Designing tool schemas — CCA-F Exam Prep

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L2.06|Designing tool schemas
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Comparison
Split screen. Left side: a clean, well-labeled tool schema with clear descriptions and typed parameters. Claude uses it perfectly, green checkmarks. Right side: a vague, poorly named schema with missing descriptions. Claude guesses wrong, red X marks everywhere. Developer pulling his hair out.

Same tool. Same model. One schema worked. The other failed 40% of the time.

A fintech team built two tools: get_balance and transfer_funds. Both worked perfectly in unit tests. But in production, Claude kept calling get_balance when users asked to send money. 40% error rate.

The fix took 15 minutes. They didn't change the code. They didn't retrain anything. They rewrote the tool descriptions.

Claude doesn't read your code. It reads your schema. If the schema is vague, Claude guesses.