From 27d318810fc00c75445d204838ee37b8e3bfb3e0 Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Tue, 20 Jan 2026 17:58:39 -0500
Subject: [PATCH] Fix FastAPI mounting examples in docs (#2962)
---
docs/deployment/http.mdx | 23 +++++++++++++++--------
docs/integrations/fastapi.mdx | 10 +++++-----
2 files changed, 20 insertions(+), 13 deletions(-)
diff --git a/docs/deployment/http.mdx b/docs/deployment/http.mdx
index 962c2e013..17ec9e825 100644
--- a/docs/deployment/http.mdx
+++ b/docs/deployment/http.mdx
@@ -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`.
+
+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.
+
+
## Mounting Authenticated Servers
diff --git a/docs/integrations/fastapi.mdx b/docs/integrations/fastapi.mdx
index 2ce28336d..abdc1d0e7 100644
--- a/docs/integrations/fastapi.mdx
+++ b/docs/integrations/fastapi.mdx
@@ -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))