crates/application/src/services/federation_event.rs

@@ -48,6 +48,29 @@ impl FederationEventService {
                     Some(u) => u,
                     None => return Ok(()),
                 };
+                // For replies to remote posts: in_reply_to_url is None but in_reply_to_id
+                // points to the locally-stored remote thought. Resolve its ap_id so the
+                // outbound Note includes inReplyTo and Mastodon threads it correctly.
+                let thought = if thought.in_reply_to_url.is_none() {
+                    if let Some(ref reply_id) = thought.in_reply_to_id {
+                        match self.thoughts.find_by_id(reply_id).await? {
+                            Some(parent) => {
+                                let parent_ap_url = parent.ap_id.unwrap_or_else(|| {
+                                    format!("{}/thoughts/{}", self.base_url, reply_id)
+                                });
+                                domain::models::thought::Thought {
+                                    in_reply_to_url: Some(parent_ap_url),
+                                    ..thought
+                                }
+                            }
+                            None => thought,
+                        }
+                    } else {
+                        thought
+                    }
+                } else {
+                    thought
+                };
                 self.ap
                     .broadcast_create(user_id, &thought, user.username.as_str())
                     .await