JSON Schema Validator
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Common questions answered

What is JSON Schema validation?

JSON Schema is a vocabulary for annotating and validating the structure of JSON data. A schema defines expected types, required fields, string patterns, number ranges, and nested object shapes. Validating data against a schema catches structural errors before they reach application logic — a practice widely used in API design, form validation, and data pipeline contracts.

This tool supports JSON Schema Draft 4, 6, 7, and 2020-12. Paste your data in one panel and your schema in the other to get instant validation results. Errors include the JSON path to the failing field, the expected type or constraint, and the actual value found. You can also generate a schema from sample data to bootstrap your validation rules.

// Schema
{"type":"object",
 "required":["name","age"],
 "properties":{
  "name":{"type":"string"},
  "age":{"type":"number",
    "minimum":0}}}

// Data
{"name":"Alice","age":-5}

✗ Validation error:
  Path: /age
  Expected: minimum 0
  Actual: -5

  1 error found.

Get the most out of this tool

• Generate a JSON Schema from sample API responses, then manually tighten the types and add required fields.
• Use JSON Schema in CI pipelines to validate configuration files before deployment.
• Start with a permissive schema and add constraints incrementally — strict schemas from day one create friction.

Understanding JSON Schema validation

JSON Schema is a vocabulary for annotating and validating JSON documents. While the plain JSON validator checks only syntax (is this valid JSON?), a JSON Schema validator checks semantics (does this JSON data conform to a defined structure?). You can specify that a field must be a string, that a number must be between 1 and 100, that an array must contain at least three elements, and that an object must have certain required properties. JSON Schema brings contract-level validation to JSON APIs.

The JSON Schema specification is maintained by the JSON Schema organization at json-schema.org, currently at Draft 2020-12 (also written as Draft 12). Earlier widely-used versions include Draft 7 and Draft 4. The drafts are additive — newer versions add new keywords and fix ambiguities. Most tooling supports Draft 7 as the lowest common denominator, while Draft 2020-12 adds powerful features like $dynamicRef for recursive schemas and prefixItems for tuple validation.

Core schema keywords include type, which specifies what JSON type a value must be (string, number, integer, boolean, array, object, null). The enum keyword restricts a value to a specific set of allowed values. The const keyword requires an exact match. The required keyword in an object schema lists properties that must be present. The properties keyword defines the schema for each named property. The additionalProperties keyword controls whether properties not listed in properties are allowed.

String validation uses keywords like minLength, maxLength, and pattern (a regular expression the string must match). The format keyword provides semantic validation for common string formats: email addresses (format: email), URLs (format: uri), date strings (format: date), and date-time strings (format: date-time). Note that format validation is optional in the specification — validators may choose to ignore it unless configured to enforce it.

Combining schemas with allOf, anyOf, oneOf, and not enables complex validation logic. allOf requires all sub-schemas to match. anyOf requires at least one to match. oneOf requires exactly one to match. not inverts a schema. These combiners let you express rules like "must be a string or null", "must match one of these three specific schemas", or "must not be an empty string" — patterns that appear constantly in real-world API contracts.

JSON Schema is the foundation of OpenAPI specifications. Every request body, response body, and query parameter in an OpenAPI 3.0 document is described using a JSON Schema subset. Swagger UI, Redoc, and other API documentation tools use these schemas to generate interactive documentation and request/response examples. Validating your actual API data against the OpenAPI-embedded JSON Schema is how you verify the implementation matches the contract.

Schema generation from data is the reverse operation: given a JSON document, infer a schema that describes its structure. This tool supports schema generation in addition to validation — paste a JSON example and the schema generator produces a Draft 7 schema with types, required fields, and basic constraints. It is a fast starting point for schema-first API development, especially when you have existing data samples but no formal specification yet.

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Additional frequently asked questions