Fast API Auto MCP — native MCP server support via mcp_url

[swe-brain]

Summary

Adds first-class MCP (Model Context Protocol) support to FastAPI. Enable with a single parameter: app = FastAPI(mcp_url="/mcp"). FastAPI automatically generates an MCP server at that path exposing all schema-included routes as callable tools, executed in-process with zero network overhead.

Trying It on an Existing FastAPI App

1. Install FastAPI from this fork + the MCP dependency

pip install "git+https://github.com/ArcInstitute/fastapi.git@claude/swe-brain/fast-api-auto-mcp" "mcp>=1.9"

This installs FastAPI from the feature branch (which has mcp_url support) alongside the MCP SDK. The mcp_url parameter doesn't exist in the PyPI release yet.

2. Add mcp_url to your app

# Before
app = FastAPI()

# After — one line change
app = FastAPI(mcp_url="/mcp")

3. Run your app normally

uvicorn myapp:app --reload

4. Connect Claude Desktop

Add this to your claude_desktop_config.json (usually at ~/Library/Application Support/Claude/claude_desktop_config.json):

{
  "mcpServers": {
    "my-api": {
      "command": "npx",
      "args": ["mcp-remote", "http://localhost:8000/mcp/"]
    }
  }
}

Restart Claude Desktop. Your API routes will appear as tools Claude can call directly.

5. Quick smoke-test (no Claude Desktop needed)

import httpx

with httpx.Client(base_url="http://localhost:8000", follow_redirects=True) as client:
    resp = client.post(
        "/mcp/",
        json={"jsonrpc": "2.0", "id": 1, "method": "tools/list", "params": {}},
        headers={"Content-Type": "application/json", "Accept": "application/json, text/event-stream"},
    )
    print(resp.json())

Note: The MCP endpoint is at /mcp/ (trailing slash). Real MCP clients follow the 307 redirect automatically; when using httpx directly pass follow_redirects=True or hit /mcp/ directly.

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Implementation Plan

Files Created

fastapi/mcp/__init__.py — public exports (MCPApp)

fastapi/mcp/generator.py — converts APIRoute objects to MCP Tool definitions:

• Iterates routes with include_in_schema=True
• Tool name: route.unique_id (e.g. read_item_items__item_id__get)
• Tool description: summary or description or "METHOD /path"
• Input schema: path params + query params as top-level fields; request body as body field
• Reuses already-generated app.openapi() schema — no re-computation

fastapi/mcp/server.py — stateless MCPApp ASGI app:

• Per request: creates a StreamableHTTPServerTransport with is_json_response_enabled=True
• Runs MCP server alongside transport in an anyio task group (stateless mode)
• Tool calls execute via httpx.AsyncClient(transport=ASGITransport(app=fastapi_app)) — in-process, zero network overhead, passes through all FastAPI middleware
• Handles path params (substituted into URL template), query params, and JSON body

Files Modified

fastapi/applications.py — added mcp_url: str | None = None parameter with full annotated_doc docstring; stores as self.mcp_url; mounts MCPApp in setup() when set; raises helpful ImportError if mcp package not installed

pyproject.toml — added [project.optional-dependencies] mcp = ["mcp>=1.9"]

Tests

tests/test_mcp.py — 7 tests × 2 backends (asyncio + trio) = 14 total, all passing:

1. test_mcp_disabled_by_default — FastAPI() without mcp_url → 404 at /mcp
2. test_mcp_custom_url — FastAPI(mcp_url="/api/mcp") mounts at custom path
3. test_mcp_tools_list — tools/list returns all schema-included routes
4. test_include_in_schema_false_excluded — include_in_schema=False routes excluded
5. test_mcp_tool_call_get — GET endpoint callable via MCP tools/call
6. test_mcp_tool_call_post_with_body — POST with Pydantic body callable via MCP
7. test_mcp_path_params — path parameters correctly substituted

Test Plan

• All 14 tests pass (pytest tests/test_mcp.py -v)
• No regressions in existing test suite
• MCP disabled by default — no breaking changes to existing apps
• Import error raised with helpful message when mcp not installed

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Implemented by swe-brain · Planning issue: https://github.com/ArcInstitute/swe-brain/issues/7

[swe-brain]

Gentle reminder: this PR has been open for 6 days awaiting review. @sullivanj91