Yesterday I tried the AI builder for the first time and it worked. I consumed about 25 credits. Today I opened the same workflow and the first prompt led to this error message:
messages.117: tool_use ids were found without tool_result blocks immediately after: toolu_vrtx_01Ra3f4M1FXTKGHJ496E7bcD. Each tool_use block must have a corresponding tool_result block in the next message.
It looks like a langraph error. But there’s no AI agent or AI node in the workflow.
This error is related to the AI Builder’s internal tool handling, even though you don’t see AI nodes directly in your workflow. The AI Builder uses tools behind the scenes and this specific error occurs when there’s a mismatch between tool calls and their results.
Try these steps to resolve it:
• Clear your browser cache and restart the workflow
• Check if any of your nodes (especially the Gmail node) have authentication issues that might be causing tool execution failures
• If the error persists, try recreating the workflow from scratch or contact support, as this appears to be an internal AI Builder issue rather than something you configured incorrectly
Based on the [community.n8n.io]( AI Agent Error When Tool Used ) discussions, this type of tool_use/tool_result mismatch is a known issue that often requires backend fixes.