fix(mcp): fail the node when an MCP tool returns isError

apps/application/flow/step_node/mcp_node/impl/base_mcp_node.py

@@ -47,6 +47,12 @@ def execute(self, mcp_servers, mcp_server, mcp_tool, mcp_tool_id, mcp_source, to
 
         backend = SandboxMCPBackend(servers)
         res = asyncio.run(backend.call_tool(mcp_server, mcp_tool, params))
+        # A failed MCP tool call is still a normal JSON-RPC result with isError set and the
+        # error text in content. Raise like the other nodes do, so the run stops (or takes
+        # the exception branch) instead of passing the error text downstream as a result.
+        if res.isError:
+            message = '\n'.join(content.text for content in res.content if getattr(content, 'text', None))
+            raise Exception(f"MCP tool {mcp_tool} execution failed" + (f': {message}' if message else ''))
         return NodeResult(
             {'result': [content.text for content in res.content], 'tool_params': params, 'mcp_tool': mcp_tool}, {})
 

apps/application/test_mcp_node.py

@@ -0,0 +1,75 @@
+# coding=utf-8
+import asyncio
+import importlib.util
+import unittest
+from pathlib import Path
+from types import SimpleNamespace
+from unittest.mock import Mock, patch
+
+
+class McpNodeToolErrorTestCase(unittest.TestCase):
+    def load_node_module(self, call_tool):
+        # Isolate Django and the MCP sandbox so the node can be tested without a database.
+        backend = SimpleNamespace(call_tool=call_tool)
+        dependencies = {
+            'application.flow.backend.sandbox_mcp': SimpleNamespace(SandboxMCPBackend=Mock(return_value=backend)),
+            'application.flow.i_step_node': SimpleNamespace(
+                NodeResult=lambda node_variable, workflow_variable: SimpleNamespace(
+                    node_variable=node_variable, workflow_variable=workflow_variable
+                )
+            ),
+            'application.flow.step_node.mcp_node.i_mcp_node': SimpleNamespace(IMcpNode=object),
+            'common.utils.shared_resource_auth': SimpleNamespace(
+                get_runtime_user_id=Mock(), validate_authorized_tool_ids=Mock()
+            ),
+            'common.utils.tool_code': SimpleNamespace(
+                ToolExecutor=Mock(return_value=SimpleNamespace(validate_mcp_transport=Mock()))
+            ),
+            'django.db.models': SimpleNamespace(QuerySet=Mock()),
+            'tools.models': SimpleNamespace(Tool=Mock()),
+        }
+        source = Path(__file__).parent / 'flow/step_node/mcp_node/impl/base_mcp_node.py'
+        spec = importlib.util.spec_from_file_location('base_mcp_node_under_test', source)
+        module = importlib.util.module_from_spec(spec)
+        with patch.dict('sys.modules', dependencies):
+            spec.loader.exec_module(module)
+        return module
+
+    @staticmethod
+    def text_content(*texts):
+        return [SimpleNamespace(text=text) for text in texts]
+
+    def execute(self, result):
+        async def call_tool(mcp_server, mcp_tool, params):
+            return result
+
+        module = self.load_node_module(call_tool)
+        return module.BaseMcpNode().execute('{}', 'demo', 'close_ticket', None, 'select', {})
+
+    def test_raising_node_error_when_the_tool_reports_is_error(self):
+        result = SimpleNamespace(
+            isError=True, content=self.text_content('Ticket T-1001 cannot be closed: it has open sub-tasks')
+        )
+        with self.assertRaises(Exception) as raised:
+            self.execute(result)
+        self.assertEqual(
+            str(raised.exception),
+            'MCP tool close_ticket execution failed: Ticket T-1001 cannot be closed: it has open sub-tasks',
+        )
+
+    def test_keeping_the_error_text_without_a_content_message(self):
+        result = SimpleNamespace(isError=True, content=[])
+        with self.assertRaises(Exception) as raised:
+            self.execute(result)
+        self.assertEqual(str(raised.exception), 'MCP tool close_ticket execution failed')
+
+    def test_returning_the_tool_result_when_is_error_is_not_set(self):
+        result = SimpleNamespace(isError=False, content=self.text_content('T-1001 is open'))
+        node_result = self.execute(result)
+        self.assertEqual(node_result.node_variable['result'], ['T-1001 is open'])
+        self.assertEqual(node_result.node_variable['mcp_tool'], 'close_ticket')
+        self.assertEqual(node_result.workflow_variable, {})
+
+
+if __name__ == '__main__':
+    unittest.main()