# Additional properties

When OpenAPI schemas use the [`additionalProperties`](https://json-schema.org/understanding-json-schema/reference/object#additional-properties) 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.

```yaml filename="openapi.yaml"
components:
  schemas:
    FlexibleObject:
      type: object
      properties:
        title:
          type: string
        description:
          type: string
      additionalProperties: true
```

### Generated type structure

```typescript

    description: string;
    additionalProperties: Record<string, any>;  // Internal field - do not interact directly
}
```

### Usage

```typescript
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:

```yaml filename="openapi.yaml" 
components:
  schemas:
    StringOnlyObject:
      type: object
      properties:
        title:
          type: string
        description:
          type: string
      additionalProperties:
        type: string
```