diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..2bf9991
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,17 @@
+"""Test-wide isolation: a developer's local `.env` (e.g. OPENROUTER_API_KEY) must never make tests call a real
+LLM provider — that costs money and makes results depend on the network. Agent tests pass explicit
+providers/settings; everything else runs with the chat agent off."""
+
+import pytest
+
+from app.config import get_settings
+
+
+@pytest.fixture(autouse=True)
+def _no_live_llm(monkeypatch: pytest.MonkeyPatch):
+    monkeypatch.setenv("CHAT_AGENT", "off")
+    monkeypatch.setenv("OPENROUTER_API_KEY", "")
+    monkeypatch.setenv("LLM_PROVIDER", "mock")
+    get_settings.cache_clear()
+    yield
+    get_settings.cache_clear()