When I say REST, I basically mean HTTP/JSON. I know. Shame on me.

But there's a pretty large body of RFCs that define what makes a good REST API. For instance POST should create things, PUT should re-define them, PATCH should incrementally update them, GET must be side-effect-free, etc. And there's a ton of standard HTTP headers. Pair it with OpenAPI and the whole thing is self-documenting. You can achieve the same effect with gRPC + reflection, or GraphQL + reflection, and there's plenty of mature tooling and interoperability for each system.

By comparison, MCP actually has relatively few conventions. LLMs are very familiar with REST and OpenAPI already. Just use that.