docs/patterns/fastapi.mdx

---
title: FastAPI Integration
sidebarTitle: FastAPI
description: Automatically create FastMCP servers directly from FastAPI applications.
icon: square-bolt
---

If you build your APIs using the popular [FastAPI](https://fastapi.tiangolo.com/) framework, FastMCP offers a seamless way to expose your FastAPI application as an MCP server. This leverages the OpenAPI integration internally but simplifies the setup significantly.

## The Goal: FastAPI App -> MCP Server

FastAPI automatically generates an OpenAPI specification for your application. FastMCP uses this built-in capability to create an MCP server that mirrors your API routes.

- FastAPI path operations (`@app.get`, `@app.post`, etc.) become MCP tools, resources, or templates.
- Pydantic models used in FastAPI for request/response validation are used to generate MCP schemas.
- Communication happens directly in memory, making it very efficient.

## Creating from FastAPI App

Use the `FastMCP.from_fastapi()` class method. You only need your FastAPI `app` instance.

```python
import asyncio
from fastapi import FastAPI
from pydantic import BaseModel
from fastmcp import FastMCP, Client # Import FastMCP and Client

# 1. Define your FastAPI application
api_app = FastAPI(title="MyFastAPIApp")

class Item(BaseModel):
    name: str
    price: float
    is_offer: bool | None = None

@api_app.get("/")
def read_root():
    return {"Hello": "World"}

@api_app.get("/items/{item_id}") # -> Resource Template
def read_item(item_id: int, q: str | None = None):
    # This will become resource://openapi/read_item_items__item_id__get/{item_id}
    return {"item_id": item_id, "q": q, "description": f"Details for item {item_id}"}

@api_app.post("/items/") # -> Tool
def create_item(item: Item):
    # This will become the 'create_item_items__post' tool
    print(f"Creating item: {item.name}")
    return {"item_name": item.name, "status": "created"}

# 2. Create the FastMCP server directly from the FastAPI app
# This is an async class method
async def create_mcp_server_from_fastapi():
    mcp_server = await FastMCP.from_fastapi(
        app=api_app,
        name="FastAPI_MCP_Bridge" # Optional name for the MCP server
    )
    return mcp_server

# 3. (Example) Run the MCP server and test with an in-memory client
async def run_and_test():
    server = await create_mcp_server_from_fastapi()
    print(f"Created MCP server '{server.name}' from FastAPI app '{api_app.title}'")

    # List discovered components
    tools = await server.list_tools()
    templates = await server.list_resource_templates()
    print("Discovered Tools:", [t.name for t in tools])
    print("Discovered Templates:", [t.uriTemplate for t in templates])

    # Test using an in-memory client
    client = Client(server) # Uses FastMCPTransport
    async with client:
        # Call the tool derived from POST /items/
        create_result = await client.call_tool(
            "create_item_items__post",
            {"name": "MCP Special", "price": 99.99} # Pydantic model fields become args
        )
        print("Create Item Tool Result:", create_result[0].text) # JSON string

        # Read the resource derived from GET /items/{item_id}
        read_result = await client.read_resource(
            "resource://openapi/read_item_items__item_id__get/42" # Match template URI
        )
        print("Read Item Resource Result:", read_result[0].text) # JSON string

    # In a real scenario, you might run the MCP server via stdio or sse
    # print("Running MCP server via stdio...")
    # server.run()

if __name__ == "__main__":
    # Requires fastapi, uvicorn, httpx:
    # uv pip install "fastapi[all]" httpx
    try:
        asyncio.run(run_and_test())
    except ImportError as e:
        print(f"Error: {e}. Please install required packages: uv pip install \"fastapi[all]\" httpx")

# Example Output might include:
# Created MCP server 'FastAPI_MCP_Bridge' from FastAPI app 'MyFastAPIApp'
# Discovered Tools: ['read_root___get', 'create_item_items__post']
# Discovered Templates: ['resource://openapi/read_item_items__item_id__get/{item_id}']
# Create Item Tool Result: {"item_name": "MCP Special", "status": "created"}
# Read Item Resource Result: {"item_id": 42, "q": null, "description": "Details for item 42"}
```

### How it Works Internally

1.  **OpenAPI Generation**: `from_fastapi` asks the FastAPI `app` for its OpenAPI schema dictionary (`app.openapi()`).
2.  **In-Memory Client**: It creates an `httpx.AsyncClient` configured with an `ASGITransport`. This special transport allows `httpx` to call the FastAPI application directly in memory without needing a running web server process.
3.  **OpenAPI Integration**: It calls `FastMCP.from_openapi()`, passing the generated schema and the in-memory `httpx` client.
4.  **MCP Server Creation**: The standard OpenAPI integration logic then proceeds to parse the schema and create the `Tool`, `Resource`, and `ResourceTemplate` components that wrap calls to the in-memory FastAPI app.

This provides a highly efficient way to expose your FastAPI logic through the MCP protocol, leveraging FastAPI's routing, dependency injection, and validation features.