Tool call
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A tool call is the model asking to run a function, followed by your code returning a result that re-enters the conversation.
Each call is a round trip that resends the accumulated context, so a multi-tool sequence costs more than its individual steps suggest. Unbounded tool output is a common cause of runaway cost — a tool returning a whole file has permanently enlarged every subsequent request.
Truncate or summarise tool results before they enter context.