fix(mcp): use SelectorEventLoop for the MCP loop thread on Windows#61445
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ahmadmdabit wants to merge 1 commit into
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fix(mcp): use SelectorEventLoop for the MCP loop thread on Windows#61445ahmadmdabit wants to merge 1 commit into
ahmadmdabit wants to merge 1 commit into
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The dedicated MCP event-loop thread was built with asyncio.new_event_loop(), which on Windows returns a ProactorEventLoop. Its subprocess-pipe reads hang the stdio_client initialize handshake, so MCP discovery times out and no MCP tools register. Build an explicit SelectorEventLoop on win32, matching the existing win32 SelectorEventLoop handling in cli.py and web_server.py. Adds a regression test asserting the loop is not a ProactorEventLoop on Windows.
alt-glitch
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type/bug
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What
Build an explicit
asyncio.SelectorEventLoop()for the dedicated MCP event-loop thread on Windows, instead ofasyncio.new_event_loop()(which returns aProactorEventLoopon win32). Non-Windows behavior is unchanged.Why
The
ProactorEventLoop's subprocess-pipe reads hang the MCP SDKstdio_clientinitializehandshake, so on Windows MCP discovery times out and no MCP tools register. This mirrors the existing win32SelectorEventLoophandling incli.pyandhermes_cli/web_server.py.Evidence: a raw stdio pipe to the server responds in ~0.6s; the same MCP SDK
ClientSessioncall raisesTimeoutErroroninitializeunder the default (Proactor) loop but returnsINITIALIZED OK, TOOLS: 14underWindowsSelectorEventLoopPolicy.Fixes #61444.
How to test
pytest tests/tools/test_mcp_loop_windows_selector.py -vPlatforms tested
elsebranch keepsasyncio.new_event_loop()); the new regression test is win32-gated.