You can still keep using Rest API with swagger docs and tell the AI to read the swagger docs. It's the same thing. The entire Rest API specification is a lot more flexible than the JSON RPC format /mcp uses.
Personally, for just tools, I wire up tool calls with context captured via system prompt.
ideally what you would have is skills + scripts that the agent can load progressively to interact with the api
or even better: a single cli tool the agent is aware of, and which combines `--help` output with `tldr` tips, when the command is incomplete, and prints out helpful error messages when the incantation is wrong