Explainer1 min readUpdated June 29, 2026

What is JSON Schema?

Short answer

JSON Schema is itself JSON that describes what valid JSON should look like: which fields are required, what type each value must be, and what constraints apply. Validators use it to check data automatically.

A description of valid data

Instead of checking data by hand, you write a schema that states the rules, then let a validator confirm any document follows them. The schema is written in JSON, so the same tooling reads both the data and its description.

{
  "type": "object",
  "required": ["id", "name"],
  "properties": {
    "id": { "type": "integer" },
    "name": { "type": "string", "minLength": 1 }
  }
}

A small schema

What it can express

The type of each value: object, array, string, number, boolean, or null
Which properties are required
Constraints like minimum, maximum, length, and patterns
Enumerated allowed values and nested object shapes

Why it is usefulA shared schema becomes a contract between systems. APIs validate incoming data, editors offer autocomplete, and teams agree on a single source of truth for a format.

Try it: JSON Schema GeneratorGenerate a JSON Schema from a sample JSON document locally.Open tool

References

Questions

Is JSON Schema written in JSON?

Yes. A schema is a JSON document, which means the same parsers and tools that handle your data can also handle its schema.

Do I write a schema by hand?

You can, but it is often faster to generate a draft from a representative sample and then tighten the constraints, which is what the schema generator helps with.

Does this send my data anywhere?

No. Utilumo's developer tools parse and transform input inside the browser tab. Nothing is uploaded, stored, or logged.