diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_debug_enabled.py b/tests/test_debug_enabled.py
new file mode 100644
index 0000000..3f94433
--- /dev/null
+++ b/tests/test_debug_enabled.py
@@ -0,0 +1,84 @@
+"""Regression tests for the _debug_enabled helper.
+
+Both app.cm_api and app.cm_web_view define a private _debug_enabled()
+function that parses the CM_DEBUG environment variable. They are
+intentionally duplicated (only two call sites; no shared utility module
+exists). This test runs the same parametrized cases against every copy
+to catch drift if one is updated without the other.
+"""
+
+import os
+import unittest
+from unittest import mock
+
+# Import the modules at top-level (before any mock.patch.dict with
+# clear=True), so module-load-time os.getenv() reads see the real
+# environment. The patches inside individual tests then only affect the
+# helper's runtime read of CM_DEBUG.
+import app.cm_api
+import app.cm_web_view
+
+
+# Modules expected to expose a private _debug_enabled() helper.
+# Add new entries here if more Flask entrypoints adopt the same toggle.
+HELPER_MODULES = (
+    app.cm_web_view,
+    app.cm_api,
+)
+
+
+# (env_value, expected_result). env_value=None means CM_DEBUG is unset.
+CASES = (
+    (None, False),
+    ("", False),
+    ("false", False),
+    ("False", False),
+    ("FALSE", False),
+    ("0", False),
+    ("no", False),
+    ("anything-else", False),
+    ("true", True),
+    ("True", True),
+    ("TRUE", True),
+    ("1", True),
+    ("yes", True),
+    ("YES", True),
+    ("  true  ", True),
+)
+
+
+class DebugEnabledTests(unittest.TestCase):
+    def _resolve(self, module):
+        return getattr(module, "_debug_enabled", None)
+
+    def test_helper_exists_on_every_module(self):
+        for module in HELPER_MODULES:
+            with self.subTest(module=module.__name__):
+                helper = self._resolve(module)
+                self.assertTrue(
+                    callable(helper),
+                    f"{module.__name__}._debug_enabled must be callable",
+                )
+
+    def test_parses_cm_debug_consistently(self):
+        for module in HELPER_MODULES:
+            helper = self._resolve(module)
+            if helper is None:
+                self.fail(
+                    f"{module.__name__}._debug_enabled is missing — "
+                    "make test_helper_exists_on_every_module pass first"
+                )
+            for env_value, expected in CASES:
+                with self.subTest(module=module.__name__, env=env_value):
+                    env = {} if env_value is None else {"CM_DEBUG": env_value}
+                    with mock.patch.dict(os.environ, env, clear=True):
+                        self.assertEqual(
+                            helper(),
+                            expected,
+                            f"{module.__name__}._debug_enabled() should be "
+                            f"{expected!r} for CM_DEBUG={env_value!r}",
+                        )
+
+
+if __name__ == "__main__":
+    unittest.main()