Hi @aschrijver, I had a glimpse at it before. It’s very verbose (3500+ words) but terse on validation: Guide for new ActivityPub implementers
It doesn’t mention any aid to validate, right? Verbosity without reliable aid makes onboarding a horror, frankly. I found most documentation seen so far as useful to a newcomer as the Oxford Dictionary for a non-english-speaker to ask for the daytime. Work it through in total and get a doctoral degree after decades.