In a REST approach, I'd expect an LLM to need some kind of initial entry point, much like needing the initial URL for the HN home page. From there the LLM should be able to parse and discover possible actions, call those actions, and understand the response only by parsing the results and any schemas provided.
In any case, we're on the same page about REST. OpenAPI was mentioned though and I don't think that's ever actually compatible with that type of usage.
I don't think I've ever encountered an API following OpenAPI that was completely self describing and could be discovered and navigated from a single entry URL. They're always paired with some swagger docs or other external content. That instantly makes them not RESTful.
But you're totally right that my comment didn't fit. Might have meant it for the grandparent comment? Not sure.