Additional properties

When OpenAPI schemas use the additionalProperties keyword, Speakeasy generates SDKs that handle these flexible object types automatically.

Using additionalProperties in SDKs

Important: additionalProperties is an internal implementation detail. Pass objects with all properties directly - the SDK handles serialization and validation automatically.

The Speakeasy approach

Speakeasy sets additionalProperties to false by default unless explicitly defined otherwise. This encourages fully-typed objects with clear documentation. Most developers prefer closed objects by default, but setting additionalProperties: false in every schema would be tedious. Most backend frameworks that generate OpenAPI schemas don’t add additionalProperties: false, even when that’s the intended behavior.

Using additionalProperties: true

Set additionalProperties: true to accept arbitrary fields beyond the defined properties. This is useful for accepting unpredictable fields or allowing future extensions without schema updates.

components:
  schemas:
    FlexibleObject:
      type: object
      properties:
        title:
          type: string
        description:
          type: string
      additionalProperties: true

Generated type structure

export type FlexibleObject = {
    title: string;
    description: string;
    additionalProperties: Record<string, any>;  // Internal field - do not interact directly
}

Usage

await sdk.items.create({
  title: "My Item",
  description: "A flexible item",
  customField: "custom value",
  anotherField: 123,
  metadata: { key: "value" }
});

Using typed additionalProperties

Constrain additional properties to a specific type:

components:
  schemas:
    StringOnlyObject:
      type: object
      properties:
        title:
          type: string
        description:
          type: string
      additionalProperties:
        type: string