Hello,
I’m glad to announce a first release of activitypub* packages, implementing (well, trying to implement some flavor of) both server-to-server and client-to-server activitypub protocols.
Documentation is available from the web site.
The activitypub_server package installs TAPS, an experimental server, handling some common activities. Accounts hosted by this server can at least follow and be followed by mastodon instances, and post and receive activities (Create, Announce, Like, Undo, …).
The library of the activitypub_client package can be used by a client application to post and receive activities to and from a server (though it was only tested with TAPS). See a simple example here.
A GUI client (based on Stk is installed by the activitypub_gui package. It requires a client configuration file as described here. You can drop IRIs/URLs of an actor in the window to open a tab and be able to follow this actor. The GUI also allows to create and post notes with attachments. This client is still very experimental and will be developed more in the future.
EDIT: the package should be available soon in opam.