fix: fix mcp output_schema is optional (#39453)

Co-authored-by: yunlu.wen <yunlu.wen@dify.ai>

api/core/tools/mcp_tool/tool.py

@@ -107,6 +107,8 @@ class MCPTool(Tool):
         if self.entity.output_schema and result.structuredContent:
             for k, v in result.structuredContent.items():
                 yield self.create_variable_message(k, v)
+        elif result.structuredContent:
+            yield self.create_json_message(result.structuredContent)
 
     def _process_text_content(self, content: TextContent) -> Generator[ToolInvokeMessage, None, None]:
         """Process text content and yield appropriate messages."""

api/tests/unit_tests/tools/test_mcp_tool.py

@@ -135,6 +135,19 @@ class TestMCPToolInvoke:
         values = {m.message.variable_name: m.message.variable_value for m in var_msgs}
         assert values == {"a": 1, "b": "x"}
 
+    def test_invoke_yields_json_when_structured_content_has_no_output_schema(self, orm_session: Session) -> None:
+        tool = _make_mcp_tool()
+        result = CallToolResult(content=[], structuredContent={"a": 1, "b": "x"})
+
+        with patch.object(tool, "invoke_remote_mcp_tool", return_value=result):
+            messages = list(tool._invoke(session=orm_session, user_id="test_user", tool_parameters={}))
+
+        assert len(messages) == 1
+        msg = messages[0]
+        assert msg.type == ToolInvokeMessage.MessageType.JSON
+        assert isinstance(msg.message, ToolInvokeMessage.JsonMessage)
+        assert msg.message.json_object == {"a": 1, "b": "x"}
+
 
 class TestMCPToolUsageExtraction:
     """Test usage metadata extraction from MCP tool results."""