diff --git a/tests/hostcfgd/conftest.py b/tests/hostcfgd/conftest.py
index 4e41ca07..67ffb30b 100644
--- a/tests/hostcfgd/conftest.py
+++ b/tests/hostcfgd/conftest.py
@@ -5,3 +5,18 @@
 @pytest.fixture(autouse=True, scope='session')
 def mock_get_device_runtime_metadata():
     device_info.get_device_runtime_metadata = mock.MagicMock(return_value={})
+
+@pytest.fixture(autouse=True, scope='session')
+def mock_open_proc_cmdline():
+    """Automatically mock opening /proc/cmdline for all tests."""
+    EMPTY_LINE = "\n"
+    original_open = open
+
+    def mock_open_side_effect(filename, *args, **kwargs):
+        if filename == "/proc/cmdline":
+            return mock.mock_open(read_data=EMPTY_LINE)()
+        else:
+            return original_open(filename, *args, **kwargs)
+
+    with mock.patch("builtins.open", side_effect=mock_open_side_effect):
+        yield