JSON Schema Generator

JSON Schema generator - a draft-07 schema from sample JSON

The JSON Schema generator turns a sample JSON object into a ready draft-07 schema. You paste a representative slice of your data and the tool infers the type of every field, builds the properties section and the required list, and describes array elements through items. You get a starting point for validating API responses, configuration files and forms.

What JSON Schema is

JSON Schema is a language for describing the structure of JSON data, written in JSON itself. Instead of noting in a comment that id is an integer and tags is an array of strings, you state it formally and let a validator check conformance. A schema plays three roles at once: a contract, documentation a person can read, and a rule a machine can run. The keywords - type, properties, required, items, $schema - mean the same in any language.

How inference from a sample works

The tool parses the pasted JSON and walks its structure recursively. For an object it builds a properties section describing each field, and adds fields with a non-empty value to required. For an array it sets the element type through items, based on the first element. Simple values map onto JSON Schema types: a whole number becomes integer, a decimal becomes number, a logical value becomes boolean, null yields null, and text becomes string. Since that split comes from how the value is written, put a decimal point in the example for fields that are sometimes fractional.

type, properties, required, items - what the result contains

Nested objects and arrays

Nested structures are handled fully recursively: an object inside an object gets its own properties section, and an array of objects describes in items the schema of a single record with its required list. An empty object {} becomes type: object with empty properties, and an empty array [] becomes type: array with no items key, because the element type cannot be guessed from an empty array. Rather than write a made-up type, the tool leaves a slot for you to fill in.

How to generate a schema step by step

1. Paste a representative sample - the fuller the object, the more accurate the schema.
2. Click Generate schema. The result appears on the right, formatted with indentation.
3. Check the required list and remove the fields that are actually optional.
4. Add what a single sample cannot show: format, pattern, enum, numeric bounds.
5. Copy the finished schema and load it into a validator.

Draft versions and the $schema field

The $schema field declares which version of the specification the schema is read against. The generator writes the draft-07 address, widely supported by validators in PHP, JavaScript and Python. Newer releases (2019-09, 2020-12) changed how array elements are described: the array form of items became prefixItems. If your validator needs a newer version, swap the address and adjust the changed fragments.

Validating data against the schema

The schema on its own checks nothing - you need a validator that compares data against it: justinrainbow/json-schema in PHP, Ajv in JavaScript, jsonschema in Python. It reports mismatches: a missing required field, a wrong type, a value outside enum. To first check that the JSON is syntactically correct, use the JSON validator, and to explore the structure as a tree, the online JSON viewer.

What JSON Schema is used for in practice

The most common use is API input validation: the server rejects data that does not match the schema. A schema is also API documentation, describes form rules consistently on client and server, and in configuration files hints at allowed keys and types. Before it lands in a repository, tidy its layout with the JSON formatter.

Frequently asked questions