fix: omit empty Attributes on UpdateItem UPDATED_NEW/UPDATED_OLD

crates/engine/src/update_item.rs

@@ -196,12 +196,12 @@ pub async fn handle_update_item(
         ReturnValues::None => None,
         ReturnValues::AllOld => old_item,
         ReturnValues::AllNew => new_item,
-        ReturnValues::UpdatedOld => {
-            old_item.map(|item| filter_to_updated_attrs(&item, &actions, &maps))
-        }
-        ReturnValues::UpdatedNew => {
-            new_item.map(|item| filter_to_updated_attrs(&item, &actions, &maps))
-        }
+        ReturnValues::UpdatedOld => old_item
+            .map(|item| filter_to_updated_attrs(&item, &actions, &maps))
+            .filter(|item| !item.is_empty()),
+        ReturnValues::UpdatedNew => new_item
+            .map(|item| filter_to_updated_attrs(&item, &actions, &maps))
+            .filter(|item| !item.is_empty()),
     };
 
     let output = UpdateItemOutput {

tests/test_item_operations.py

@@ -454,6 +454,53 @@ def test_update_item_ne_comparison_missing_attribute(self, dynamodb_client, upd_
         resp = dynamodb_client.get_item(TableName=upd_table, Key={"pk": {"S": "ne-missing"}})
         assert resp["Item"]["data"]["S"] == "ok"
 
+    def test_update_item_remove_with_updated_new_omits_attributes(self, dynamodb_client, upd_table):
+        """REMOVE leaves nothing in UPDATED_NEW: Attributes field must be omitted, not returned as {}."""
+        dynamodb_client.put_item(
+            TableName=upd_table,
+            Item={"pk": {"S": "remove-empty"}, "map_attr": {"M": {"child": {"S": "old"}}}},
+        )
+        resp = dynamodb_client.update_item(
+            TableName=upd_table,
+            Key={"pk": {"S": "remove-empty"}},
+            UpdateExpression="REMOVE map_attr",
+            ReturnValues="UPDATED_NEW",
+        )
+        assert "Attributes" not in resp, f"expected Attributes omitted, got {resp.get('Attributes')!r}"
+
+    def test_update_item_set_new_attribute_with_updated_old_omits_attributes(
+        self, dynamodb_client, upd_table
+    ):
+        """SET on a brand-new attribute has no prior value: UPDATED_OLD must omit Attributes."""
+        dynamodb_client.put_item(
+            TableName=upd_table,
+            Item={"pk": {"S": "set-new-old"}},
+        )
+        resp = dynamodb_client.update_item(
+            TableName=upd_table,
+            Key={"pk": {"S": "set-new-old"}},
+            UpdateExpression="SET fresh_attr = :v",
+            ExpressionAttributeValues={":v": {"S": "new"}},
+            ReturnValues="UPDATED_OLD",
+        )
+        assert "Attributes" not in resp, f"expected Attributes omitted, got {resp.get('Attributes')!r}"
+
+    def test_update_item_legacy_delete_with_updated_new_omits_attributes(
+        self, dynamodb_client, upd_table
+    ):
+        """Legacy AttributeUpdates DELETE on a Map mirrors REMOVE: UPDATED_NEW must omit Attributes."""
+        dynamodb_client.put_item(
+            TableName=upd_table,
+            Item={"pk": {"S": "legacy-delete"}, "map_attr": {"M": {"child": {"S": "old"}}}},
+        )
+        resp = dynamodb_client.update_item(
+            TableName=upd_table,
+            Key={"pk": {"S": "legacy-delete"}},
+            AttributeUpdates={"map_attr": {"Action": "DELETE"}},
+            ReturnValues="UPDATED_NEW",
+        )
+        assert "Attributes" not in resp, f"expected Attributes omitted, got {resp.get('Attributes')!r}"
+
 
 # ---------------------------------------------------------------------------
 # DeleteItem tests