Good question! Right now the client and server talk over HTTP using routes that conform with the Activity Pub spec (everything resource under `/ap` on the server should be a valid Activity Stream document conforming to Activity Pub spec).

However:

1) this isn't tested with any other Activity Pub client, so there are probably some implementation differences (errors) to iron out.

2) Currently the server doesn't implement the full Activity Pub protocol, so an existing client will probably make calls that will 404.

3) The server doesn't use any kind of authentication becasue Chatter Net doesn't rely on any server authentication. Clients authenticate messages based on signatures, so it doesn't really matter where the messages come from. Activity Pub doesn't seem to say anything about how authentication should work, so this should in theory be compatible. But in practice, existing Activity Pub implementations seem to rely on certain authentication patterns.

But I would definitely like to get the project to be compatible Fediverse clients. So this is something I want to work towards.