Even if your sub-workflow runs successfully and ends with a Set or Code node, the AI Agent still shows this kind of error.
This is because when a sub-workflow is invoked as a tool, it expects a proper response structure.
This behavior has been observed and discussed in the community previously:
Use “Respond to a Webhook” node (Respond to Webhook | n8n Docs) to control the response to incoming webhooks. Try to set “First incoming Item” as response
