JSON Schema Validator
Validate JSON against a JSON Schema online. See exactly which fields fail validation with clear error messages. Free, no signup, runs in browser.
JSON Schema Validation — Catching Data Errors Before They Cause Bugs
A bug that could be caught at the API boundary should never reach your database. JSON Schema validation is the gatekeeper — it inspects incoming data against a defined contract and rejects anything that doesn't conform before your application code ever touches it. This shifts error detection from runtime crashes to clear, actionable validation messages at the point of entry.
Schema validation vs type checking
TypeScript types catch errors at compile time in your code. JSON Schema validation catches errors at runtime in your data. They solve different problems. A TypeScript interface defines what shape your code expects. A JSON Schema validates that external data — from users, APIs, webhooks — actually matches that shape before you process it. Both are necessary; neither replaces the other.
What AJV validates
This tool uses AJV (Another JSON Validator), the most widely-used JSON Schema library. It validates types (string, number, boolean, array, object), string constraints (minLength, maxLength, pattern), number ranges (minimum, maximum), required properties, array item types, string formats (email, date, uri), and enum values. Error messages include the exact field path where validation failed.
Adding validation to your API
In Express.js with AJV: compile your schema once at startup, then call validate(requestBody) in your route handler. If validation fails, return a 422 status with the error details. In FastAPI (Python), JSON Schema validation is built in via Pydantic models. In any framework, validating input at the API boundary is cheaper than debugging corrupted data in your database weeks later.