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This error occurs when passing mismatched, insufficient, or excessive @[ToolMessage] objects to a model during tool calling operations. The error stems from a fundamental requirement: an assistant message with tool_calls must be followed by tool messages responding to each tool_call_id. When a model returns an AIMessage with tool calls, you must provide exactly one corresponding @[ToolMessage] for each tool call, with matching tool_call_id values.

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Common causes

  • Insufficient responses: If a model requests two tool executions but you only provide one response message, the model rejects the incomplete message chain
  • Duplicate responses: Providing multiple @[ToolMessage] objects for the same tool call ID results in rejection, as does having unmatched IDs
  • Orphaned tool messages: Sending a @[ToolMessage] without a preceding AIMessage containing tool calls violates protocol requirements
TODO

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Troubleshooting

To resolve this error:
  • Count matching pairs: Ensure one @[ToolMessage] exists per tool call in the preceding AIMessage
  • Verify IDs: Confirm each ToolMessage.tool_call_id matches an actual tool call identifier
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