Index: src/routes/posts.js
===================================================================
--- src/routes/posts.js	(revision 7c44c1227391a3e62327e785d8a67b661aaae489)
+++ src/routes/posts.js	(revision ffa53cc6a9b4e745a21bddde2ecd0e463566def8)
@@ -538,5 +538,5 @@
     // Resolve + deliver in the background so Send responds instantly.
     ActivityPubService.resolveRemoteNote(uri)
-      .then((parent) => parent && ActivityPubService.deliverReply(site, { postId: '', postSlug: null, parent, text }))
+      .then((parent) => parent && ActivityPubService.deliverReply(site, { postId: parent.localPostId || '', postSlug: null, parent, text }))
       .catch((e) => console.warn('[AP] remote reply failed:', e.message));
   }