diff --git a/router/tests/test_save_best_model_to_disk.py b/router/tests/test_save_best_model_to_disk.py
new file mode 100644
index 0000000..ebf67b2
--- /dev/null
+++ b/router/tests/test_save_best_model_to_disk.py
@@ -0,0 +1,41 @@
+import pytest
+from unittest.mock import patch, mock_open
+from router import main
+
+def test_save_best_model_to_disk_success():
+    best_model = {"model_name": "agent-gemma", "score": 100}
+
+    mock_file = mock_open()
+
+    with patch("builtins.open", mock_file), \
+         patch("json.dump") as mock_json_dump, \
+         patch("datetime.datetime") as mock_datetime:
+
+        # Setup mock datetime
+        mock_now = mock_datetime.now.return_value
+        mock_now.isoformat.return_value = "2023-10-25T12:00:00+00:00"
+
+        main._save_best_model_to_disk(best_model)
+
+        # Verify file opened correctly
+        mock_file.assert_called_once_with("/config/router_dir/best_free_model.json", "w")
+
+        # Verify json.dump called with correct payload
+        expected_payload = {
+            "model_name": "agent-gemma",
+            "score": 100,
+            "updated_at": "2023-10-25T12:00:00Z"
+        }
+        mock_json_dump.assert_called_once_with(expected_payload, mock_file(), indent=2)
+
+def test_save_best_model_to_disk_exception_handled():
+    best_model = {"model_name": "agent-gemma"}
+
+    mock_file = mock_open()
+    mock_file.side_effect = PermissionError("Cannot write to disk")
+
+    # Should not raise exception
+    with patch("builtins.open", mock_file):
+        main._save_best_model_to_disk(best_model)
+
+    mock_file.assert_called_once_with("/config/router_dir/best_free_model.json", "w")