One thing I do not see with Stainless is also using it to generate the server side of the API.
We use OpenAPI as a design doc, and generate client SDKs and Datos for the server from the same spec. This gives us pretty solid control over interoperability.
Yeah, Stainless is more focused on public SDKs, which can be a lot trickier and more demanding.
Some more details replying to your other comment here: >>40148629