Mastodon reply to a message

Software Mastodon
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Hi #MastoAdmin and ActivityPub developers :waving_hand:

I’m currently testing an OSSN-based social platform that federates via ActivityPub. Hashtags and Likes are received properly (via signed Create and Like activities), but replies to our posts aren’t being received.

Here’s what I need help with:

:white_check_mark: Our posts include a valid Note with:

  • to: https://www.w3.org/ns/activitystreams#Public
  • url: pointing to our canonical post URL
  • id: full post URL (same as url)
  • published, attributedTo, etc.

:puzzle_piece: When replying from Mastodon to one of our posts, the ActivityPub message does contain an inReplyTo with the full URL of the original post.

However:
:cross_mark: No Create activity with Note and inReplyTo is received on our inbox endpoint.
:white_check_mark: We do receive Like, Undo, and Delete activities without problems.

Question:
:backhand_index_pointing_right: Under what exact conditions does Mastodon send a federated reply (as a Create with Note and inReplyTo) to the original post’s inbox?
Do we need to:

  • support Accept for incoming Follow?
  • add a cc field with the replier’s inbox?
  • expose a replies collection?
  • be whitelisted somehow?

Thanks for any pointers!