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