FastMCP connected with Fast API results 400 error #1797
Description
Description
Everything was working fine, until i connected FastMCP with Fast API and started using connecting with my LangGraph application.
I'm getting the error:
Error in post_writer
Traceback (most recent call last):
File "/Users/shan/Documents/.env/lib/python3.11/site-packages/mcp/client/streamable_http.py", line 413, in post_writer
await handle_request_async()
File "/Users/shan/Documents//.env/lib/python3.11/site-packages/mcp/client/streamable_http.py", line 407, in handle_request_async
await self._handle_post_request(ctx)
File "/Users/shan/Documents//.env/lib/python3.11/site-packages/mcp/client/streamable_http.py", line 278, in _handle_post_request
response.raise_for_status()
File "/Users/shan/Documents//.env/lib/python3.11/site-packages/httpx/_models.py", line 829, in raise_for_status
raise HTTPStatusError(message, request=request, response=self)
httpx.HTTPStatusError: Client error '400 Bad Request' for url 'http://0.0.0.0:8001/mcp'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/400
Error initializing RecipeGraph:
Session termination failed: 400
Traceback (most recent call last):
File "/Users/shan/Documents/src/llm_graph.py", line 446, in <module>
asyncio.run(main())
File "/Users/shan/.pyenv/versions/3.11.10/lib/python3.11/asyncio/runners.py", line 190, in run
return runner.run(main)
^^^^^^^^^^^^^^^^
File "/Users/shan/.pyenv/versions/3.11.10/lib/python3.11/asyncio/runners.py", line 118, in run
return self._loop.run_until_complete(task)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/shan/.pyenv/versions/3.11.10/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
return future.result()
^^^^^^^^^^^^^^^
File "/Users/shan/Documents/src/llm_graph.py", line 409, in main
graph_system = await RecipeGraph.ainit([],inputs,'123') # type: ignore
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/shan/Documents///src/llm_graph.py", line 115, in ainit
raise e
File "/Users/shan/Documents//src/llm_graph.py", line 109, in ainit
tools = await asyncio.wait_for(load_mcp_tools(client.session), timeout=30)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/shan/.pyenv/versions/3.11.10/lib/python3.11/asyncio/tasks.py", line 489, in wait_for
return fut.result()
^^^^^^^^^^^^
File "/Users/shan/Documents/.env/lib/python3.11/site-packages/langchain_mcp_adapters/tools.py", line 172, in load_mcp_tools
tools = await _list_all_tools(session)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/shan/Documents//.env/lib/python3.11/site-packages/langchain_mcp_adapters/tools.py", line 81, in _list_all_tools
list_tools_page_result = await session.list_tools(cursor=current_cursor)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/shan/Documents//.env/lib/python3.11/site-packages/mcp/client/session.py", line 386, in list_tools
result = await self.send_request(
^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/shan/Documents/.env/lib/python3.11/site-packages/mcp/shared/session.py", line 261, in send_request
await self._write_stream.send(SessionMessage(message=JSONRPCMessage(jsonrpc_request), metadata=metadata))
File "/Users/shan/Documents/.env/lib/python3.11/site-packages/anyio/streams/memory.py", line 256, in send
raise BrokenResourceError from None
anyio.BrokenResourceError
Running Application with
gunicorn src.mcp_server:app -w 4 -k "uvicorn.workers.UvicornWorker" --bind "0.0.0.0:8001"
Example Server
...
mcp = FastMCP("RecipeSubstitute")
mcp_app = mcp.http_app(path='/mcp',transport="streamable-http")
app = FastAPI(title="E-commerce API", lifespan=mcp_app.lifespan)
app.mount("/", mcp_app)
cache = SimpleMemoryCache(ttl=3600) # Set TTL to 1 hour
...Example Client
...
if not config.OPENAI_API_KEY:
raise ValueError("OPENAI_API_KEY is required in configuration")
llm = AzureChatOpenAI(
model=config.OPENAI_MODEL,
api_key=SecretStr(config.OPENAI_API_KEY or ""),
azure_endpoint=config.OPENAI_API_BASE,
api_version=config.OPENAI_API_VERSION,
streaming=False,
)
client = Client(transport=StreamableHttpTransport(sse_read_timeout=60,url=config.MCP_SERVER_URL))
await asyncio.wait_for(client._connect(), timeout=30)
tools = await asyncio.wait_for(load_mcp_tools(client.session), timeout=30)
logger.info(f"Tools loaded: {', '.join(tool.name for tool in tools)}")
return cls(llm, client, tools, ingredients, recipe_filters, 'RecipeSubstitute', session_id)
except Exception as e:
print(f"Error initializing RecipeGraph: {e}")
raise e
...
Version Information
FastMCP version: 2.12.2
MCP version: 1.13.1
Python version: 3.11.10
Platform: macOS-15.6.1-arm64-arm-64bit
FastMCP root path: /Users/shan/Documents/callexcell/agent_assist/.env/lib/python3.11/site-packages