diff --git a/tests/__init__.py b/tests/__init__.py
index e69de29b..9edbc0fa 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -0,0 +1,13 @@
+"""No test downloads a release asset unless it says so.
+
+The CI job sets RETROBIOS_OFFLINE; a local run of the documented command
+did not, and e2e fixtures queried the large-files release and wrote its
+answers into the shared .cache/large. A test that serves its own assets
+clears the variable for itself.
+"""
+
+from __future__ import annotations
+
+import os
+
+os.environ.setdefault("RETROBIOS_OFFLINE", "1")
diff --git a/tests/test_audit_regressions.py b/tests/test_audit_regressions.py
index 8fceaf89..5eed1c0a 100644
--- a/tests/test_audit_regressions.py
+++ b/tests/test_audit_regressions.py
@@ -3367,5 +3367,11 @@ class EveryToolRefusesAFlagItWouldDrop(unittest.TestCase):
                     self.assertEqual(proc.returncode, 2, proc.stdout + proc.stderr)
 
 
+
+class TestsRunOffline(unittest.TestCase):
+    def test_the_package_forbids_downloads(self):
+        self.assertTrue(os.environ.get("RETROBIOS_OFFLINE"))
+
+
 if __name__ == "__main__":
     unittest.main()