First of all, this implies client-side validation and handling errors. Irrelevant to the communication technology in use, applications can't rely on external systems to provide responses that are not malformed or not simply wrong.

This doesn't mean that clients immediately have to reject all responses that are not perfect in the application's understanding. Responses that contain more information or slightly different formats than expected, but are still understandable, should not lead to immediate errors. Following the principle to be conservative in what you do and liberal in what you accept, messages that contain just enough for the application to do its job should be accepted. For example, additional, unknown properties in JSON responses should not lead to refusing to map the object.