feat(mcp): dynamic tool discovery via notifications/tools/list_changed#3812
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feat(mcp): dynamic tool discovery via notifications/tools/list_changed#3812
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When a connected MCP server sends a ToolListChangedNotification (per the MCP spec), Hermes now automatically re-fetches the tool list, deregisters removed tools, and registers new ones — without requiring a restart. This enables MCP servers with dynamic toolsets (e.g. GitHub MCP with GITHUB_DYNAMIC_TOOLSETS=1) to add/remove tools at runtime. Changes: - registry.py: add ToolRegistry.deregister() for nuke-and-repave refresh - mcp_tool.py: extract _register_server_tools() from _discover_and_register_server() as a shared helper for both initial discovery and dynamic refresh - mcp_tool.py: add _make_message_handler() and _refresh_tools() on MCPServerTask, wired into all 3 ClientSession sites (stdio, new HTTP, deprecated HTTP) - Graceful degradation: silently falls back to static discovery when the MCP SDK lacks notification types or message_handler support - 8 new tests covering registration, refresh, handler dispatch, and deregister Salvaged from PR #1794 by shivvor2.
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Summary
Implement MCP spec's
notifications/tools/list_changednotification handler. When a connected MCP server sends this notification (e.g., GitHub MCP withGITHUB_DYNAMIC_TOOLSETS=1), Hermes automatically re-fetches the tool list, deregisters removed tools, and registers new ones — without requiring a gateway restart or/mcp refresh.Changes
tools/registry.pyToolRegistry.deregister(name)— removes a tool and cleans up its toolset check if it was the last tool in that toolset. Used by the nuke-and-repave refresh strategy.tools/mcp_tool.pyToolListChangedNotification,ServerNotification, etc.) with graceful degradation for older SDK versions_check_message_handler_support()— inspectsClientSessionconstructor to verify the SDK version acceptsmessage_handler_register_server_tools()— extracted from_discover_and_register_server()as a shared helper used by both initial discovery and dynamic refresh. Handles filtering, collision guards, utility tools, toolset creation, and hermes-* injection.MCPServerTask._make_message_handler()— builds a notification callback that dispatches on type;ToolListChangedNotificationtriggers refresh, prompt/resource changes are logged stubsMCPServerTask._refresh_tools()— nuke-and-repave under_refresh_lock: fetch new tools, remove old from registry + hermes-* toolsets, re-register freshmessage_handlerwired into all 3ClientSessionconstruction sites (stdio, new HTTP, deprecated HTTP)tests/tools/test_mcp_dynamic_discovery.py(new, 8 tests)deregister()edge casesBackward compatibility
message_handlersupport, the feature silently degrades to existing static-discovery behaviormcp_serversconfig format is unchangedSalvaged from PR #1794 by @shivvor2.