The API library I wrote for my last couple projects required the developer to fill in the openapi spec specifics and said spec was the part of the API itself, making it difficult to add something to the API that wasn't also in the spec.

Incoming request params, became validated and casted object properties. Outgoing response params were validated and casted according to spec.

In the end I think it worked really well, and loved not needing to maintain the spec separately. The annoying bit was adjusting the library when the spec changed.

And some gnarly bits of the spec that weren't easy to implement logically.

At any rate, it also made for a similar experience of considering the client experience while writing/maintaining the api.