fix(mcp): preserve chart form data on update

superset/mcp_service/chart/tool/update_chart.py

@@ -229,11 +229,15 @@ def _build_update_payload(
     )
 
     if parsed_config is not None:
+        existing_form_data = _get_existing_form_data(chart)
         new_form_data = map_config_to_form_data(
             parsed_config, dataset_id=effective_dataset_id
         )
         new_form_data.pop("_mcp_warnings", None)
-        merge_table_column_config(_get_existing_form_data(chart), new_form_data)
+        merge_table_column_config(existing_form_data, new_form_data)
+        new_form_data = _merge_replacement_config(
+            existing_form_data, new_form_data, parsed_config
+        )
 
         chart_name = (
             request.chart_name

tests/unit_tests/mcp_service/chart/tool/test_update_chart.py

@@ -36,6 +36,7 @@ from superset.mcp_service.chart.schemas import (
     FilterConfig,
     GenerateChartResponse,
     LegendConfig,
+    MixedTimeseriesChartConfig,
     TableChartConfig,
     UpdateChartRequest,
     XYChartConfig,
@@ -738,6 +739,36 @@ class TestBuildUpdatePayload:
         # query_context must be cleared so get_chart_data uses updated params
         assert result["query_context"] is None
 
+    def test_config_update_preserves_unrelated_mixed_timeseries_settings(self):
+        """Save payload retains settings outside the simplified config schema."""
+        config = MixedTimeseriesChartConfig(
+            x=ColumnRef(name="ds"),
+            y=[ColumnRef(name="new_primary", aggregate="SUM")],
+            y_secondary=[ColumnRef(name="new_secondary", aggregate="SUM")],
+        )
+        request = UpdateChartRequest(identifier=1, config=config)
+        chart = Mock(
+            id=1,
+            datasource_id=7,
+            slice_name="Year-over-year metrics",
+            params=json.dumps(
+                {
+                    "viz_type": "mixed_timeseries",
+                    "time_compare": ["1 year ago"],
+                    "comparison_type_b": "percentage",
+                    "y_axis_format": ",.2f",
+                }
+            ),
+        )
+
+        result = _build_update_payload(request, chart, parsed_config=config)
+
+        assert isinstance(result, dict)
+        saved_form_data = json.loads(result["params"])
+        assert saved_form_data["time_compare"] == ["1 year ago"]
+        assert saved_form_data["comparison_type_b"] == "percentage"
+        assert saved_form_data["y_axis_format"] == ",.2f"
+
     def test_add_columns_preserves_existing_columns_and_metrics(self):
         """An additive update does not require reconstructing the table."""
         request = UpdateChartRequest(