diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..5f7da2b
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,43 @@
+"""Keep the test suite away from the developer's real DevOrch data.
+
+Several modules compute ``Path.home() / ".devorch"`` at import time, and some tests (for example
+``Permissions().set_tool_permission(...)``) save through those paths. Without isolation, running
+``pytest`` overwrote the developer's real ``~/.devorch/permissions.yaml`` (replacing their saved rules
+with ``edit: allow`` / ``filesystem: allow``) and wrote to their real sessions folder.
+
+This must run before the application modules are imported, hence environment variables set at import
+time of this conftest rather than a fixture.
+"""
+
+import os
+import shutil
+import tempfile
+
+_TEST_HOME = tempfile.mkdtemp(prefix="devorch-test-home-")
+
+# Path.home() reads HOME on POSIX and USERPROFILE on Windows
+os.environ["HOME"] = _TEST_HOME
+os.environ["USERPROFILE"] = _TEST_HOME
+os.environ.pop("HOMEDRIVE", None)
+os.environ.pop("HOMEPATH", None)
+# never read from or write to the developer's real keychain
+os.environ["PYTHON_KEYRING_BACKEND"] = "keyring.backends.null.Keyring"
+# and no real API keys leak into tests
+for _var in (
+    "OPENAI_API_KEY",
+    "ANTHROPIC_API_KEY",
+    "GOOGLE_API_KEY",
+    "GROQ_API_KEY",
+    "MISTRAL_API_KEY",
+    "OPENROUTER_API_KEY",
+    "TOGETHER_API_KEY",
+    "GITHUB_TOKEN",
+    "DEEPSEEK_API_KEY",
+    "MOONSHOT_API_KEY",
+    "CUSTOM_API_KEY",
+):
+    os.environ.pop(_var, None)
+
+
+def pytest_sessionfinish(session, exitstatus):
+    shutil.rmtree(_TEST_HOME, ignore_errors=True)
diff --git a/tests/test_isolation.py b/tests/test_isolation.py
new file mode 100644
index 0000000..48b65f7
--- /dev/null
+++ b/tests/test_isolation.py
@@ -0,0 +1,41 @@
+"""The suite must never read or write the developer's real ~/.devorch."""
+
+import os
+from pathlib import Path
+
+from config import permissions as perm_mod
+from config import settings as settings_mod
+from core import sessions as sessions_mod
+from core.memory import MEMORY_DIR
+
+
+def test_home_is_a_throwaway_directory():
+    home = Path.home()
+    assert "devorch-test-home-" in home.name
+    assert home == Path(os.environ["HOME"])
+
+
+def test_application_paths_resolve_under_the_test_home():
+    home = Path.home()
+    for path in (
+        perm_mod.PERMISSIONS_FILE,
+        settings_mod.CONFIG_FILE,
+        sessions_mod.DB_PATH,
+        MEMORY_DIR,
+    ):
+        assert home in path.parents, f"{path} is outside the test HOME"
+
+
+def test_saving_permissions_does_not_touch_the_real_file():
+    from config.permissions import PermissionLevel, Permissions
+
+    Permissions().set_tool_permission("edit", PermissionLevel.ALLOW)  # saves to PERMISSIONS_FILE
+    assert perm_mod.PERMISSIONS_FILE.exists()
+    assert Path.home() in perm_mod.PERMISSIONS_FILE.parents
+
+
+def test_keyring_is_the_null_backend():
+    import keyring
+
+    assert keyring.get_password("devorch", "openai") is None
+    assert type(keyring.get_keyring()).__module__ == "keyring.backends.null"