I've just released a simple #ActivityPub debugging tool. It's hosted on Glitch so you don't have to worry about spinning up servers or SSL certs etc. You clone your own copy of the project, set it up, and you can create ActivityPub accounts that can send any raw JSON payload you specify to its followers. I use it for testing new and novel ActivityPub objects, and to test compatibility with messages sent from remote servers without needing to create an account on them.
Oh and here's the thread I made on #SocialHub if you're an AP developer who's using that.
@darius the hero we don't deserve :)
@darius this looks like it will be very useful. Thank you!
@darius this sounds great, thank you!
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