You are building a structured data extraction system using Claude.

The schema already supports the correct representation of missing evidence: null . The remaining defect is behavioral. Claude must be explicitly instructed that absence of information is a valid outcome and that values may be populated only when directly supported by the supplied article.

Anthropic’s hallucination-reduction guidance recommends explicitly permitting Claude to express uncertainty, grounding outputs in the source, and retracting claims that lack supporting evidence. It also recommends restricting the model to the provided documents rather than allowing unsupported external knowledge. ( https://docs.anthropic.com/en/docs/test-and-evaluate/strengthen-guardrails/reduce-hallucinations ) Option C translates those controls directly into the extraction contract: when no source evidence exists, the model returns null .

Option A may improve baseline performance but does not remove the architectural ambiguity that encourages completion of missing fields. Option B is counterproductive because non-nullable required fields force the model to provide values even when the article contains none. Structured validation would confirm that the response is syntactically valid while accepting fabricated content. Option D adds cost, latency, and another probabilistic model decision; a second model can repeat or endorse the original hallucination.

A robust implementation should supplement the instruction with source spans or quotations for populated fields and automated checks where feasible. Nevertheless, among the options, explicit null behavior is the direct and most effective correction.

Official references/topics: Reduce Hallucinations; External-Knowledge Restriction; Nullable JSON Schema Fields; Grounded Extraction.