PRODUCTION SCENARIO
A support assistant's prompt, written for an older model, says "CRITICAL: You MUST use the lookup tool whenever in doubt." After the team moves the assistant to Claude Opus 4.6, it calls the tool on nearly every greeting.
What prompt change should the team make?
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Show answer and explanation
Answer: Soften the wording to "Use this tool when..." and drop the when-in-doubt rule
Claude Opus 4.5 and 4.6 are more responsive to the system prompt than earlier models, so language written to stop a tool from undertriggering now makes it overtrigger; the documented fix is to dial back aggressive wording, and the guide names if-in-doubt instructions as a direct cause. Repeating the rule in the tool description raises the pressure rather than lowering it. A cost warning adds a second emphatic instruction to a prompt that is already being followed too literally. And tool_choice any forces a call on every turn, the opposite of the fix.