seekItem is a new spec for finding objects by id inside collections:

https://swicg.github.io/activitypub-api/seekitem

It can help with catching up on a collection, or with determining membership in a collection.

In our ActivityPub API Task Force meeting today, we decided to move this spec forward to CG-DRAFT status. That means that we intend to make it a CG-FINAL report, and we're soliciting implementations and comments.

I've implemented it twice on the server side, in activitypub-bot and onepage.pub. It was easy, since I already had internal functions for seeking items inside a collection with indexed lookup, so O(log N) complexity.

I am working on using it on the client side; for example, confirming that if A follows B the B is in A's following collection and A is in B's followers collection. It's a hassle with serial access, but I think it can be quite quick with seekItem.