@helge ActivityPub requires application/ld+json; profile="https://www.w3.org/ns/activitystreams"

https://www.w3.org/TR/activitypub/#retrieving-objects

The shorter form application/activity+json is nicer, but media type is one of the things about which specification is not ambiguous, so I always use the required one.

application/activity+json is (or was) used by Mastodon, so of course everyone copy-pasted it into their projects.