additionalProperties JSON Schema Keyword JSON Schema

Object

Schema template

JSON Schema

{
  "type": "object",
  "properties": {
    "id": {
      "type": "string"
    }
  },
  "required": [
    "id"
  ],
  "additionalProperties": false
}

Valid example JSON

{
  "id": "item_1"
}

Invalid example JSON

{
  "id": "item_1",
  "extra": true
}

Fields

Field	Type	Required	Description
`additionalProperties`	boolean \| schema	Yes	false rejects unknown fields. A schema validates unknown fields instead of banning them.

How to use this schema

Use additionalProperties: false for strict API payloads and configuration files. Use a schema value when arbitrary keys are allowed but their values must follow a rule.

Version support and combinations

This keyword or pattern is commonly supported across modern JSON Schema workflows. Check your validator for Draft-04, Draft-07, 2019-09, and 2020-12 behavior before relying on newer composition or conditional features.

Common combination: pair this reference with type, required, and properties so validation errors are specific enough for API clients and form users.

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additionalProperties JSON Schema Keyword