I think it is probably a bad idea to just restrict it to things with 'type': "Application", "Group", "Organization", "Person", and "Service". Restricting it to just those would mean you couldn't have new actor types (and sub-types) in the future.

So then, do we do it in a duck-typing way? And if "yes", how?

Maybe if something has an "inbox" OR and "outbox" it is an Actor. I.e., it could have just one of those.

#ActivityPub #ActivityStreams #FediDev