i think an Agent is an LLM that interacts with the outside world via a protocol like MCP, that's a kind of REST-like protocol with a detailed description for the LLM on how to use it. An example is an MCP server that knows how to look up the price for a given stock ticker, so it enables the LLM to tell the current price for that ticker.

see: https://github.com/luigiajah/mcp-stocks

The implementation: https://github.com/luigiajah/mcp-stocks/blob/main/main.py

Each MCP endpoint comes with a detailed comment - that comment will be part of the metadata published by the MCP server / extension. The LLM reads this instruction when the MCP extension is added by the end user, so it will know how to call it.

The main difference between REST an MCP is that MCP can maintain state for the current session (that's an option), while REST is supposed to be inherently stateless.

I think most of the other protocols are a variation of MCP.