diff --git a/tests/utils/test_utils.py b/tests/utils/test_utils.py
index 5bbe3be3..054c010c 100644
--- a/tests/utils/test_utils.py
+++ b/tests/utils/test_utils.py
@@ -31,21 +31,48 @@ class TestHelpers(unittest.TestCase):
         dict2 = {"b": {"d": 3}, "e": 4}
         merged = helpers.merge_dicts(dict1, dict2, deep=True)
         self.assertEqual(merged, {"a": 1, "b": {"c": 2, "d": 3}, "e": 4})
+
     def test_flatten_dict(self):
-        data = {"a": {"b": 1, "c": 2}}
+        """Basic nested flattening with multiple sibling keys."""
+        data = {"a": {"b": 1, "c": 2}, "d": 3}
         result = helpers.flatten_dict(data)
-        self.assertEqual(result, {"a.b": 1, "a.c": 2})
+        self.assertEqual(result, {"a.b": 1, "a.c": 2, "d": 3})
+
+    def test_flatten_dict_deeply_nested(self):
+        """Deeply nested structure is fully flattened."""
+        self.assertEqual(
+            helpers.flatten_dict({"a": {"b": {"c": 1}}}),
+            {"a.b.c": 1},
+        )
+
+    def test_flatten_dict_custom_separator(self):
+        """Custom separator is used in generated keys."""
+        self.assertEqual(
+            helpers.flatten_dict({"a": {"b": 1}}, sep="__"),
+            {"a__b": 1},
+        )
+
+    def test_flatten_dict_empty(self):
+        """Empty input returns empty output."""
+        self.assertEqual(helpers.flatten_dict({}), {})
 
     def test_flatten_dict_key_collision(self):
-        data = {
-            "a.b": 1,
-            "a": {
-                "b": 2
-            }
-        }
+        """#1010 regression: a top-level key containing the separator must not
+        silently overwrite a value produced from a nested dict when both resolve
+        to the same flattened key.  Before the fix, {'a.b': 1, 'a': {'b': 2}}
+        silently dropped one value; now it raises ValueError."""
+        with self.assertRaises(ValueError) as ctx:
+            helpers.flatten_dict({"a.b": 1, "a": {"b": 2}})
+        self.assertIn("Key collision", str(ctx.exception))
+        self.assertIn("a.b", str(ctx.exception))
 
-        with self.assertRaises(ValueError):
-            helpers.flatten_dict(data)
+    def test_flatten_dict_no_false_positive(self):
+        """Similar-looking keys that produce distinct flattened keys must not
+        trigger the collision guard."""
+        self.assertEqual(
+            helpers.flatten_dict({"a.b": 1, "a": {"c": 2}}),
+            {"a.b": 1, "a.c": 2},
+        )
 
     def test_safe_import_returns_module_and_flag(self):
         module, available = helpers.safe_import("json")