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Fix FastAPI mounting examples in docs (#2962)

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Jeremiah Lowin 2026-01-20 17:58:39 -05:00 • committed by GitHub
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2 changed files with 20 additions and 13 deletions
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23
docs/deployment/http.mdx
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@ -344,13 +344,6 @@ Here's a quick example showing how to add MCP to an existing FastAPI application
from fastapi import FastAPI
from fastmcp import FastMCP
# Your existing API
api = FastAPI()
@api.get("/api/status")
def status():
return {"status": "ok"}
# Create your MCP server
mcp = FastMCP("API Tools")
@ -359,14 +352,28 @@ def query_database(query: str) -> dict:
"""Run a database query"""
return {"result": "data"}
# Create the MCP ASGI app with path="/" since we'll mount at /mcp
mcp_app = mcp.http_app(path="/")
# Create FastAPI app with MCP lifespan (required for session management)
api = FastAPI(lifespan=mcp_app.lifespan)
@api.get("/api/status")
def status():
return {"status": "ok"}
# Mount MCP at /mcp
api.mount("/mcp", mcp.http_app())
api.mount("/mcp", mcp_app)
# Run with: uvicorn app:api --host 0.0.0.0 --port 8000
```
Your existing API remains at `http://localhost:8000/api` while MCP is available at `http://localhost:8000/mcp`.
<Warning>
Just like with Starlette, you **must** pass the lifespan from the MCP app to FastAPI. Without this, the session manager won't initialize properly and requests will fail.
</Warning>
## Mounting Authenticated Servers
<VersionBadge version="2.13.0" />

10
docs/integrations/fastapi.mdx
View file

@ -390,14 +390,14 @@ def get_user(user_id: int):
When mounting MCP servers, always pass the lifespan context:
```python
# Correct - lifespan passed
mcp_app = mcp.http_app(path='/mcp')
# Correct - lifespan passed, path="/" since we mount at /mcp
mcp_app = mcp.http_app(path="/")
app = FastAPI(lifespan=mcp_app.lifespan)
app.mount("/mcp", mcp_app)
app.mount("/mcp", mcp_app) # MCP endpoint at /mcp
# Incorrect - missing lifespan
app = FastAPI()
app.mount("/mcp", mcp.http_app()) # Session manager won't initialize
app.mount("/mcp", mcp.http_app(path="/")) # Session manager won't initialize
```
If you're mounting an authenticated MCP server under a path prefix, see [Mounting Authenticated Servers](/deployment/http#mounting-authenticated-servers) for important OAuth routing considerations.
@ -427,7 +427,7 @@ async def app_lifespan(app: FastAPI):
# Create MCP server
mcp = FastMCP("Tools")
mcp_app = mcp.http_app()
mcp_app = mcp.http_app(path="/")
# Combine both lifespans
app = FastAPI(lifespan=combine_lifespans(app_lifespan, mcp_app.lifespan))