diff --git a/.github/actions/setup-python-env/action.yml b/.github/actions/setup-python-env/action.yml
index 8e764b5..f4ea0e5 100644
--- a/.github/actions/setup-python-env/action.yml
+++ b/.github/actions/setup-python-env/action.yml
@@ -35,6 +35,8 @@ runs:
           args="$args --group docs"
         fi
         if [[ -n "$EXTRA_ARGS" ]]; then
+          # Validate each whitespace-split token: must start with '-' and contain only safe flag characters.
+          # NOTE: '--' (end-of-options marker) is intentionally allowed; positional args would fail validation.
           for arg in $EXTRA_ARGS; do
             if [[ ! "$arg" =~ ^-[a-zA-Z0-9=._:/@+-]+$ ]]; then
               echo "::error::Unsafe extra-args token: '$arg'. Each token must start with '-' and contain only safe flag characters." >&2
@@ -42,5 +44,5 @@ runs:
             fi
           done
         fi
-        # shellcheck disable=SC2086  # intentional word-splitting for uv flags
+        # shellcheck disable=SC2086  # intentional word-splitting for uv flags (all tokens validated above)
         uv sync $args $EXTRA_ARGS