diff --git a/src/mcp/client/stdio/__init__.py b/src/mcp/client/stdio/__init__.py
index 50bceddec..e3532e988 100644
--- a/src/mcp/client/stdio/__init__.py
+++ b/src/mcp/client/stdio/__init__.py
@@ -32,6 +32,7 @@
         "HOMEPATH",
         "LOCALAPPDATA",
         "PATH",
+        "PATHEXT",
         "PROCESSOR_ARCHITECTURE",
         "SYSTEMDRIVE",
         "SYSTEMROOT",