René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Exception in `ClientSessionGroup` if `streamable_http` MCP server is not available","articleBody":"**Describe the bug**\nWhen I use the `ClientSessionGroup` for a remote MCP server via `remote_http` for a remote server that is not running, I get exception `RuntimeError: Attempted to exit cancel scope in a different task than it was entered in`\n\n\u003cdetails\u003e\n\u003csummary\u003e\n(complete output collapsed)\n\u003c/summary\u003e\n\n```\nan error occurred during closing of asynchronous generator \u003casync_generator object streamablehttp_client at 0xffffa53755d0\u003e\nasyncgen: \u003casync_generator object streamablehttp_client at 0xffffa53755d0\u003e\n  + Exception Group Traceback (most recent call last):\n  |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py\", line 772, in __aexit__\n  |     raise BaseExceptionGroup(\n  |         \"unhandled errors in a TaskGroup\", self._exceptions\n  |     ) from None\n  | BaseExceptionGroup: unhandled errors in a TaskGroup (2 sub-exceptions)\n  +-+---------------- 1 ----------------\n    | Traceback (most recent call last):\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_transports/default.py\", line 101, in map_httpcore_exceptions\n    |     yield\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_transports/default.py\", line 394, in handle_async_request\n    |     resp = await self._pool.handle_async_request(req)\n    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_async/connection_pool.py\", line 256, in handle_async_request\n    |     raise exc from None\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_async/connection_pool.py\", line 236, in handle_async_request\n    |     response = await connection.handle_async_request(\n    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |         pool_request.request\n    |         ^^^^^^^^^^^^^^^^^^^^\n    |     )\n    |     ^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_async/connection.py\", line 101, in handle_async_request\n    |     raise exc\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_async/connection.py\", line 78, in handle_async_request\n    |     stream = await self._connect(request)\n    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_async/connection.py\", line 124, in _connect\n    |     stream = await self._network_backend.connect_tcp(**kwargs)\n    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_backends/auto.py\", line 31, in connect_tcp\n    |     return await self._backend.connect_tcp(\n    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |     ...\u003c5 lines\u003e...\n    |     )\n    |     ^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_backends/anyio.py\", line 113, in connect_tcp\n    |     with map_exceptions(exc_map):\n    |          ~~~~~~~~~~~~~~^^^^^^^^^\n    |   File \"/usr/local/lib/python3.13/contextlib.py\", line 162, in __exit__\n    |     self.gen.throw(value)\n    |     ~~~~~~~~~~~~~~^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpcore/_exceptions.py\", line 14, in map_exceptions\n    |     raise to_exc(exc) from exc\n    | httpcore.ConnectError: All connection attempts failed\n    | \n    | The above exception was the direct cause of the following exception:\n    | \n    | Traceback (most recent call last):\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/streamable_http.py\", line 391, in handle_request_async\n    |     await self._handle_post_request(ctx)\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/streamable_http.py\", line 251, in _handle_post_request\n    |     async with ctx.client.stream(\n    |                ~~~~~~~~~~~~~~~~~^\n    |         \"POST\",\n    |         ^^^^^^^\n    |     ...\u003c2 lines\u003e...\n    |         headers=headers,\n    |         ^^^^^^^^^^^^^^^^\n    |     ) as response:\n    |     ^\n    |   File \"/usr/local/lib/python3.13/contextlib.py\", line 214, in __aenter__\n    |     return await anext(self.gen)\n    |            ^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_client.py\", line 1583, in stream\n    |     response = await self.send(\n    |                ^^^^^^^^^^^^^^^^\n    |     ...\u003c4 lines\u003e...\n    |     )\n    |     ^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_client.py\", line 1629, in send\n    |     response = await self._send_handling_auth(\n    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |     ...\u003c4 lines\u003e...\n    |     )\n    |     ^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_client.py\", line 1657, in _send_handling_auth\n    |     response = await self._send_handling_redirects(\n    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |     ...\u003c3 lines\u003e...\n    |     )\n    |     ^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_client.py\", line 1694, in _send_handling_redirects\n    |     response = await self._send_single_request(request)\n    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_client.py\", line 1730, in _send_single_request\n    |     response = await transport.handle_async_request(request)\n    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_transports/default.py\", line 393, in handle_async_request\n    |     with map_httpcore_exceptions():\n    |          ~~~~~~~~~~~~~~~~~~~~~~~^^\n    |   File \"/usr/local/lib/python3.13/contextlib.py\", line 162, in __exit__\n    |     self.gen.throw(value)\n    |     ~~~~~~~~~~~~~~^^^^^^^\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/httpx/_transports/default.py\", line 118, in map_httpcore_exceptions\n    |     raise mapped_exc(message) from exc\n    | httpx.ConnectError: All connection attempts failed\n    +---------------- 2 ----------------\n    | Traceback (most recent call last):\n    |   File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/streamable_http.py\", line 492, in streamablehttp_client\n    |     yield (\n    |     ...\u003c3 lines\u003e...\n    |     )\n    | GeneratorExit\n    +------------------------------------\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/streamable_http.py\", line 464, in streamablehttp_client\n    async with anyio.create_task_group() as tg:\n               ~~~~~~~~~~~~~~~~~~~~~~~^^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py\", line 778, in __aexit__\n    if self.cancel_scope.__exit__(type(exc), exc, exc.__traceback__):\n       ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py\", line 457, in __exit__\n    raise RuntimeError(\n    ...\u003c2 lines\u003e...\n    )\nRuntimeError: Attempted to exit cancel scope in a different task than it was entered in\nTraceback (most recent call last):\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/streams/memory.py\", line 111, in receive\n    return self.receive_nowait()\n           ~~~~~~~~~~~~~~~~~~~^^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/streams/memory.py\", line 106, in receive_nowait\n    raise WouldBlock\nanyio.WouldBlock\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"/workspaces/test.py\", line 14, in \u003cmodule\u003e\n    asyncio.run(main())\n    ~~~~~~~~~~~^^^^^^^^\n  File \"/usr/local/lib/python3.13/asyncio/runners.py\", line 195, in run\n    return runner.run(main)\n           ~~~~~~~~~~^^^^^^\n  File \"/usr/local/lib/python3.13/asyncio/runners.py\", line 118, in run\n    return self._loop.run_until_complete(task)\n           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^\n  File \"/usr/local/lib/python3.13/asyncio/base_events.py\", line 719, in run_until_complete\n    return future.result()\n           ~~~~~~~~~~~~~^^\n  File \"/workspaces/test.py\", line 10, in main\n    await client_session_group.connect_to_server(server_params)\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/session_group.py\", line 232, in connect_to_server\n    server_info, session = await self._establish_session(server_params)\n                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/session_group.py\", line 267, in _establish_session\n    result = await session.initialize()\n             ^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/client/session.py\", line 133, in initialize\n    result = await self.send_request(\n             ^^^^^^^^^^^^^^^^^^^^^^^^\n    ...\u003c15 lines\u003e...\n    )\n    ^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/mcp/shared/session.py\", line 283, in send_request\n    response_or_error = await response_stream_reader.receive()\n                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/streams/memory.py\", line 119, in receive\n    await receive_event.wait()\n  File \"/workspaces/test/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py\", line 1774, in wait\n    await self._event.wait()\n  File \"/usr/local/lib/python3.13/asyncio/locks.py\", line 213, in wait\n    await fut\nasyncio.exceptions.CancelledError: Cancelled by cancel scope ffffa51c4050\n```\n\u003c/details\u003e\n\n\n**To Reproduce**\n\nI tested this with this script:\n\n```python\nimport asyncio\n\nfrom mcp.client.session_group import ClientSessionGroup, StreamableHttpParameters\n\n\nasync def main():\n    async with ClientSessionGroup() as client_session_group:\n        server_params = StreamableHttpParameters(url=\"http://localhost:3001/mcp/\")\n        try:\n            await client_session_group.connect_to_server(server_params)\n        except Exception:\n            print(\"this is never reached\")\n\n\nif __name__ == \"__main__\":\n    asyncio.run(main())\n```\n\n**Expected behavior**\n\nI would like to be able to catch any error that happens during `connect_to_server` and handle it within my application code - in my case I am using the `ClientSessionGroup` for multiple MCP servers and want to provide the tools of the running MCP servers to the LLM.\n\n\n**Additional context**\n- I am using version `1.9.3`\n\n","author":{"url":"https://github.com/hzeus","@type":"Person","name":"hzeus"},"datePublished":"2025-06-10T09:49:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":12},"url":"https://github.com/915/python-sdk/issues/915"}