@sean @alex In Mastodon, the code to receive the circle is 1 commit + fix below.

Add support for dereferencing objects through bearcaps #14683
https://github.com/tootsuite/mastodon/pull/14683
https://github.com/tootsuite/mastodon/pull/14981

See below for the technology used.
Bearcaps
https://docs.joinmastodon.org/spec/bearcaps/

The implementation of the circle has not yet been finalized as to how it will be positioned in Mastodon. However, simply adding the code for Bearcaps to Pleroma makes it more interoperable.