additional-properties-false

Reports object schemas that explicitly set additionalProperties: true.

Attribute	Value
Category	Schema
Maturity	Gold
OpenAPI	Swagger 2.0; OpenAPI 3.0, 3.1, 3.2
Starter	Off
Lenient	Warning
Recommended	Warning
Strict	Warning
Complete	Error

Intent

Closed object schemas make payload fields explicit. Consumers can validate unknown fields consistently, while API owners can evolve the contract deliberately.

Flags

This rule reports a schema only when both conditions hold:

• resolved schema type is object;
• additionalProperties is the Boolean value true.

Does not flag

• additionalProperties: false.
• An omitted additionalProperties field.
• A schema-valued additionalProperties definition.
• Non-object schemas.

The rule therefore catches explicit open-object declarations; it does not require every object to declare additionalProperties: false.

See it fail

openapi: 3.0.3
info:
  title: Payments API
  version: 1.0.0
paths: {}
components:
  schemas:
    Payment:
      type: object
      additionalProperties: true

Specmatic reports:

Object schemas with `additionalProperties` should set it to false.

Fix it

Close the object and define supported fields:

components:
  schemas:
    Payment:
      type: object
      additionalProperties: false
      properties:
        id:
          type: string

Configure

specmatic-linter.yaml

profiles:
  default:
    rules:
      extends:
        - recommended
      override:
        additional-properties-false: error

Nearby rules

• no-unconstrained-additional-properties
• invalid-additional-properties-usage
• no-schema-type-mismatch