[jimbobimbo]

Sorry, could you elaborate? If I'm creating an API using, say, ASP.NET Core or Go, I can generate OpenAPI spec out of actual implementation. How this "IDL" fits into the workflow? Is this another output in addition to OpenAPI spec?

[bterlson]

TypeSpec is designed primarily as an API first tool as opposed to being an output. In the context of ASP.NET and HTTP/REST APIs, our goal is that you can write your spec and generate much of the service implementation and clients. From this same source of truth you could also emit clients or service implementations in other API definition formats like OpenAPI, schemas like JSON Schema, and other things besides.

[BillyTheKing]

In my (limited) experience so far with TypeSpec - it really shines in an API first approach, so you define your API before you implement it, but not so much the other way around.