JSON Schema Validator - Free Online Tool

Validate JSON data against a JSON Schema instantly. Perfect for AI function calling and API validation. Free, browser-based.

About JSON Schema Validator

JSON Schema is a standard for describing the structure of JSON data. It is widely used for API validation, configuration file validation, and increasingly for AI function calling and structured output.

This validator supports JSON Schema Draft 2020-12 and earlier versions. Paste your JSON data and schema to instantly see validation results with detailed error messages.

JSON Schema is increasingly used in AI applications. OpenAI's function calling requires JSON Schema to define tool parameters. Anthropic's tool use follows the same pattern. By validating your schemas before deployment, you catch type mismatches, missing required fields, and constraint errors that would otherwise cause runtime failures in production.

Common schema features include type validation (string, number, boolean, array, object), string patterns via regex, numeric ranges with minimum and maximum, array length constraints with minItems and maxItems, and conditional schemas using if-then-else. The Draft 2020-12 specification adds vocabulary support and dynamic references for complex schema reuse.

This validator processes everything in your browser using the Ajv library, the fastest JSON Schema validator for JavaScript. Paste your data and schema side by side to see instant validation results. Error messages include the exact JSON path of each failing field, making it straightforward to locate and fix issues.

How the JSON Schema Validator Works

  1. 01Paste your JSON data in the left panel
  2. 02Paste or select a JSON Schema in the right panel
  3. 03Click Validate to check the data against the schema
  4. 04View detailed error messages showing exactly which fields fail and why

Using JSON Schema for Data Validation

JSON Schema defines the structure, types, and constraints your JSON data must follow - like a contract for your API. Use 'required' to enforce mandatory fields, 'enum' for allowed values, and 'pattern' for string formats like email or UUID. Draft 2020-12 is the latest specification. Integrating schema validation into your API endpoints catches malformed requests early and generates clear error messages automatically.

When to Use the JSON Schema Validator

Use this tool when developing APIs that accept JSON input, when setting up AI function calling with structured output, or when debugging why an API rejects your data. It is essential for validating configuration files, testing webhook payloads, and ensuring data conforms to a contract before processing.

Common Use Cases

  • Validating AI function calling parameter schemas before deployment
  • Testing API request and response payloads against documented schemas
  • Debugging webhook integration failures caused by unexpected data formats
  • Verifying configuration files conform to expected structure before deployment

Expert Tips

  • Set 'additionalProperties: false' on objects to catch unexpected fields that indicate a data mapping error.
  • Use 'description' fields in your schema - they serve as inline documentation and improve AI model accuracy in function calling.
  • Test your schema with both valid and intentionally invalid data to confirm that validation errors are caught correctly.

Frequently Asked Questions

Which JSON Schema draft version should I use?→
Draft 2020-12 is the latest and most feature-complete version. However, many tools and APIs still use Draft-07 or Draft 2019-09. OpenAI's function calling uses a subset of Draft 2020-12. Check your target platform's documentation to confirm which draft it supports.
What is the difference between JSON Schema validation and JSON formatting?→
JSON formatting (prettifying or minifying) changes how JSON looks without altering its content. JSON Schema validation checks whether the data structure and values conform to a defined contract - correct types, required fields, value ranges, and patterns. Use our JSON Formatter for formatting and this tool for structural validation.
Can I validate nested objects and arrays?→
Yes. JSON Schema supports deeply nested validation. Use 'properties' for object fields, 'items' for array elements, and combine with 'required', 'minItems', 'maxItems', 'additionalProperties: false', and other constraints to precisely define your expected data structure.
How do I use JSON Schema with AI function calling?→
OpenAI and Anthropic use JSON Schema to define the parameters that AI models can pass to your functions. Define each parameter's type, description, and constraints in a schema. The AI model then generates structured JSON that conforms to your schema, which your code can parse reliably.

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