fix(mcp): prevent DetachedInstanceError in get_chart_preview

superset/mcp_service/chart/tool/get_chart_preview.py

@@ -28,7 +28,7 @@
 
 from superset.commands.exceptions import CommandException
 from superset.exceptions import OAuth2Error, OAuth2RedirectError, SupersetException
-from superset.extensions import event_logger
+from superset.extensions import db, event_logger
 from superset.mcp_service.chart.ascii_charts import (
     generate_ascii_chart,
     generate_ascii_table,
@@ -1140,6 +1140,15 @@ def __init__(self, fd: Dict[str, Any]):
                     )
                 chart = find_chart_by_identifier(request.identifier)
 
+                # Eagerly refresh all attributes while the session is still
+                # active.  SQLAlchemy expires object attributes after any
+                # commit; if a downstream operation commits before the strategy
+                # classes access chart attributes, a DetachedInstanceError will
+                # be raised.  Calling refresh() here ensures all column values
+                # are loaded into the object's __dict__ upfront.
+                if chart is not None:
+                    db.session.refresh(chart)
+
                 # If not found and looks like a form_data_key, try transient
                 if (
                     not chart
@@ -1371,6 +1380,20 @@ def __init__(self, form_data: Dict[str, Any]):
 
         return _sanitize_chart_preview_for_llm_context(result)
 
+    except SQLAlchemyError as e:
+        # Catch DetachedInstanceError and other SQLAlchemy errors that can
+        # surface when the ORM session expires or commits mid-request.
+        await ctx.error(
+            "Chart preview failed due to database session error: "
+            "identifier=%s, error_type=%s, error=%s"
+            % (request.identifier, type(e).__name__, str(e))
+        )
+        logger.exception("SQLAlchemy error in get_chart_preview: %s", e)
+        return ChartError(
+            error="Database session error while generating chart preview. "
+            "Please retry the request.",
+            error_type="InternalError",
+        )
     except (
         CommandException,
         SupersetException,

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

@@ -595,3 +595,191 @@ async def test_ascii_art_variations(self):
 """
 
         # These demonstrate the expected ASCII formats for different chart types
+
+
+class TestDetachedInstanceError:
+    """Tests that DetachedInstanceError is handled gracefully.
+
+    When the SQLAlchemy session commits mid-request, ORM objects expire and
+    become detached.  Accessing lazy attributes on a detached Slice raises
+    DetachedInstanceError.  The tool must:
+    1. Call db.session.refresh() immediately after loading the chart so all
+       column values are loaded upfront before any downstream operation.
+    2. Catch SQLAlchemyError (the base class) and return a ChartError
+       instead of propagating the exception.
+    """
+
+    @pytest.mark.asyncio
+    async def test_session_refresh_called_after_chart_load(self):
+        """db.session.refresh() is invoked right after find_chart_by_identifier."""
+        import importlib
+        from contextlib import nullcontext
+        from unittest.mock import MagicMock, patch
+
+        from superset.mcp_service.chart.schemas import URLPreview
+        from superset.utils import json
+
+        get_chart_preview_module = importlib.import_module(
+            "superset.mcp_service.chart.tool.get_chart_preview"
+        )
+
+        mock_chart = MagicMock()
+        mock_chart.id = 42
+        mock_chart.slice_name = "Sales Chart"
+        mock_chart.viz_type = "table"
+        mock_chart.datasource_id = 1
+        mock_chart.datasource_type = "table"
+        mock_chart.params = "{}"
+
+        refresh_calls: list[object] = []
+
+        def _fake_refresh(obj: object) -> None:
+            refresh_calls.append(obj)
+
+        url_preview = URLPreview(
+            preview_url="http://localhost/explore/?slice_id=42",
+            width=800,
+            height=600,
+        )
+
+        with (
+            patch.object(
+                get_chart_preview_module,
+                "find_chart_by_identifier",
+                return_value=mock_chart,
+            ),
+            patch.object(
+                get_chart_preview_module.db,
+                "session",
+                **{"refresh.side_effect": _fake_refresh},
+            ),
+            patch.object(
+                get_chart_preview_module,
+                "validate_chart_dataset",
+                return_value=MagicMock(is_valid=True, warnings=[]),
+            ),
+            patch.object(
+                get_chart_preview_module.event_logger,
+                "log_context",
+                return_value=nullcontext(),
+            ),
+            # Return a real URLPreview so Pydantic model validation succeeds
+            patch.object(
+                get_chart_preview_module.PreviewFormatGenerator,
+                "generate",
+                return_value=url_preview,
+            ),
+            patch(
+                "superset.mcp_service.utils.url_utils.get_superset_base_url",
+                return_value="http://localhost",
+            ),
+        ):
+            from fastmcp import Client
+
+            from superset.mcp_service.app import mcp
+            from superset.mcp_service.chart.schemas import GetChartPreviewRequest
+
+            with patch("superset.mcp_service.auth.get_user_from_request") as mu:
+                mu.return_value = MagicMock(id=1, username="admin")
+                with patch(
+                    "superset.mcp_service.auth.check_tool_permission", return_value=True
+                ):
+                    async with Client(mcp) as client:
+                        response = await client.call_tool(
+                            "get_chart_preview",
+                            {
+                                "request": GetChartPreviewRequest(
+                                    identifier=42, format="url"
+                                ).model_dump()
+                            },
+                        )
+
+        data = json.loads(response.content[0].text)
+        # The tool should succeed — not return a ChartError
+        assert "error_type" not in data, (
+            f"Expected ChartPreview but got ChartError: {data.get('error')}"
+        )
+        assert data.get("chart_id") == 42
+
+        assert len(refresh_calls) == 1, (
+            "db.session.refresh() should be called once after loading the chart"
+        )
+        assert refresh_calls[0] is mock_chart
+
+    @pytest.mark.asyncio
+    async def test_detached_instance_error_returns_chart_error(self):
+        """DetachedInstanceError during preview generation returns ChartError."""
+        import importlib
+        from contextlib import nullcontext
+        from unittest.mock import MagicMock, patch
+
+        from sqlalchemy.orm.exc import DetachedInstanceError
+
+        get_chart_preview_module = importlib.import_module(
+            "superset.mcp_service.chart.tool.get_chart_preview"
+        )
+
+        mock_chart = MagicMock()
+        mock_chart.id = 7
+        mock_chart.slice_name = "Broken Chart"
+        mock_chart.viz_type = "bar"
+        mock_chart.datasource_id = 3
+        mock_chart.datasource_type = "table"
+        mock_chart.params = "{}"
+
+        with (
+            patch.object(
+                get_chart_preview_module,
+                "find_chart_by_identifier",
+                return_value=mock_chart,
+            ),
+            patch.object(
+                get_chart_preview_module.db,
+                "session",
+                **{"refresh.return_value": None},
+            ),
+            patch.object(
+                get_chart_preview_module,
+                "validate_chart_dataset",
+                return_value=MagicMock(is_valid=True, warnings=[]),
+            ),
+            patch.object(
+                get_chart_preview_module.event_logger,
+                "log_context",
+                return_value=nullcontext(),
+            ),
+            # Simulate the session expiring inside the strategy
+            patch.object(
+                get_chart_preview_module.PreviewFormatGenerator,
+                "generate",
+                side_effect=DetachedInstanceError(),
+            ),
+            patch(
+                "superset.mcp_service.utils.url_utils.get_superset_base_url",
+                return_value="http://localhost",
+            ),
+        ):
+            from fastmcp import Client
+
+            from superset.mcp_service.app import mcp
+            from superset.mcp_service.chart.schemas import GetChartPreviewRequest
+            from superset.utils import json
+
+            with patch("superset.mcp_service.auth.get_user_from_request") as mu:
+                mu.return_value = MagicMock(id=1, username="admin")
+                with patch(
+                    "superset.mcp_service.auth.check_tool_permission", return_value=True
+                ):
+                    async with Client(mcp) as client:
+                        response = await client.call_tool(
+                            "get_chart_preview",
+                            {
+                                "request": GetChartPreviewRequest(
+                                    identifier=7, format="ascii"
+                                ).model_dump()
+                            },
+                        )
+
+        data = json.loads(response.content[0].text)
+        assert data["error_type"] == "InternalError"
+        assert "session" in data["error"].lower() or "retry" in data["error"].lower()