I don't think you and your parent mean the same thing. The client "has to know what to do in advance" in the sense that the human building the application needs to know what user interface they want to build, and know how the API works (in advance) in order to use it to populate the interface.

If you have an extremely dumb interface that just displays fields in API responses, then ok... but that's a very trivial UI that accounts for... basically no useful products ever.