Your invoice extraction tool's JSON schema marks purchase_order_number as required. Downstream reconciliation finds that on invoices with no PO, the output contains values like "PO-0000" or a copy of the invoice number. Schema validation passes on all of them. What change best fixes this?

Answer: C — Make purchase_order_number optional and nullable in the schema, and instruct Claude to return null when the document contains no purchase order number.

A required field pushes the model to fabricate a value when the source doesn't contain one. Making the field optional or nullable removes that pressure, as section 6 describes. (a) keeps the pressure and adds a sentinel string downstream systems have to special-case. (b) retries for information that isn't in the document, which can't succeed, and invented values can still match a format check. (d) moves the field into a separate call but leaves the same required-field pressure in place.