Unevaluated properties and items

JSON Schema 2020-12-style unevaluatedProperties and unevaluatedItems apply to object keys and array positions that were not already “evaluated” by properties, patternProperties, additionalProperties, items, prefixItems, composition, and related keywords.

They are useful when you want forbid extra keys or constrain extra array elements after tuples or composition.

`unevaluatedProperties`

With explicit `properties`

type: object
properties:
  a:
    type: string
unevaluatedProperties: false

Valid: { a: hello }
Invalid: an extra key such as b.

With `allOf`

Annotations from branches of allOf can merge which names count as evaluated. Keys allowed by any subschema may be present; unevaluatedProperties: false then rejects any other keys.

allOf:
  - properties:
      a:
        type: string
  - unevaluatedProperties: false

a is allowed; b is not.

With `anyOf`

Successful branches contribute evaluated property names from their schemas. For example, if the instance matches the first branch, only keys allowed by that branch (and merged annotations) count as evaluated; extra keys can still fail unevaluatedProperties: false.

anyOf:
  - properties:
      a:
        type: string
  - properties:
      b:
        type: string
unevaluatedProperties: false

Valid: a: ok or b: ok (each branch’s allowed keys only).
Invalid: a: ok together with an extra unevaluated key such as c: extra.

`unevaluatedItems`

After `prefixItems`

Positions beyond the tuple are “unevaluated” unless covered by items.

type: array
prefixItems:
  - type: integer
unevaluatedItems:
  type: string

Valid: [1, foo, bar]
Invalid: a non-string after the first integer, e.g. [1, 2].

Without `items` or `prefixItems`

Every index is unevaluated, so the schema applies to all elements:

type: array
unevaluatedItems:
  type: integer

Valid: [1, 2]
Invalid: [1, hi] (second item is not an integer).

Unevaluated properties and items