).await
+            }
+
+            DomainEvent::BoostAdded { boost_id: _, user_id, thought_id } => {
+                let thought = match self.thoughts.find_by_id(thought_id).await? {
+                    Some(t) => t,
+                    None => return Ok(()),
+                };
+                let object_ap_id = thought.ap_id.clone().unwrap_or_else(|| {
+                    format!("{}/thoughts/{}", self.base_url, thought_id)
+                });
+                self.ap.broadcast_announce(user_id, &object_ap_id).await
+            }
+
+            _ => Ok(()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use async_trait::async_trait;
+    use domain::{
+        errors::DomainError,
+        events::DomainEvent,
+        models::thought::{Thought, Visibility},
+        models::user::User,
+        ports::OutboundFederationPort,
+        testing::TestStore,
+        value_objects::*,
+    };
+    use std::sync::{Arc, Mutex};
+
+    // ── Spy port ─────────────────────────────────────────────────────────────
+
+    #[derive(Default)]
+    struct SpyPort {
+        created:   Mutex<Vec<ThoughtId>>,
+        deleted:   Mutex<Vec<String>>,
+        updated:   Mutex<Vec<ThoughtId>>,
+        announced: Mutex<Vec<String>>,
+    }
+
+    #[async_trait]
+    impl OutboundFederationPort for SpyPort {
+        async fn broadcast_create(&self, _: &UserId, thought: &Thought, _: &str) -> Result<(), DomainError> {
+            self.created.lock().unwrap().push(thought.id.clone());
+            Ok(())
+        }
+        async fn broadcast_delete(&self, _: &UserId, ap_id: &str) -> Result<(), DomainError> {
+            self.deleted.lock().unwrap().push(ap_id.to_string());
+            Ok(())
+        }
+        async fn broadcast_update(&self, _: &UserId, thought: &Thought, _: &str) -> Result<(), DomainError> {
+            self.updated.lock().unwrap().push(thought.id.clone());
+            Ok(())
+        }
+        async fn broadcast_announce(&self, _: &UserId, ap_id: &str) -> Result<(), DomainError> {
+            self.announced.lock().unwrap().push(ap_id.to_string());
+            Ok(())
+        }
+    }
+
+    fn alice() -> User {
+        User::new_local(
+            UserId::new(),
+            Username::new("alice").unwrap(),
+            Email::new("alice@ex.com").unwrap(),
+            PasswordHash("h".into()),
+        )
+    }
+
+    fn local_thought(author_id: UserId) -> Thought {
+        Thought::new_local(
+            ThoughtId::new(), author_id,
+            Content::new_local("hello").unwrap(),
+            None, Visibility::Public, None, false,
+        )
+    }
+
+    fn svc(store: &TestStore, spy: Arc<SpyPort>) -> FederationEventService {
+        FederationEventService {
+            thoughts: Arc::new(store.clone()),
+            users: Arc::new(store.clone()),
+            ap: spy,
+            base_url: "https://example.com".to_string(),
+        }
+    }
+
+    #[tokio::test]
+    async fn thought_created_broadcasts_create() {
+        let store = TestStore::default();
+        let alice = alice();
+        let thought = local_thought(alice.id.clone());
+        store.users.lock().unwrap().push(alice.clone());
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::ThoughtCreated {
+                thought_id: thought.id.clone(),
+                user_id: alice.id.clone(),
+                in_reply_to_id: None,
+            })
+            .await
+            .unwrap();
+
+        assert_eq!(spy.created.lock().unwrap().len(), 1);
+        assert_eq!(spy.created.lock().unwrap()[0], thought.id);
+    }
+
+    #[tokio::test]
+    async fn remote_thought_created_does_not_broadcast() {
+        let store = TestStore::default();
+        let alice = alice();
+        // Remote thought: local = false, ap_id = Some(...)
+        let mut thought = local_thought(alice.id.clone());
+        thought.local = false;
+        thought.ap_id = Some("https://remote.example/notes/1".into());
+        store.users.lock().unwrap().push(alice.clone());
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::ThoughtCreated {
+                thought_id: thought.id.clone(),
+                user_id: alice.id.clone(),
+                in_reply_to_id: None,
+            })
+            .await
+            .unwrap();
+
+        assert!(spy.created.lock().unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn thought_deleted_broadcasts_delete_with_constructed_ap_id() {
+        let store = TestStore::default();
+        let alice = alice();
+        let tid = ThoughtId::new();
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::ThoughtDeleted {
+                thought_id: tid.clone(),
+                user_id: alice.id.clone(),
+            })
+            .await
+            .unwrap();
+
+        let deleted = spy.deleted.lock().unwrap();
+        assert_eq!(deleted.len(), 1);
+        assert_eq!(deleted[0], format!("https://example.com/thoughts/{}", tid));
+    }
+
+    #[tokio::test]
+    async fn thought_updated_broadcasts_update() {
+        let store = TestStore::default();
+        let alice = alice();
+        let thought = local_thought(alice.id.clone());
+        store.users.lock().unwrap().push(alice.clone());
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::ThoughtUpdated {
+                thought_id: thought.id.clone(),
+                user_id: alice.id.clone(),
+            })
+            .await
+            .unwrap();
+
+        assert_eq!(spy.updated.lock().unwrap().len(), 1);
+    }
+
+    #[tokio::test]
+    async fn boost_of_local_thought_announces_constructed_url() {
+        let store = TestStore::default();
+        let alice = alice();
+        let thought = local_thought(alice.id.clone()); // ap_id = None
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::BoostAdded {
+                boost_id: BoostId::new(),
+                user_id: alice.id.clone(),
+                thought_id: thought.id.clone(),
+            })
+            .await
+            .unwrap();
+
+        let announced = spy.announced.lock().unwrap();
+        assert_eq!(announced.len(), 1);
+        assert_eq!(announced[0], format!("https://example.com/thoughts/{}", thought.id));
+    }
+
+    #[tokio::test]
+    async fn boost_of_remote_thought_announces_remote_ap_id() {
+        let store = TestStore::default();
+        let alice = alice();
+        let mut thought = local_thought(alice.id.clone());
+        thought.local = false;
+        thought.ap_id = Some("https://mastodon.social/users/bob/statuses/123".into());
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::BoostAdded {
+                boost_id: BoostId::new(),
+                user_id: alice.id.clone(),
+                thought_id: thought.id.clone(),
+            })
+            .await
+            .unwrap();
+
+        let announced = spy.announced.lock().unwrap();
+        assert_eq!(announced[0], "https://mastodon.social/users/bob/statuses/123");
+    }
+
+    #[tokio::test]
+    async fn unrelated_events_are_noop() {
+        let store = TestStore::default();
+        let spy = Arc::new(SpyPort::default());
+        let svc = svc(&store, spy.clone());
+
+        svc.process(&DomainEvent::UserBlocked {
+            blocker_id: UserId::new(),
+            blocked_id: UserId::new(),
+        }).await.unwrap();
+
+        assert!(spy.created.lock().unwrap().is_empty());
+        assert!(spy.deleted.lock().unwrap().is_empty());
+        assert!(spy.updated.lock().unwrap().is_empty());
+        assert!(spy.announced.lock().unwrap().is_empty());
+    }
+}
diff --git a/crates/application/src/services/mod.rs b/crates/application/src/services/mod.rs
index 480fdc6..6116915 100644
--- a/crates/application/src/services/mod.rs
+++ b/crates/application/src/services/mod.rs
@@ -1,2 +1,5 @@
+pub mod federation_event;
 pub mod notification_event;
+
+pub use federation_event::FederationEventService;
 pub use notification_event::NotificationEventService;