`tools/call` with string `arguments` breaks mcp server until restart

[Sillocan]

Describe the bug

When making a tools/call JSON-RPC request to the mcp server, passing the arguments parameter as a string instead of an object causes the server to stop responding to all HTTP requests. The server does not return an informative error, and requires a restart to recover.

This makes it possible for a single malformed client request to break the server for everyone.

See the MRE at https://github.com/Sillocan/fastmcp-mre-breaking-task-group-with-invalid-arguments/blob/main/python_sdk/simple_echo_mre.py

To Reproduce
Steps to reproduce the behavior:

1. Run the following command in your shell:

   uv run --with=git+https://github.com/Sillocan/fastmcp-mre-breaking-task-group-with-invalid-arguments#subdirectory=python_sdk mre

2. Observe that a single malformed request (arguments is a string) returns 200, but further requests return 500 and the server must be restarted.
3. See error output below.

Expected behavior

• The server should return a clear error response (such as HTTP 400) when a request has an invalid format—specifically, when arguments is a string instead of a dict/object.
• The server should remain operational and handle all further requests correctly, even after processing an invalid or malformed request.

Logs

$ uv run --with=git+https://github.com/Sillocan/fastmcp-mre-breaking-task-group-with-invalid-arguments#subdirectory=python_sdk mre
 Built fastmcp-mre-breaking-task-group-with-invalid-arguments @ git+https://github.com/Sillocan/fastmcp-mre-breaking-task-group-with-invalid-arguments@3eeb959518a5eaa685c7619ebc7b20f5330f6235#subdirectory=python_sdk
Installed 32 packages in 5ms
[05/27/25 12:40:16] DEBUG Using selector: EpollSelector selector_events.py:64
INFO: Started server process [1783696]
INFO: Waiting for application startup.
 INFO StreamableHTTP session manager started streamable_http_manager.py:109
INFO: Application startup complete.
INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
[05/27/25 12:40:17] DEBUG connect_tcp.started host='127.0.0.1' port=8000 local_address=None timeout=5.0 socket_options=None _trace.py:87
 DEBUG connect_tcp.complete return_value=<httpcore._backends.anyio.AnyIOStream object at 0x7fcbb8fe4ec0> _trace.py:87
 DEBUG send_request_headers.started request=<Request [b'POST']> _trace.py:87
 DEBUG send_request_headers.complete _trace.py:87
 DEBUG send_request_body.started request=<Request [b'POST']> _trace.py:87
 DEBUG send_request_body.complete _trace.py:87
 DEBUG receive_response_headers.started request=<Request [b'POST']> _trace.py:87
 DEBUG Stateless mode: Creating new transport for this request streamable_http_manager.py:159
INFO: 127.0.0.1:51112 - "POST /mcp/ HTTP/1.1" 200 OK
 DEBUG receive_response_headers.complete return_value=(b'HTTP/1.1', 200, b'OK', [(b'date', b'Tue, 27 May 2025 19:40:16 GMT'), (b'server', b'uvicorn'), (b'cache-control', b'no-cache, no-transform'), _trace.py:87
 (b'connection', b'keep-alive'), (b'content-type', b'text/event-stream'), (b'x-accel-buffering', b'no'), (b'Transfer-Encoding', b'chunked')]) 
 INFO HTTP Request: POST http://127.0.0.1:8000/mcp/ "HTTP/1.1 200 OK" _client.py:1740
 DEBUG receive_response_body.started request=<Request [b'POST']> _trace.py:87
 DEBUG Closing SSE writer streamable_http.py:486
 INFO StreamableHTTP session manager shutting down streamable_http_manager.py:113
 DEBUG Got event: http.disconnect. Stop streaming. sse.py:182
ERROR: + Exception Group Traceback (most recent call last):
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/routing.py", line 692, in lifespan
 | async with self.lifespan_context(app) as maybe_state:
 | ~~~~~~~~~~~~~~~~~~~~~^^^^^
 | File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/contextlib.py", line 235, in __aexit__
 | await self.gen.athrow(value)
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/streamable_http_manager.py", line 106, in run
 | async with anyio.create_task_group() as tg:
 | ~~~~~~~~~~~~~~~~~~~~~~~^^
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
 | raise BaseExceptionGroup(
 | "unhandled errors in a TaskGroup", self._exceptions
 | ) from None
 | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
 +-+---------------- 1 ----------------
 | Exception Group Traceback (most recent call last):
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/streamable_http_manager.py", line 171, in run_stateless_server
 | async with http_transport.connect() as streams:
 | ~~~~~~~~~~~~~~~~~~~~~~^^
 | File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/contextlib.py", line 235, in __aexit__
 | await self.gen.athrow(value)
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/streamable_http.py", line 843, in connect
 | async with anyio.create_task_group() as tg:
 | ~~~~~~~~~~~~~~~~~~~~~~~^^
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
 | raise BaseExceptionGroup(
 | "unhandled errors in a TaskGroup", self._exceptions
 | ) from None
 | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
 +-+---------------- 1 ----------------
 | Exception Group Traceback (most recent call last):
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/streamable_http.py", line 916, in connect
 | yield read_stream, write_stream
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/streamable_http_manager.py", line 174, in run_stateless_server
 | await self.app.run(
 | ...<4 lines>...
 | )
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/lowlevel/server.py", line 495, in run
 | async with AsyncExitStack() as stack:
 | ~~~~~~~~~~~~~~^^
 | File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/contextlib.py", line 768, in __aexit__
 | raise exc
 | File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/contextlib.py", line 751, in __aexit__
 | cb_suppress = await cb(*exc_details)
 | ^^^^^^^^^^^^^^^^^^^^^^
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/shared/session.py", line 220, in __aexit__
 | return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
 | raise BaseExceptionGroup(
 | "unhandled errors in a TaskGroup", self._exceptions
 | ) from None
 | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
 +-+---------------- 1 ----------------
 | Traceback (most recent call last):
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/session.py", line 147, in _receive_loop
 | await super()._receive_loop()
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/shared/session.py", line 354, in _receive_loop
 | validated_request = self._receive_request_type.model_validate(
 | message.message.root.model_dump(
 | by_alias=True, mode="json", exclude_none=True
 | )
 | )
 | File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/pydantic/main.py", line 705, in model_validate
 | return cls.__pydantic_validator__.validate_python(
 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
 | obj, strict=strict, from_attributes=from_attributes, context=context, by_alias=by_alias, by_name=by_name
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 | )
 | ^
 | pydantic_core._pydantic_core.ValidationError: 23 validation errors for ClientRequest
 | PingRequest.method
 | Input should be 'ping' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | InitializeRequest.method
 | Input should be 'initialize' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | InitializeRequest.params.protocolVersion
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | InitializeRequest.params.capabilities
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | InitializeRequest.params.clientInfo
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | CompleteRequest.method
 | Input should be 'completion/complete' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | CompleteRequest.params.ref
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | CompleteRequest.params.argument
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | SetLevelRequest.method
 | Input should be 'logging/setLevel' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | SetLevelRequest.params.level
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | GetPromptRequest.method
 | Input should be 'prompts/get' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | GetPromptRequest.params.arguments
 | Input should be a valid dictionary [type=dict_type, input_value='{"text": "imnestedtext"}', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/dict_type
 | ListPromptsRequest.method
 | Input should be 'prompts/list' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | ListResourcesRequest.method
 | Input should be 'resources/list' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | ListResourceTemplatesRequest.method
 | Input should be 'resources/templates/list' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | ReadResourceRequest.method
 | Input should be 'resources/read' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | ReadResourceRequest.params.uri
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | SubscribeRequest.method
 | Input should be 'resources/subscribe' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | SubscribeRequest.params.uri
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | UnsubscribeRequest.method
 | Input should be 'resources/unsubscribe' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 | UnsubscribeRequest.params.uri
 | Field required [type=missing, input_value={'name': 'echo', 'argumen...text": "imnestedtext"}'}, input_type=dict]
 | For further information visit https://errors.pydantic.dev/2.11/v/missing
 | CallToolRequest.params.arguments
 | Input should be a valid dictionary [type=dict_type, input_value='{"text": "imnestedtext"}', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/dict_type
 | ListToolsRequest.method
 | Input should be 'tools/list' [type=literal_error, input_value='tools/call', input_type=str]
 | For further information visit https://errors.pydantic.dev/2.11/v/literal_error
 +------------------------------------

 DEBUG receive_response_body.complete _trace.py:87
 DEBUG response_closed.started _trace.py:87
 DEBUG response_closed.complete _trace.py:87
breaking_response.status_code=200 breaking_response.text=''
 DEBUG send_request_headers.started request=<Request [b'GET']> _trace.py:87
 DEBUG send_request_headers.complete _trace.py:87
 DEBUG send_request_body.started request=<Request [b'GET']> _trace.py:87
 DEBUG send_request_body.complete _trace.py:87
 DEBUG receive_response_headers.started request=<Request [b'GET']> _trace.py:87
INFO: 127.0.0.1:51112 - "GET /mcp/ HTTP/1.1" 500 Internal Server Error
ERROR: Exception in ASGI application
Traceback (most recent call last):
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
 result = await app( # type: ignore[func-returns-value]
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 self.scope, self.receive, self.send
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 )
 ^
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
 return await self.app(scope, receive, send)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/applications.py", line 112, in __call__
 await self.middleware_stack(scope, receive, send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/middleware/errors.py", line 187, in __call__
 raise exc
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/middleware/errors.py", line 165, in __call__
 await self.app(scope, receive, _send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
 await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
 raise exc
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
 await app(scope, receive, sender)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/routing.py", line 714, in __call__
 await self.middleware_stack(scope, receive, send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/routing.py", line 734, in app
 await route.handle(scope, receive, send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/starlette/routing.py", line 460, in handle
 await self.app(scope, receive, send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/fastmcp/server.py", line 786, in handle_streamable_http
 await self.session_manager.handle_request(scope, receive, send)
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/mcp/server/streamable_http_manager.py", line 137, in handle_request
 raise RuntimeError("Task group is not initialized. Make sure to use run().")
RuntimeError: Task group is not initialized. Make sure to use run().
 DEBUG receive_response_headers.complete return_value=(b'HTTP/1.1', 500, b'Internal Server Error', [(b'date', b'Tue, 27 May 2025 19:40:16 GMT'), (b'server', b'uvicorn'), (b'content-length', b'21'), _trace.py:87
 (b'content-type', b'text/plain; charset=utf-8')]) 
 INFO HTTP Request: GET http://127.0.0.1:8000/mcp/ "HTTP/1.1 500 Internal Server Error" _client.py:1740
 DEBUG receive_response_body.started request=<Request [b'GET']> _trace.py:87
 DEBUG receive_response_body.complete _trace.py:87
 DEBUG response_closed.started _trace.py:87
 DEBUG response_closed.complete _trace.py:87
 DEBUG close.started _trace.py:87
 DEBUG close.complete _trace.py:87
get_response.status_code=500 get_response.text='Internal Server Error'
Traceback (most recent call last):
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/bin/mre", line 12, in <module>
 sys.exit(main())
 ~~~~^^
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/simple_echo_mre.py", line 69, in main
 asyncio.run(run_and_break_the_mcp())
 ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
 File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/asyncio/runners.py", line 195, in run
 return runner.run(main)
 ~~~~~~~~~~^^^^^^
 File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/asyncio/runners.py", line 118, in run
 return self._loop.run_until_complete(task)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
 File "${HOME}/.local/share/uv/python/cpython-3.13.2-linux-x86_64-gnu/lib/python3.13/asyncio/base_events.py", line 725, in run_until_complete
 return future.result()
 ~~~~~~~~~~~~~^^
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/simple_echo_mre.py", line 62, in run_and_break_the_mcp
 await breakit()
 File "${HOME}/.cache/uv/archive-v0/WBNLPmHcXZu1RxZu0HKOl/lib/python3.13/site-packages/simple_echo_mre.py", line 54, in breakit
 assert get_response.status_code < 300, "MCP is fully broken"
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: MCP is fully broken

Desktop (please complete the following information):

• OS: Linux
• Browser [e.g. chrome, safari]
• Version: 1.9.1

Additional context

• Related minimal reproduction repository and instructions

[Sillocan]

The source of this appears to be caused by a lack of an exeception handler when validating the _receive_request_type

python-sdk/src/mcp/shared/session.py

Line 355 in 532b117

validated_request = self._receive_request_type.model_validate(

Edit: It appears there is a PR which should hopefully address this issue #822

[johnw188]

Yup, TY for the report, we are pushing in a fix!

[Sillocan]

Yup, TY for the report, we are pushing in a fix!

Just validated the MRE does not occur when using your branch. Thanks!

[Sillocan]

With #822 merged in this is resolved