fix(mcp): use SelectorEventLoop for the MCP loop thread on Windows

tests/tools/test_mcp_loop_windows_selector.py

@@ -0,0 +1,25 @@
+"""Regression: the MCP event loop must not be a ProactorEventLoop on Windows.
+
+The MCP stdio client spawns servers as subprocesses and reads their stdout
+pipe. Windows' ProactorEventLoop hangs those subprocess-pipe reads during the
+`initialize` handshake, so discovery times out and no MCP tools register.
+`_ensure_mcp_loop` therefore builds an explicit SelectorEventLoop on win32,
+matching the win32 SelectorEventLoop handling in cli.py and web_server.py.
+"""
+import asyncio
+import sys
+
+
+def test_mcp_loop_is_selector_on_windows():
+    import tools.mcp_tool as mcp_tool
+
+    mcp_tool._ensure_mcp_loop()
+    try:
+        loop = mcp_tool._mcp_loop
+        assert loop is not None
+        if sys.platform == "win32":
+            proactor = getattr(asyncio, "ProactorEventLoop", None)
+            assert proactor is None or not isinstance(loop, proactor)
+            assert isinstance(loop, asyncio.SelectorEventLoop)
+    finally:
+        mcp_tool._stop_mcp_loop()

tools/mcp_tool.py

@@ -3570,7 +3570,17 @@ def _ensure_mcp_loop():
     with _lock:
         if _mcp_loop is not None and _mcp_loop.is_running():
             return
-        _mcp_loop = asyncio.new_event_loop()
+        # On Windows the default event loop is the ProactorEventLoop, whose
+        # subprocess-pipe reads hang the MCP stdio_client handshake (the
+        # `initialize` request is sent but its response is never surfaced,
+        # so discovery times out and no MCP tools register). The
+        # SelectorEventLoop reads stdio subprocess pipes correctly. Use it
+        # explicitly for the dedicated MCP loop thread on Windows, matching
+        # the win32 SelectorEventLoop handling in cli.py and web_server.py.
+        if sys.platform == "win32":
+            _mcp_loop = asyncio.SelectorEventLoop()
+        else:
+            _mcp_loop = asyncio.new_event_loop()
         _mcp_loop.set_exception_handler(_mcp_loop_exception_handler)
         _mcp_thread = threading.Thread(
             target=_mcp_loop.run_forever,