MCP + FastAPI demo

Small example showing how a normal FastAPI REST API becomes an MCP server that an LLM client (Claude Desktop, Claude Code, etc.) can call as tools.

What's here

• main.py — a tiny "Tasks" REST API (/tasks, /tasks/{id}, ...) built with plain FastAPI, plus 3 lines at the bottom that wrap it with fastapi-mcp.
• fastapi-mcp introspects your routes (via their operation_id) and auto-generates an MCP tool for each one — no separate tool definitions to hand-write. It mounts an MCP endpoint at /mcp alongside your normal REST routes.

How MCP fits in

MCP (Model Context Protocol) is a standard way for an LLM client to discover and call "tools" exposed by a server. Normally you'd hand-write a tool schema for each capability. fastapi-mcp skips that by reusing the OpenAPI schema FastAPI already generates from your Pydantic models and route signatures — so each REST endpoint becomes both a normal HTTP route and an MCP tool, for free.

Run it

cd ~/mcp-fastapi-demo
python3 -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
uvicorn main:app --reload

• REST API + docs: http://127.0.0.1:8000/docs
• MCP endpoint: http://127.0.0.1:8000/mcp

Try it as a normal REST API first

curl -X POST http://127.0.0.1:8000/tasks -H 'Content-Type: application/json' -d '{"title": "learn MCP"}'
curl http://127.0.0.1:8000/tasks

Connect an MCP client

Option A: MCP Inspector (fastest way to poke at it)

npx @modelcontextprotocol/inspector

Point it at http://127.0.0.1:8000/mcp as an SSE/HTTP server and you'll see list_tasks, create_task, get_task, complete_task, delete_task show up as callable tools — one per FastAPI route.

Option B: Claude Desktop

Add to your Claude Desktop MCP config (claude_desktop_config.json):

{
  "mcpServers": {
    "tasks": {
      "url": "http://127.0.0.1:8000/mcp"
    }
  }
}

Restart Claude Desktop, and you should be able to ask it to list/create tasks — it'll call your FastAPI endpoints under the hood.

Key takeaway

operation_id in each @app.get/post/... decorator becomes the MCP tool name, and the summary becomes its description. Add a new FastAPI route with an operation_id and it automatically becomes a new MCP tool — that's the whole trick.