Help：Agent NEVER get any result from workflow tool

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

1, I created a Agent, and add a workflow in "tool" area(enabled for sure), let's name it Workflow_Tool.
2, I asked this Agent to use Workflow_Tool to address content issue, the Agent did call the tool.
3, BUT, when the tool is done, the Agent always say that the Workflow_Tool returned NULL {}, which is impossible.
4, I checked the workflow run history. history clearly shows that workflow received the task, finished it, and RETURNED result.

So, why the Agent can't handle the return value of a workflow tool? is there any bugs of default API definition? Please help, thanks a lot!

2. Additional context or comments

No response