The schema is creating a structural incentive for fabrication. When a field is declared required, the output must contain a value even when the source document contains no corresponding evidence. Structured Outputs can guarantee that Claude’s response conforms to a JSON Schema, but schema conformance does not establish that every generated value is factually supported. Anthropic’s documentation shows that the required array determines which properties must be present; therefore, source-dependent properties that may legitimately be absent should not be included as required fields. ( https://platform.claude.com/docs/en/build-with-claude/structured-outputs )

Option B corrects the problem at the contract level. Claude can omit the unavailable property rather than inventing content merely to produce valid JSON. A nullable representation could also be used when downstream systems require a stable key set, but forcing an unsupported non-null value is architecturally unsound.

Option A still requires placeholder generation and does not resolve the mismatch between the schema and available evidence. Option C relies on model-generated confidence, which is not a substitute for grounding. Option D is a useful secondary control, but it does not constitute the requested schema-design change. Anthropic recommends allowing uncertainty and requiring factual claims to be grounded in the provided material. ( https://docs.anthropic.com/en/docs/test-and-evaluate/strengthen-guardrails/reduce-hallucinations )

Official references/topics: Structured Outputs—JSON Schema design; Reduce Hallucinations—allowing uncertainty and grounding claims.