ClientSessionGroup raises KeyError when connecting a server that exposes no components #3578

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

What I ran into

ClientSessionGroup._aggregate_components deletes the session from
_session_exit_stacks whenever a connected server exposes no prompts, resources,
or tools:

if not any((prompts_temp, resources_temp, tools_temp)):
    del self._session_exit_stacks[session]

Via connect_with_session(server_info, session) the caller supplies a session
that was never registered in _session_exit_stacks, so this raises KeyError.
Via connect_to_server it drops the freshly-registered exit stack, so a later
disconnect_from_server can't close that session's transport.

Why it matters

A valid MCP server can legitimately register zero tools/resources/prompts (e.g.
one that only does sampling/roots), and list_* failures are swallowed into
empty dicts too. In both cases connecting an otherwise-fine server either crashes
or leaks a transport until group teardown.

Minimal reproduction

group = ClientSessionGroup()
# server_info + an in-memory session whose list_tools/resources/prompts are empty
await group.connect_with_session(server_info, session)  # -> KeyError

Suggested fix

Remove the empty-server cleanup block so an empty server is aggregated like any
other. Happy to open a PR (branch already prepared).

Example Code

Python & MCP Python SDK

Python 3.13.2
MCP Python SDK: main @ f1b6589 (dev build 0.0.1.dev1063)