Nothing. MCP and HTTP APIs and CLI tools without the good parts. They lack the robustness of the OpenAPI spec, including security standardization, and are more complex to run than simple CLI utilities without any authentication.

I have done it many times, using the swagger.json as a "discovery service" and then having the agent utilize that API. A good OpenAPI spec was working perfectly fine for me all the way back when OpenAI introduced GPTs.

If we standardized on a discovery/ endpoint, or something like that, as a more compact description of the API to reduce token usage compared to consuming the somewhat bloated full OpenAPI spec, you would have everything you need right there.

The MCP side quest for AI has been one of the most annoying things in AI in recent years. Complete waste of time.