What Is JSON? — AI Glossary

JSON (JavaScript Object Notation) is a lightweight, text-based data format used to send structured information between computers. It is the most common data format in modern web development and the dominant format for AI API requests and responses — when you call Claude, ChatGPT, or any major AI model through an API, the data going back and forth is almost always JSON.

JSON was created by Douglas Crockford in the early 2000s and standardized as ECMA-404 in 2013. It became dominant because it is simple enough for humans to read, strict enough for computers to parse reliably, and supported natively by every major programming language. The format uses two structures: objects (key-value pairs wrapped in curly braces) and arrays (ordered lists wrapped in square brackets).

A simple JSON example:

{
  "name": "Claude Opus 4.7",
  "context_window": 1000000,
  "pricing": {
    "input_per_million": 5,
    "output_per_million": 25
  },
  "capabilities": ["text", "vision", "code"]
}

Why JSON matters for AI:

• API requests and responses: Every major AI API uses JSON to receive your prompt and return the model’s response.
• Structured outputs: Modern AI models can be asked to return JSON directly, making it easy to plug AI outputs into databases, dashboards, and downstream code.
• Tool calling and function calling: When AI models invoke external tools (web search, code execution, calendar APIs), the tool call is typically expressed as JSON.
• Configuration files: Settings for Claude Code, plugin manifests, MCP server configurations, and similar AI infrastructure all use JSON.
• Schema validation: JSON Schema lets you specify exactly what shape valid JSON output should take, and the AI can be constrained to match it.

Related terms: API (Application Programming Interface), Schema, Endpoint, Structured Output, Model Context Protocol (MCP), Large Language Model (LLM).

Learn more: Claude API for Beginners, OpenAI API for Beginners, What Is a Large Language Model?. Get a daily AI fundamentals issue in our free Beginners in AI newsletter.