Extremely pleased to announce the release of the ActivityPub Fuzzer! This is a tool that runs in a local development environment and (partially) emulates known ActivityPub software. It can even create a fake local "public fire hose" shaped like what you might see if you hooked your in-development software up to the real Fediverse.
I built this because I wanted ways to test in-progress fedi software for compatibility without actually making it live and breaking things.