Notes no longer delivered on Mastodon

ActivityPub Server to Server
1

I’m currently working on a service with ActivityPub support and mainly reused a custom implementation I’ve created some time ago (GitHub: GitHub - H4kor/fedi-games: Framework for Games running on ActivityPub Site:https://games.rerere.org/). During testing I’ve noticed that my messages are no longer showing up on mastodon. Here is an example that was accepted previously, but now does not show up in my stream:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "http://joinmastodon.org/ns#"
  ],
  "id": "https://games.rerere.org/games/rps/1753891843/activity",
  "type": "Create",
  "to": [
    "https://23.social/users/h4kor"
  ],
  "published": "2025-07-30T16:10:43Z",
  "actor": "https://games.rerere.org/games/rps",
  "object": {
    "id": "https://games.rerere.org/games/rps/1753891843",
    "type": "Note",
    "content": "I choose: 📄<br><br>I WIN! 😈",
    "attributedTo": "https://games.rerere.org/games/rps",
    "inReplyTo": "https://23.social/users/h4kor/statuses/114943055830461383",
    "tag": [
      {
        "id": "https://23.social/users/h4kor",
        "type": "Mention",
        "href": "https://23.social/users/h4kor"
      }
    ],
    "to": [
      "https://23.social/users/h4kor"
    ],
    "published": "2025-07-30T16:10:43Z"
  }
}

Have there been changes on what is accepted? the POST to the inbox is accepted with status code 202.

2

which server(s) are you testing on? fwiw, the note contents does look like spam to someone not familiar with it.

3

I’ve tested on 23.social, chaos.social and mastodon.social

Also tested with other content and public notes. Nothing shows up.

The account is also following the sender and this is in reply to a (private) mention

4

Found the problem:

Mastodon seems to be more pedantic about the Content-Type header.
This patch resolved the issue:

-       req.Header.Set("Accept", "application/ld+json")
+       req.Header.Set("Accept", "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"")
+       req.Header.Set("Content-Type", "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"")