I’m running into an issue with the Agent Node (v3.1 stable) using an Anthropic model with Extended Thinking enabled and a Think tool connected.
As soon as the Think tool is attached, the agent fails to determine what to do and throws the error: “undefined”
This setup worked fine before the update. After upgrading to v3.1, it broke. Seems like a regression/bug to me.
Has anyone else encountered this? Any workarounds?
@tobiasai Welcome
Because using both at the same time in n8n v3.1 causes a conflict, either disable the model’s Extended Thinking feature or remove the external Think tool.
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Do you know what the reason ist? i mean thinking is for some cases very important but if i can not reference the row id in my database i can not save the output 
a little bit confusing why n8n make this logik…
@tobiasai This looks like a regression in 3.1 when combining Anthropic Extended Thinking with a Think tool the agent gets confused and returns undefined / not sure what item to use. For now, a workaround is to use only one of them at a time (either Extended Thinking without the Think tool, or the Think tool with Extended Thinking turned off). Hope this helps.
Thanks for the quick answer. Yes I saw this. Do you know the think tool? Is it only to make the thoughts of the agent visible or does it have an impact on the action?
Because we use it at this time only to make the thought visible but I don’t know if it has a confident impact on the output quality
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hi @tobiasai
Welcome to the n8n community!
The Think tool in n8n is mainly for visibility and debugging: it exposes the agent’s reasoning but does not directly improve output quality or influence action selection. The model already “thinks” internally even without it, and with Anthropic models, Extended Thinking covers that reasoning at the model level. So if you’re only using the Think tool to see the thoughts, disabling it won’t reduce output quality and avoids the current conflict in v3.1.