[Bug]: MCP SSE is failing with "Unexpected message" #1316
Description
crawl4ai version
0.7.0
Expected Behavior
When called upon, MCP SSE server responds properly
Current Behavior
MCP server is added to Claude Code and shows as "connected", however, trying to use it results in Error: All connection attempts failed
Snippet of the docker compose logs is below.
I'm trying to use the mcp for getting markdown of a webpage, no additional params.
Is this reproducible?
Yes
Inputs Causing the Bug
I was using "https://www.numberanalytics.com/blog/advanced-propaganda-detection", but pretty sure URL has nothing to do with thisSteps to Reproduce
Code snippets
OS
Linux/Docker
Python version
3.11
Browser
No response
Browser version
No response
Error logs & Screenshots (if applicable)
crawl4ai | [2025-07-17 17:57:40 +0000] [13] [ERROR] Exception in ASGI application
crawl4ai | + Exception Group Traceback (most recent call last):
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/_utils.py", line 77, in collapse_excgroups
crawl4ai | | yield
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 183, in __call__
crawl4ai | | async with anyio.create_task_group() as task_group:
crawl4ai | | ^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
crawl4ai | | raise BaseExceptionGroup(
crawl4ai | | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
crawl4ai | +-+---------------- 1 ----------------
crawl4ai | | Traceback (most recent call last):
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
crawl4ai | | result = await app( # type: ignore[func-returns-value]
crawl4ai | | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
crawl4ai | | return await self.app(scope, receive, send)
crawl4ai | | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
crawl4ai | | await super().__call__(scope, receive, send)
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/applications.py", line 113, in __call__
crawl4ai | | await self.middleware_stack(scope, receive, send)
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/errors.py", line 186, in __call__
crawl4ai | | raise exc
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/errors.py", line 164, in __call__
crawl4ai | | await self.app(scope, receive, _send)
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 182, in __call__
crawl4ai | | with recv_stream, send_stream, collapse_excgroups():
crawl4ai | | ^^^^^^^^^^^^^^^^^^^^
crawl4ai | | File "/usr/local/lib/python3.12/contextlib.py", line 158, in __exit__
crawl4ai | | self.gen.throw(value)
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
crawl4ai | | raise exc
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 185, in __call__
crawl4ai | | await response(scope, wrapped_receive, send)
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 223, in __call__
crawl4ai | | async for chunk in self.body_iterator:
crawl4ai | | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 169, in body_stream
crawl4ai | | assert message["type"] == "http.response.body", f"Unexpected message: {message}"
crawl4ai | | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | | AssertionError: Unexpected message: {'type': 'http.response.start', 'status': 200, 'headers': [(b'content-length', b'4'), (b'content-type', b'application/json')]}
crawl4ai | +------------------------------------
crawl4ai |
crawl4ai | During handling of the above exception, another exception occurred:
crawl4ai |
crawl4ai | Traceback (most recent call last):
crawl4ai | File "/usr/local/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
crawl4ai | result = await app( # type: ignore[func-returns-value]
crawl4ai | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | File "/usr/local/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
crawl4ai | return await self.app(scope, receive, send)
crawl4ai | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | File "/usr/local/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
crawl4ai | await super().__call__(scope, receive, send)
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/applications.py", line 113, in __call__
crawl4ai | await self.middleware_stack(scope, receive, send)
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/errors.py", line 186, in __call__
crawl4ai | raise exc
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/errors.py", line 164, in __call__
crawl4ai | await self.app(scope, receive, _send)
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 182, in __call__
crawl4ai | with recv_stream, send_stream, collapse_excgroups():
crawl4ai | ^^^^^^^^^^^^^^^^^^^^
crawl4ai | File "/usr/local/lib/python3.12/contextlib.py", line 158, in __exit__
crawl4ai | self.gen.throw(value)
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
crawl4ai | raise exc
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 185, in __call__
crawl4ai | await response(scope, wrapped_receive, send)
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 223, in __call__
crawl4ai | async for chunk in self.body_iterator:
crawl4ai | File "/usr/local/lib/python3.12/site-packages/starlette/middleware/base.py", line 169, in body_stream
crawl4ai | assert message["type"] == "http.response.body", f"Unexpected message: {message}"
crawl4ai | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
crawl4ai | AssertionError: Unexpected message: {'type': 'http.response.start', 'status': 200, 'headers': [(b'content-length', b'4'), (b'content-type', b'application/json')]}
crawl4ai | 2025-07-17 17:57:43,378 - mcp.server.lowlevel.server - INFO - Processing request of type ListToolsRequest
crawl4ai | 2025-07-17 17:57:43,389 - mcp.server.lowlevel.server - INFO - Processing request of type ListResourcesRequest
crawl4ai | 2025-07-17 17:58:35,778 - mcp.server.lowlevel.server - INFO - Processing request of type CallToolRequest