typespec VS core-crypto

It does not exist, but it will be worked on fairly soon. You can track progress on GitHub [1]. It will be as you suggest - a one time conversion, after which you can iterate. We have this workflow for inside Azure, but the converter is Azure-specific (e.g. converts to TypeSpec that uses our extensive Azure-specific component library). It will take a bit to generalize.

[1]: https://github.com/microsoft/typespec/issues/3038

Afaik there's nothing that's widely used and of which I'd say I prefer it. But I'm having hopes for https://github.com/microsoft/typespec (former cadl). The reason being that yaml/JSON specs get huge very quickly, OpenAPI has to fight with restrictions of yaml/JSON ($refs, splitting up into files, oneOf, etc.) and tooling is ok but not great.

You've touched on some use cases I've thought about and led me to ask the question.

I also recently saw Microsoft take a higher level approach with CADL/Typespec: https://microsoft.github.io/typespec/ which could be cool if it reaches wide adoption.

This one just crossed my feed today: Cadl, "... a language for describing cloud service APIs and generating other API description languages, client and service code, documentation, and other assets."

We're using it at Wire to target Android and iOS; For WASM we use wasm-bindgen with a custom TypeScript wrapper to make it better to use (Rust Results are notoriously bad over the WASM FFI so we deconstruct errors on the rust side as JSON and reconstruct them on the TS side to get "rich errors").