What is the difference between a validation keyword and an applicator?

A validation keyword directly asserts a constraint on the instance, like minimum or maxLength. An applicator applies subschemas to parts of the instance, like properties or items, delegating the actual validation to those subschemas.

What does additionalProperties do?

additionalProperties applies a schema to object members not matched by properties or patternProperties. Set it to false to forbid unlisted properties, or to a schema to constrain them. It only sees properties the sibling keywords did not match.

How does $ref work in JSON Schema 2020-12?

$ref is a URI reference to another schema, resolved against the current base URI ($id). In 2020-12 $ref can sit alongside other keywords in the same schema object, unlike older drafts where it replaced them.

What is the difference between prefixItems and items?

In 2020-12 prefixItems validates array elements positionally (tuple style), and items then applies to every element after the prefix. Older drafts overloaded items as an array for the tuple case.

What are annotation keywords?

Annotation keywords like title, description, default, examples and deprecated carry no validation logic. They attach metadata to a schema for documentation and tooling, and are collected as annotations during evaluation.

JSON Schema Keywords Reference

JSON Schema keywords (2020-12)

JSON Schema is a vocabulary for validating and annotating JSON documents. The 2020-12 draft organizes its keywords into vocabularies: core, validation, applicators, and annotations. This reference lists the keywords developers reach for most, with the JSON types each applies to and the exact assertion it makes.

How it works

Evaluation walks the schema against an instance. A validation keyword directly asserts a constraint — minimum checks a number, pattern matches a string against a regex, required lists mandatory object members. These produce a pass/fail boolean.

An applicator keyword does not assert anything itself; it applies subschemas to parts of the instance. properties maps names to subschemas, items applies a schema to array elements, and allOf/anyOf/oneOf combine subschemas logically. The instance is valid only if the delegated subschemas validate.

Core keywords structure the document: $id sets the base URI, $ref/$dynamicRef reference other schemas, and $defs holds reusable subschemas. A key 2020-12 change is that $ref may now coexist with sibling keywords rather than replacing them.

Annotation keywords (title, description, default, examples, deprecated, readOnly) attach metadata without affecting validity, and are gathered as annotations for tooling.

Tips and example

A schema mixing several keyword groups:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "age": { "type": "integer", "minimum": 0 },
    "tags": { "type": "array", "items": { "type": "string" } }
  },
  "required": ["age"],
  "additionalProperties": false
}

Remember that additionalProperties: false only sees properties not matched by properties or patternProperties — a frequent source of confusion. Filter the table below by group to confirm what any keyword applies to before relying on it.

JSON Schema Keywords Reference

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JSON Schema keywords (2020-12)

How it works

Tips and example