diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 00000000..0610a629
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,52 @@
+"""Test configuration to keep the suite hermetic with respect to user config.
+
+Several command modules read global settings through ConfigManager at import
+time (for example the click ``--output_format`` default in
+``surfactant/cmd/generate.py``). ConfigManager is a process-wide singleton
+that, with the default ``config_dir=None``, reads the developer's real user
+config such as ``~/.config/surfactant/config.toml``. An invalid value there
+(for example ``core.output_format = ""``) then makes otherwise-correct tests
+fail, which is exactly the problem reported in issue #470.
+
+To make the suite independent of any global settings, point the config and
+data lookups at empty temporary directories. This is done at module import
+(before test modules import the command modules whose defaults are evaluated
+eagerly) and again per test for anything created at runtime.
+"""
+
+import os
+import tempfile
+
+import pytest
+
+from surfactant.configmanager import ConfigManager
+
+# Redirect config/data lookups to empty temp dirs before any test module
+# imports command modules that read config defaults at import time.
+_ISOLATED_CONFIG_DIR = tempfile.mkdtemp(prefix="surfactant-test-config-")
+_ISOLATED_DATA_DIR = tempfile.mkdtemp(prefix="surfactant-test-data-")
+os.environ["XDG_CONFIG_HOME"] = _ISOLATED_CONFIG_DIR
+os.environ["XDG_DATA_HOME"] = _ISOLATED_DATA_DIR
+os.environ["APPDATA"] = _ISOLATED_CONFIG_DIR
+os.environ["LOCALAPPDATA"] = _ISOLATED_DATA_DIR
+ConfigManager._instances.clear()  # pylint: disable=protected-access
+
+
+@pytest.fixture(autouse=True)
+def isolate_surfactant_config(tmp_path, monkeypatch):
+    """Give every test a clean, isolated ConfigManager backed by empty dirs."""
+    config_home = tmp_path / "config"
+    data_home = tmp_path / "data"
+    config_home.mkdir()
+    data_home.mkdir()
+
+    # Unix-like lookups (see ConfigManager._get_config_file_path / get_data_dir_path)
+    monkeypatch.setenv("XDG_CONFIG_HOME", str(config_home))
+    monkeypatch.setenv("XDG_DATA_HOME", str(data_home))
+    # Windows lookups
+    monkeypatch.setenv("APPDATA", str(config_home))
+    monkeypatch.setenv("LOCALAPPDATA", str(data_home))
+
+    ConfigManager._instances.clear()  # pylint: disable=protected-access
+    yield
+    ConfigManager._instances.clear()  # pylint: disable=protected-access