Agreed. The state since the original success may have been mutated by another request since (or simply changed state as a result of any other async process), an

Agreed. The state since the original success may have been mutated by another request since (or simply changed state as a result of any other async process), and the response may simply be a rendering of the latest server state. The response of the server absolutely should not be specified as being identical, since that's not how idempotent requests work. At that point, you're really just specifying a caching key for verbs that have side effects.