I'm running into what I think is an unresolved area of ambiguity as it relates to json:api's spec constraints around:

• Full linkage in compound documents, and
• Server-initiated sparse fieldsets.

Full linkage between a response's primary data and included documents is required in all cases, except:

...when relationship fields that would otherwise contain linkage data are excluded via sparse fieldsets.

In the sparse fieldsets section, we learn servers may respond by applying its own sparse fielset rule, in the absence of one being sent in the request query string:

If a client does not specify the set of fields for a given resource type, the server MAY send all fields, a subset of fields, or no fields for that resource type.

Does this mean, then, that the server may forego full linkage if it has opted to respond with a subset of fields?

If not, I think we need to add some sort of requirement stating that the obligation to send full linkage cannot be foregone by server-initiated sparse fielsets. (My invented term, but I think we need to call it something for purposes of this discussion.)