I don't understand how if you're on a team in which you control both the front end and back end, you wouldn't understand these factors - "Will the server send IDs as ints or strings? Will it accept either? Which params are optional? What shape does the data come back in, an array of results, or an objects with results nested somewhere? etc"

Surely you would know all the answers to these questions because your team control the entire stack.

If it was the case that the FE and BE teams were separate, then these would be reasonable considerations, but I just don't see it in a team dealing with full control.