Our first attempt at a TypeSpec-like thing was in fact a TypeScript dialect! It works great for simple cases, but at the limit it falls over for a number of reasons. HTTP needs a lot of metadata that isn't found in TypeScript, things like HTTP verbs, query/path params, headers, routes, etc. API patterns also need their own metadata, like for pagination you need to know various bits of metadata that describe how to paginate an endpoint. Also many things in the TypeScript type system and stdlib don't apply in the context of API descriptions, so we needed a subset.

So after going down this road for a while, we found we were in a place where the syntax was actually very complex, with metadata spread between deeply nested generic type instantiations and JSDoc comments, and didn't feel at all like TypeScript. This didn't align with our goals of having something simple and easy to learn for all devs.

So I wrote the first implementation of a custom language in a day and folks really liked the direction. 4 years later, here we are!