fix(clientsessiongroup): only query negotiated capabilities#2753
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ClientSessionGroup._aggregate_components queried prompts, resources, and tools unconditionally on every connect, ignoring the ServerCapabilities returned by initialize(). A server that advertised only some of these (e.g. tools) returned JSON-RPC "Method not found" for the rest, which was swallowed into spurious WARNING logs. The MCP lifecycle spec requires clients to only use capabilities that were successfully negotiated. Gate each list_* call on the matching capability from session.initialize_result.capabilities, falling back to the prior unconditional behavior when initialize_result is absent so the existing MCPError handler still covers servers that advertise a capability but fail the method. Fixes modelcontextprotocol#2689 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
Fixes #2689
ClientSessionGroupqueried prompts, resources, and tools unconditionally on every connect, ignoring theServerCapabilitiesreturned byinitialize(). A server advertising only some capabilities (e.g. tools) returned JSON-RPCMethod not foundfor the rest, which was swallowed into spuriousWARNINGlogs — contrary to the lifecycle spec: clients MUST "Only use capabilities that were successfully negotiated."Changes
_aggregate_componentsnow gates eachlist_*call on the matching capability fromsession.initialize_result.capabilities.MCPErrorfallback is preserved for servers that advertise a capability but still fail the method; a missinginitialize_resultfalls back to the prior unconditional behavior.test_client_session_group_skips_unsupported_capabilities, parametrized over tools/prompts/resources-only servers.The same code path exists on
v1.xand likely warrants a[v1.x]backport.Note
I followed the suggestion strictly, but I think the fallback behavior for "misbehaving servers" has questionable value. Sharing as-is for feedback.
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