During testing, when a customer says, “I need a refund for my recent purchase,” the...

Option A corrects the missing tool contract that permits Claude to guess a required parameter. The process_refund description should state its prerequisite explicitly: invoke lookup_order, use only the exact order_id returned by that successful call, and ask for additional identifying information when lookup cannot resolve an order. An input example can demonstrate the correct two-tool sequence.

Anthropic’s tool-definition guidance identifies detailed descriptions as the most important factor in tool performance. Descriptions should explain when a tool should and should not be used, what every parameter means, and all important limitations. The tool-use overview confirms that Claude selects and populates tools based on the request and their descriptions.

Option B forces some tool call but does not tell Claude which tool must run first or where the identifier originates. Option C is an essential defence-in-depth control, but it detects the fabricated value after Claude has already made the incorrect decision. Option D works only when the customer supplies an identifier and does not cover “recent purchase.” Explicit prerequisite and provenance instructions directly prevent the model from inventing the required argument.