--- a/extensions/toolchain.bzl
+++ b/extensions/toolchain.bzl
@@ -65,6 +65,6 @@ _platform_tag = tag_class(
         "os": attr.string(
             mandatory = True,
-            values = ["linux", "macos"],
+            values = ["linux", "macos", "windows"],
         ),
         "arch": attr.string(
             mandatory = True,
