If you truly care about that, the truly REST solution would be to define separate MediaTypes and use Accepts headers to specify which MediaType is desired. So

If you truly care about that, the truly REST solution would be to define separate MediaTypes and use Accepts headers to specify which MediaType is desired. So if you wanted comments you would send a request with a header like "Accept: application/resource_with_comments", and you'd send a Header like "Accept: application/resource_without_comments". The thing is, few people want this level of control in practice. This would generally lead to a huge proliferation of types for very little practical benefit. Much simpler to have a set of well-defined types with optional fields than to define a new type for each combination of fields used today.