diff --git a/tests/test_peer_route_via.py b/tests/test_peer_route_via.py
index b8884ce..e7e7804 100644
--- a/tests/test_peer_route_via.py
+++ b/tests/test_peer_route_via.py
@@ -156,5 +156,47 @@ class TestSetPeerRouteVia(unittest.TestCase):
         self.assertEqual(relay_calls[1].args, ('new-exit', True))
 
 
+    @patch('app.cell_link_manager')
+    @patch('app.wireguard_manager')
+    @patch('app.peer_registry')
+    def test_loop_detection_returns_409(self, mock_reg, mock_wg, mock_clm):
+        """If the target cell already routes peers through us (remote_exit_relay_active=True),
+        enabling route-via would create an A→B→A loop — expect 409."""
+        loop_link = dict(_CELL_LINK, remote_exit_relay_active=True)
+        mock_reg.get_peer.return_value = _PEER
+        mock_clm.list_connections.return_value = [loop_link]
+        r = self._put('alice', {'via_cell': 'exit-cell'})
+        self.assertEqual(r.status_code, 409)
+        data = json.loads(r.data)
+        self.assertIn('loop', data['error'].lower())
+        mock_wg.update_cell_peer_allowed_ips.assert_not_called()
+
+    @patch('app.cell_link_manager')
+    @patch('app.wireguard_manager')
+    @patch('app.peer_registry')
+    def test_loop_detection_skipped_when_disabling(self, mock_reg, mock_wg, mock_clm):
+        """Disabling route-via (via_cell=null) must NOT be blocked by the loop check."""
+        loop_link = dict(_CELL_LINK, remote_exit_relay_active=True)
+        peer_with_via = dict(_PEER, route_via='exit-cell')
+        mock_reg.get_peer.return_value = peer_with_via
+        mock_reg.set_route_via.return_value = dict(_PEER, route_via=None)
+        mock_clm.list_connections.return_value = [loop_link]
+        r = self._put('alice', {'via_cell': None})
+        self.assertEqual(r.status_code, 200)
+
+    @patch('app.cell_link_manager')
+    @patch('app.wireguard_manager')
+    @patch('app.peer_registry')
+    def test_no_loop_when_remote_exit_relay_false(self, mock_reg, mock_wg, mock_clm):
+        """remote_exit_relay_active=False means no loop — route-via should succeed."""
+        safe_link = dict(_CELL_LINK, remote_exit_relay_active=False)
+        mock_reg.get_peer.return_value = _PEER
+        mock_reg.set_route_via.return_value = dict(_PEER, route_via='exit-cell')
+        mock_clm.list_connections.return_value = [safe_link]
+        mock_clm.set_exit_relay_active.return_value = safe_link
+        r = self._put('alice', {'via_cell': 'exit-cell'})
+        self.assertEqual(r.status_code, 200)
+
+
 if __name__ == '__main__':
     unittest.main()