fix(client): skip unsupported capability requests in ClientSessionGroup

[bobby-nandigam]

MCP Python SDK Contribution – Issue #2689

Respect Negotiated Capabilities in ClientSessionGroup

Overview

This contribution improves capability negotiation behavior in the MCP Python SDK.

Previously, ClientSessionGroup._aggregate_components() attempted to retrieve prompts, resources, and tools from every connected server regardless of the capabilities negotiated during initialization.

Problem

During server initialization, MCP servers advertise their supported capabilities. However, the existing implementation attempted to call:

• list_prompts()
• list_resources()
• list_tools()

without verifying whether those capabilities were available.

Solution

The implementation now checks the negotiated capabilities before requesting server components.

New Behavior

• Query prompts only when prompt capabilities are advertised.
• Query resources only when resource capabilities are advertised.
• Query tools only when tool capabilities are advertised.

This ensures that ClientSessionGroup interacts only with features explicitly supported by the connected server.

Testing

Added regression test:

test_client_session_group_skips_unsupported_capabilities

Validation

uv run pytest tests/client/test_session_group.py -v

Output:

12 passed

Files Modified

• src/mcp/client/session_group.py
• tests/client/test_session_group.py

Benefits

• Better protocol compliance
• Reduced unnecessary RPC requests
• Improved compatibility with capability-limited MCP servers
• More predictable client behavior

Related Issue

Fixes #2689

Author

Bobby Nandigam