JSON Schema

Protocol protocols published

Also known as: JSON Schema Specification, Schema Definition

Definition

A vocabulary for annotating and validating JSON documents, widely used in LLM applications to define tool parameters, structured output formats, and API request/response shapes. JSON Schema provides a standard way to describe what valid JSON looks like, enabling both documentation and runtime validation.

What this is NOT

  • Not JSON itself (JSON Schema describes JSON)
  • Not a programming language (it's a schema language)
  • Not TypeScript types (though often equivalent)

Alternative Interpretations

Different communities use this term differently:

llm-practitioners

The schema language used to define tool/function parameters for function calling and structured output. Models receive JSON Schema definitions and produce conforming JSON output.

Sources: OpenAI Function Calling documentation, JSON Schema specification (json-schema.org)

software-engineering

A specification for describing the structure of JSON data, including types, required fields, constraints, and nested structures. Used for API documentation (OpenAPI), configuration validation, and data contracts.

Sources: JSON Schema specification, OpenAPI specification

Examples

  • Tool parameter schema with type, properties, required
  • Structured output schema for data extraction
  • OpenAPI 3.0 specification using JSON Schema
  • Validating LLM output against a schema

Counterexamples

Things that might seem like JSON Schema but are not:

  • Raw JSON data (JSON Schema describes JSON, isn't JSON)
  • TypeScript interface (different language)
  • XML Schema (different format)

Relations

See Also