This topic is for collecting resources about ActivityPub that relate to implementing the Client-to-Server (C2S) parts of the specification.
Bovine C2S tests in Python
Helge implements Bovine, a modular ActivityPub server, and is writing a suite of tests for C2S in Python.
Graffiti project, Anthony Wang
Anthony wrote a blog We're Missing Half ActivityPub's Potential! about the potential of C2s and is involved with the MIT project Graffiti to research this area (also mentioned here).