@@ -1568,6 +1568,13 @@ def _prepare_entities_to_read_from_online_store(
         entityless_case,
     ) = _get_cached_request_context(registry, project, features, full_feature_names)
 
+    # Online stores may append internal fields to the requested feature lists.
+    # Keep those request-local so the cached resolution result remains unchanged.
+    grouped_refs = [
+        (feature_view, list(requested_features))
+        for feature_view, requested_features in grouped_refs
+    ]
+
     # Mutable copy — downstream code adds join keys to this set.
     requested_result_row_names = set(requested_result_row_names_frozen)
 
 
@@ -6,6 +6,7 @@
 import pytest
 
 import feast.utils as utils
+from feast.protos.feast.types.Value_pb2 import Value as ValueProto
 
 
 @pytest.fixture(autouse=True)
@@ -140,6 +141,36 @@ def test_returned_result_row_names_is_mutable_copy(self, mock_ctx):
         assert isinstance(result_row_names_1, frozenset)
         assert result_row_names_1 == result_row_names_2
 
+    @patch("feast.utils._get_online_request_context")
+    def test_grouped_refs_are_copied_per_request(self, mock_ctx):
+        """Downstream mutations must not leak into cached grouped refs."""
+        mock_ctx.return_value = _make_context()
+        registry = _make_registry(datetime.now(tz=timezone.utc))
+        entity_values = {"user_id": [ValueProto(int64_val=1)]}
+
+        first_result = utils._prepare_entities_to_read_from_online_store(
+            registry,
+            "proj",
+            ["fv:feat1"],
+            entity_values,
+            native_entity_values=False,
+        )
+        first_requested_features = first_result[1][0][1]
+        first_requested_features.append("_ts:fv")
+
+        second_result = utils._prepare_entities_to_read_from_online_store(
+            registry,
+            "proj",
+            ["fv:feat1"],
+            entity_values,
+            native_entity_values=False,
+        )
+        second_requested_features = second_result[1][0][1]
+        assert first_requested_features == ["feat1", "_ts:fv"]
+        assert second_requested_features == ["feat1"]
+        assert second_requested_features is not first_requested_features
+        mock_ctx.assert_called_once()
+
     @patch("feast.utils._get_online_request_context")
     def test_feature_service_cached_separately(self, mock_ctx):
         """FeatureService objects should be cached by name."""