`ClientSessionGroup` attempts to use capabilities that were not advertised

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

ClientSessionGroup calls session.list_tools, session.list_prompts(), and session.list_resources() unconditionally on every connect. It never consults the ServerCapabilities returned by initialize.

For a server that doesn't implement tools, prompts, or resources, the corresponding calls return a JSON-RPC Method not found error. The MCPError is squashed and a WARNING is emitted, e.g.,

WARNING:root:Could not fetch prompts: Method not found

This seems to contradict the MCP lifecycle spec (2025-11-25, Capability Negotiation / Operation):

Both parties MUST: ... Only use capabilities that were successfully negotiated.

ClientSessionGroup should inspect session.initialize_result.capabilities and only interrogate capabilities that the server advertised, e.g.,

caps = session.initialize_result.capabilities if session.initialize_result else None

if caps and caps.prompts is not None:
    prompts = (await session.list_prompts()).prompts
    ...
if caps and caps.resources is not None:
    resources = (await session.list_resources()).resources
    ...
if caps and caps.tools is not None:
    tools = (await session.list_tools()).tools
    ...

Example Code

"""Repro: tools-only server + ClientSessionGroup emits WARNINGs."""

from __future__ import annotations

import anyio

from mcp import Client, types
from mcp.client.session_group import ClientSessionGroup
from mcp.server import Server, ServerRequestContext


async def handle_list_tools(
    ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
) -> types.ListToolsResult:
    return types.ListToolsResult(
        tools=[
            types.Tool(
                name="ping",
                description="Ping",
                input_schema={"type": "object", "properties": {}},
            )
        ]
    )


async def handle_call_tool(
    ctx: ServerRequestContext, params: types.CallToolRequestParams
) -> types.CallToolResult:
    return types.CallToolResult(content=[types.TextContent(type="text", text="pong")])


async def main() -> None:
    server = Server(
        "tools-only-server",
        on_list_tools=handle_list_tools,
        on_call_tool=handle_call_tool,
    )

    async with Client(server) as client:
        # Server omits prompts/resources capabilities:
        assert client.initialize_result.capabilities.prompts is None
        assert client.initialize_result.capabilities.resources is None

        group = ClientSessionGroup()
        await group.connect_with_session(client.initialize_result.server_info, client.session)


if __name__ == "__main__":
    anyio.run(main)

Python & MCP Python SDK

Python: 3.14.4
MCP Python SDK: main @ da4c118 (also reproducible on v1.27.1)
OS: Linux 6.6.87.2-microsoft-standard-WSL2 x86_64

Bắt đầu tại mcp.client.session_group.ClientSessionGroup.connect_with_session và kiểm tra cách nó sử dụng các capabilities của initialize_result của session. Chạy bản tái hiện server tools-only được cung cấp và xác nhận rằng các prompts và resources không được quảng bá không bị truy vấn, trong khi các tools được quảng bá vẫn khả dụng mà không có cảnh báo Method not found.