diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 4ed1aefb2..2045e3180 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -340,6 +340,13 @@ pytest backend/tests/unit/         # Unit tests only
 pytest backend/tests/ --cov=backend  # With coverage
 ```
 
+`conftest.py` redirects `DATABASE_URL` to a throwaway SQLite file before any app
+module is imported, and aborts the run if that redirect did not take. Your `.env`
+is ignored for the duration, so the suite cannot reach the database you develop
+against — it exercises code paths that open their own sessions, and some of those
+write. Don't undo that override to "test against real data": point `DATABASE_URL`
+at a copy instead.
+
 **Frontend** — tests use [Vitest](https://vitest.dev/) and are in `frontend/src/__tests__/`:
 
 ```bash