JSON Schema Validator – Validate JSON Against a Schema

Free Online JSON Schema Validator

Validate any JSON document against a JSON Schema — instantly, with detailed error reporting for every constraint violation.

JSON Schema is a powerful standard for describing the structure, types, and constraints of JSON data. It acts as a "blueprint" or contract that your JSON must follow. This tool uses Ajv — the most popular and standards-compliant JSON Schema validator — to validate your data and report errors with exact field paths and human-readable messages.

What Is JSON Schema?

A JSON Schema is itself a JSON document that defines the rules for another JSON document. For example:

{
  "$schema": "https://json-schema.org/draft/2019-09/schema",
  "type": "object",
  "required": ["name", "email", "age"],
  "properties": {
    "name":  { "type": "string", "minLength": 1 },
    "email": { "type": "string", "format": "email" },
    "age":   { "type": "integer", "minimum": 0, "maximum": 150 }
  },
  "additionalProperties": false
}

This schema says: the JSON must be an object, with required fields name, email, and age of specific types and constraints — and no extra fields allowed.

When Should You Use JSON Schema Validation?

Testing API request/response payloads during development
Validating configuration files (CI/CD pipelines, app configs)
Enforcing data contracts between microservices
Checking mock data matches your TypeScript interfaces
Debugging why a JSON payload is being rejected by your backend
Generating documentation from schemas (OpenAPI, Swagger)

Supported JSON Schema Drafts

Draft-07 — Most widely used; supported by almost all tooling
Draft 2019-09 — Adds $anchor, unevaluatedProperties, and more
Draft 2020-12 — Latest standard with prefixItems for arrays

Common JSON Schema Keywords

Keyword	Purpose	Example
`type`	Data type constraint	`"type": "string"`
`required`	Mandatory keys on an object	`"required": ["id","name"]`
`properties`	Object property definitions	`"properties": {"id": {"type":"integer"}}`
`minimum / maximum`	Numeric range	`"minimum": 0, "maximum": 100`
`minLength / maxLength`	String length	`"minLength": 3, "maxLength": 50`
`pattern`	Regex pattern for strings	`"pattern": "^[A-Z]{2}\\d{6}$"`
`format`	Semantic format (email, date, uri…)	`"format": "email"`
`enum`	Allowed values list	`"enum": ["active","inactive","pending"]`
`additionalProperties`	Allow/block extra keys	`"additionalProperties": false`
`items`	Array item schema	`"items": { "type": "string" }`

Pro Tips

Use "Generate from Data" to auto-create a base schema from your JSON, then refine it
Add "additionalProperties": false to catch unexpected fields in API payloads
Use "format": "email", "date-time", or "uri" for semantic string validation
Use $ref to reuse schema definitions and avoid repetition
Draft-07 is the safest choice for maximum tool compatibility