Comment by WickyNilliams

There is json-schema which is a sort of dialect/extension of OpenAPI which offers support for fetching relations (and relations of relations etc) and selecting a subset of fields in a single request https://json-schema.org/

I used this to get a fully type safe client and API, with minimal requests. But it was a lot of work to get right and is not as mainstream as OpenAPI itself. Gql is of course much simpler to get going