diff --git a/tools/test_x.py b/tools/test_x.py
new file mode 100644
index 00000000..3fa90a79
--- /dev/null
+++ b/tools/test_x.py
@@ -0,0 +1,28 @@
+"""Tests for the repository build driver."""
+
+import subprocess
+import sys
+import unittest
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+class XPyTests(unittest.TestCase):
+    def test_unknown_command_fails_without_running_a_build(self):
+        result = subprocess.run(
+            [sys.executable, str(ROOT / "x.py"), "unrecognized-audit-command"],
+            cwd=ROOT,
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+
+        self.assertEqual(result.returncode, 1)
+        self.assertIn("Unknown command: unrecognized-audit-command", result.stdout)
+        self.assertIn("Usage: x.py", result.stdout)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/x.py b/x.py
index ce8df546..d08cb8a7 100644
--- a/x.py
+++ b/x.py
@@ -1337,6 +1337,7 @@ def main():
     else:
         print("Unknown command:", cmd)
         print("Usage: x.py [install | build | package | release | clean | gui] [target...]")
+        sys.exit(1)
 
 if __name__ == "__main__":
     main()