diff --git a/src/test/test_main_invocation.py b/src/test/test_main_invocation.py
new file mode 100644
index 0000000..913a3da
--- /dev/null
+++ b/src/test/test_main_invocation.py
@@ -0,0 +1,10 @@
+from pathlib import Path
+
+
+def test_main_uses_static_fill_call():
+    """Guard against reintroducing constructor/call mismatch for Fill."""
+    main_path = Path(__file__).resolve().parents[1] / "main.py"
+    content = main_path.read_text(encoding="utf-8")
+
+    assert "Fill.fill_form(" in content
+    assert "Fill(user_input=user_input).fill_form(" not in content