If you want to precisely capture when a field must be present (or will always be set on the server), the field_behavior annotation captures more of the nuance than proto2's required fields: https://github.com/googleapis/googleapis/blob/master/google/...

You could (e.g.) annotate all key fields as IDENTIFIERs. Client code can assume those will always be set in server responses, but are optional when making an RPC request to create that resource.

(This may just work in theory, though – I’m not sure which code generators have good support for field_behavior.)