I've started formalizing what I'm using for filtering on #fedbox into a FEP[1].
> I’d like to discuss an easy way to query a collection, for example, using a well-defined query parameter or an HTTP header. You’d send a request to the collection ID URL, specifying the ID of the object you’re checking for.
In my implementation you use iri={url} of the item you're searching. If it's there you receive a collection with just it as an element.
@grishka @activitypub
An example of it working is this (search for 2 activities): https://federated.id/inbox?iri=https://federated.id/activities/7db0d6fc-a007-4eee-998a-a28babf7b577&iri=https://federated.id/activities/486c7b6d-7feb-406f-bb4b-d64224c5d3f5