How to Connect FastAPI to Claude Desktop in 5 Minutes

What you will build

In this guide you will connect a FastAPI service to Claude Desktop using MCP. You will export your OpenAPI spec, generate an MCP server, and register the tool catalog so Claude can call your endpoints.

You do not need to change your FastAPI routes. The workflow builds a thin MCP layer on top of your existing API.

Step 1: Confirm OpenAPI is enabled

FastAPI exposes OpenAPI at /openapi.json by default. If you disabled docs, re-enable them for build time or export the spec through a script.

from fastapi import FastAPI

app = FastAPI(title="LegacyAI Demo API")

@app.get("/customers")
def list_customers():
    return [{"id": "c_1", "name": "Acme"}]

Step 2: Export the OpenAPI spec

You can fetch the spec from a running service or export it directly during CI. Save it as openapi.json for the MCP generator.

curl http://localhost:8000/openapi.json -o openapi.json

Step 3: Generate the MCP server with LegacyAI

Upload openapi.json to LegacyAI or use the CLI to generate an MCP server. The generator maps each path to a tool with a structured schema and safe defaults.

LegacyAI also detects auth schemes and lets you enforce read-only mode or scoped access before you go live.

Step 4: Register the MCP server in Claude Desktop

Claude Desktop reads an MCP configuration file that points to your MCP server. The exact config file location depends on your OS, but the structure is the same.

{
  "mcpServers": [
    {
      "name": "LegacyAI FastAPI",
      "command": "node",
      "args": ["./legacyai-mcp-server.js"],
      "env": {
        "API_BASE_URL": "https://api.yourcompany.com",
        "API_TOKEN": "${API_TOKEN}"
      }
    }
  ]
}

Step 5: Test a tool call

Open Claude Desktop, refresh tools, and ask the agent to call your endpoint. Start with a read-only route, confirm the request, and validate the response payload.

If the response fails, check the MCP server logs for validation errors. Most issues come from missing auth headers or mismatched parameter names.

Production tips

• Use read-only mode by default and enable writes only after testing.
• Rotate API keys and scope tokens per tool or environment.
• Enable logging so you can audit every tool call.
• Sync the MCP server when your OpenAPI spec changes.