You could get pretty far with a set of agent-focused routes mounted under e.g. an /agents path in your API.

There'd be a little extra friction compared to MCP – the agent would presumably have to find and download and read the OpenAPI/Swagger spec, and the auth story might be a little clunkier – but you could definitely do it, and I'm sure many people do.

Beyond that, there are a few concrete things MCP provides that I'm a fan of:

- first-class integration with LLM vendors/portals (Claude, ChatGPT, etc), where actual customers are frequently spending their time and attention

- UX support via the MCP Apps protocol extension (this hasn't really entered the zeitgeist yet, but I'm quite bullish on it)

- code mode (if using FastMCP)

- lots of flexibility on tool listings – it's trivial to completely show/hide tools based on access controls, versus having an AI repeatedly stumble into an API endpoint that its credentials aren't valid for

I could keep going, but the point is that while it's possible to use another tool for the job and get _something_ up and running, MCP (and FastMCP, as a great implementation) is purpose built for it, with a lot of little considerations to help out.