crates/adapters/activitypub-base/src/service.rs

@@ -200,6 +200,68 @@ impl ActivityPubService {
         Ok(())
     }
 
+    /// Fan out an Undo(Announce) activity to all accepted followers.
+    pub async fn broadcast_undo_announce_to_followers(
+        &self,
+        local_user_id: uuid::Uuid,
+        object_ap_id: url::Url,
+    ) -> anyhow::Result<()> {
+        let data = self.federation_config.to_request_data();
+        let local_actor = get_local_actor(local_user_id, &data)
+            .await
+            .map_err(|e| anyhow::anyhow!("{e}"))?;
+
+        let followers = data.federation_repo.get_followers(local_user_id).await?;
+        let blocked = data.federation_repo.get_blocked_actors(local_user_id).await.unwrap_or_default();
+        let blocked_set: std::collections::HashSet<String> = blocked.into_iter().collect();
+        let blocked_domains = data.federation_repo.get_blocked_domains().await.unwrap_or_default();
+        let blocked_domain_set: std::collections::HashSet<String> =
+            blocked_domains.into_iter().map(|d| d.domain).collect();
+
+        let accepted: Vec<_> = followers
+            .into_iter()
+            .filter(|f| f.status == FollowerStatus::Accepted)
+            .filter(|f| !blocked_set.contains(&f.actor.url))
+            .filter(|f| {
+                let domain = url::Url::parse(&f.actor.inbox_url)
+                    .ok()
+                    .and_then(|u| u.host_str().map(|s| s.to_string()))
+                    .unwrap_or_default();
+                !blocked_domain_set.contains(&domain)
+            })
+            .collect();
+
+        if accepted.is_empty() {
+            return Ok(());
+        }
+
+        let undo_id = crate::urls::activity_url(&self.base_url).map_err(|e| anyhow::anyhow!("{e}"))?;
+        let undo = crate::activities::UndoActivity {
+            id: undo_id,
+            kind: Default::default(),
+            actor: activitypub_federation::fetch::object_id::ObjectId::from(local_actor.ap_id.clone()),
+            object: serde_json::json!({
+                "type": "Announce",
+                "actor": local_actor.ap_id.to_string(),
+                "object": object_ap_id.to_string(),
+            }),
+        };
+
+        let inboxes = collect_inboxes(&accepted);
+        let sends = activitypub_federation::activity_sending::SendActivityTask::prepare(
+            &activitypub_federation::protocol::context::WithContext::new_default(undo),
+            &local_actor,
+            inboxes,
+            &data,
+        )
+        .await?;
+        let failures = send_with_retry(sends, &data).await;
+        if !failures.is_empty() {
+            tracing::warn!(count = failures.len(), "some Undo(Announce) deliveries failed");
+        }
+        Ok(())
+    }
+
     pub async fn follow(&self, local_user_id: uuid::Uuid, handle: &str) -> anyhow::Result<()> {
         let data = self.federation_config.to_request_data();
 
@@ -1383,6 +1445,19 @@ impl domain::ports::OutboundFederationPort for ActivityPubService {
             .await
             .map_err(|e| domain::errors::DomainError::Internal(e.to_string()))
     }
+
+    async fn broadcast_undo_announce(
+        &self,
+        booster_user_id: &domain::value_objects::UserId,
+        object_ap_id: &str,
+    ) -> Result<(), domain::errors::DomainError> {
+        let user_uuid = booster_user_id.as_uuid();
+        let ap_id = url::Url::parse(object_ap_id)
+            .map_err(|e| domain::errors::DomainError::Internal(e.to_string()))?;
+        self.broadcast_undo_announce_to_followers(user_uuid, ap_id)
+            .await
+            .map_err(|e| domain::errors::DomainError::Internal(e.to_string()))
+    }
 }
 
 #[cfg(test)]

crates/application/src/services/federation_event.rs

@@ -58,6 +58,17 @@ impl FederationEventService {
                 self.ap.broadcast_announce(user_id, &object_ap_id).await
             }
 
+            DomainEvent::BoostRemoved { 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_undo_announce(user_id, &object_ap_id).await
+            }
+
             _ => Ok(()),
         }
     }
@@ -82,10 +93,11 @@ mod tests {
 
     #[derive(Default)]
     struct SpyPort {
-        created:   Mutex<Vec<ThoughtId>>,
-        deleted:   Mutex<Vec<String>>,
-        updated:   Mutex<Vec<ThoughtId>>,
-        announced: Mutex<Vec<String>>,
+        created:        Mutex<Vec<ThoughtId>>,
+        deleted:        Mutex<Vec<String>>,
+        updated:        Mutex<Vec<ThoughtId>>,
+        announced:      Mutex<Vec<String>>,
+        undo_announced: Mutex<Vec<String>>,
     }
 
     #[async_trait]
@@ -106,6 +118,10 @@ mod tests {
             self.announced.lock().unwrap().push(ap_id.to_string());
             Ok(())
         }
+        async fn broadcast_undo_announce(&self, _: &UserId, ap_id: &str) -> Result<(), DomainError> {
+            self.undo_announced.lock().unwrap().push(ap_id.to_string());
+            Ok(())
+        }
     }
 
     fn alice() -> User {
@@ -354,6 +370,50 @@ mod tests {
         assert!(spy.created.lock().unwrap().is_empty());
     }
 
+    #[tokio::test]
+    async fn boost_removed_sends_undo_announce_for_local_thought() {
+        let store = TestStore::default();
+        let alice = alice();
+        let thought = local_thought(alice.id.clone()); // ap_id = None → constructed URL
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::BoostRemoved {
+                user_id: alice.id.clone(),
+                thought_id: thought.id.clone(),
+            })
+            .await
+            .unwrap();
+
+        let undo_announced = spy.undo_announced.lock().unwrap();
+        assert_eq!(undo_announced.len(), 1);
+        assert_eq!(undo_announced[0], format!("https://example.com/thoughts/{}", thought.id));
+    }
+
+    #[tokio::test]
+    async fn boost_removed_sends_undo_announce_for_remote_thought() {
+        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/456".into());
+        store.thoughts.lock().unwrap().push(thought.clone());
+
+        let spy = Arc::new(SpyPort::default());
+        svc(&store, spy.clone())
+            .process(&DomainEvent::BoostRemoved {
+                user_id: alice.id.clone(),
+                thought_id: thought.id.clone(),
+            })
+            .await
+            .unwrap();
+
+        let undo_announced = spy.undo_announced.lock().unwrap();
+        assert_eq!(undo_announced.len(), 1);
+        assert_eq!(undo_announced[0], "https://mastodon.social/users/bob/statuses/456");
+    }
+
     #[tokio::test]
     async fn thought_updated_does_not_broadcast_if_user_missing() {
         let store = TestStore::default();

crates/domain/src/ports.rs

@@ -267,4 +267,11 @@ pub trait OutboundFederationPort: Send + Sync {
         booster_user_id: &UserId,
         object_ap_id: &str,
     ) -> Result<(), DomainError>;
+
+    /// Fan out an Undo(Announce) to followers when a boost is removed.
+    async fn broadcast_undo_announce(
+        &self,
+        booster_user_id: &UserId,
+        object_ap_id: &str,
+    ) -> Result<(), DomainError>;
 }