fix agent putout the output of workflow-tool twice (#26835)

api/core/tools/tool_engine.py

@@ -228,29 +228,39 @@ def _convert_tool_response_to_str(tool_response: list[ToolInvokeMessage]) -> str
         """
         Handle tool response
         """
-        result = ""
+        parts: list[str] = []
+        json_parts: list[str] = []
+        saw_text = False
+
         for response in tool_response:
             if response.type == ToolInvokeMessage.MessageType.TEXT:
-                result += cast(ToolInvokeMessage.TextMessage, response.message).text
+                parts.append(cast(ToolInvokeMessage.TextMessage, response.message).text)
+                saw_text = True
             elif response.type == ToolInvokeMessage.MessageType.LINK:
-                result += (
+                parts.append(
                     f"result link: {cast(ToolInvokeMessage.TextMessage, response.message).text}."
                     + " please tell user to check it."
                 )
             elif response.type in {ToolInvokeMessage.MessageType.IMAGE_LINK, ToolInvokeMessage.MessageType.IMAGE}:
-                result += (
+                parts.append(
                     "image has been created and sent to user already, "
                     + "you do not need to create it, just tell the user to check it now."
                 )
             elif response.type == ToolInvokeMessage.MessageType.JSON:
-                result += json.dumps(
-                    safe_json_value(cast(ToolInvokeMessage.JsonMessage, response.message).json_object),
-                    ensure_ascii=False,
+                json_parts.append(
+                    json.dumps(
+                        safe_json_value(cast(ToolInvokeMessage.JsonMessage, response.message).json_object),
+                        ensure_ascii=False,
+                    )
                 )
             else:
-                result += str(response.message)
+                parts.append(str(response.message))
 
-        return result
+         # If no TEXT message is present, append JSON parts to avoid duplication.
+        if not saw_text and json_parts:
+            parts.extend(json_parts)
+
+        return "".join(parts)
 
     @staticmethod
     def _extract_tool_response_binary_and_text(