This is a really interesting approach! How did you end up deciding on executable markdown as opposed to other approaches?
and moreover we had this fundamental idea that documentation and testing should be in a single source of truth to avoid documentation drift (not just the spec drift).
And hence we came up with the idea of using markdown as the baseline and doing everything around it.
We have it in beta now : https://voiden.md/download (Only Linux and Mac)
And in the future we also plan to support graphQL but not smithy for now.
Also, while a nice API document is a godsend (and sadly often missing in practice), a way to generate the consuming side of the API (again via various mechanisms) is also a very useful thing.
Re-reading the page, perhaps I got it wrong and it works the other way around? Voiden uses e.g. OpenAPI files and verifies it's still compliant with them? That would work, although it's a bit of a double effort.. still, useful in many cases.