Use structured types instead of generic strings in API definitions to improve type safety and clarity. Prefer enums over string literals, specific ID types over generic strings, and maintain consistent data type usage across request/response models. Key practices: - Use enum types for status fields instead of strings: `pub status: SubscriptionStatus` not `pub status: String` - Use specific ID types: `merchant_id: impl Into` not `merchant_id: String` - Use connector enums: `connector: Connector` not `connector: String` - Maintain URL validation with proper types: `base_url: Url` not `base_url: String` - Use consistent amount types (MinorUnit vs StringMajorUnit) based on context Example: ```rust // Good - structured types #[derive(Debug, Clone, serde::Serialize)] pub struct CreateSubscriptionResponse { pub status: SubscriptionStatus, // enum instead of String pub merchant_id: id_type::MerchantId, // specific ID type pub connector: Connector, // enum instead of String } // Avoid - generic strings pub struct CreateSubscriptionResponse { pub status: String, // unclear what values are valid pub merchant_id: String, // loses type safety pub connector: String, // no validation } ``` This approach prevents runtime errors, improves API documentation, enables better IDE support, and makes the codebase more maintainable.