For a sneak preview of my followup to the Fediverse Schema Observatory, there's now a project page on the lab website for the ActivityPub Fuzzer:
https://asml.cyber.harvard.edu/activitypub-fuzzer/
Basically it lets an ActivityPub server developer test compatibility with simulated messages from Fediverse software known to the Observatory (dozens of software projects and hundreds of versions!). The dev can do this entirely in a local dev environment.
The project is still in the works but I'll be releasing the code soon.