Swen Mulderij enhanced the labd/commercetools-node-mock repository by implementing support for the 'key' field when adding line items to carts and shopping lists. Using TypeScript and Node.js, Swen refactored the CartUpdateHandler and ShoppingListUpdateHandler to persist the 'key' alongside each line item, ensuring data integrity and closer alignment with commercetools’ real-world usage. Comprehensive tests were added to validate the new functionality across both cart and shopping list services, demonstrating a focus on robust API development and thorough testing practices. This work addressed interoperability needs and improved the fidelity of the mock service for downstream consumers and developers.