A JSON schema for a structured output includes all required fields and types but omits `additionalProperties: false`. What is the risk?
Answer: The model may include extra fields not in the schema — potentially containing unexpected data or instructions
Without additionalProperties: false, the schema allows the model to include fields beyond those specified. In most cases this is harmless — but the extra fields might contain unexpected data, reasoning traces, or in adversarial contexts, content that downstream code shouldn't process. additionalProperties: false closes the schema: the model can only include the explicitly defined fields. The API doesn't reject schemas without it (A). Response speed is not meaningfully affected (C). The specified fields are still populated correctly (D) — this is about extra fields, not missing ones.