Term
JSON Schema
The IETF standard for describing the shape of a JSON value — required fields, types, allowed enums, nested objects. The contract LLM providers use to constrain structured-output decoding.
Background
OpenAI's response_format, Anthropic's tool_use, and Gemini's responseSchema all consume JSON Schema. Providers enforce the schema at decoding time, guaranteeing parse-valid output. Best practice: keep schemas flat, name fields semantically, use enums for constrained vocabularies, and avoid deeply nested arrays-of-objects where possible.