Building reusable widgets in Flutter is easy. Building a design system that scales and stays consistent over time is not. As applications grow, design systems often turn into “widget dumping grounds”: inconsistent APIs, duplicated styles, and components that slowly drift away from the original design intent. Without clear boundaries, a solid theming foundation, and proper validation workflows, even well-intentioned systems become hard to use and harder to trust. Drawing from our experience building and evolving the design system behind the Holafly app, we’ll share real-world challenges, trade-offs, and lessons learned while scaling a production UI system. In this talk, we’ll move beyond simple widget construction to explore the architecture of a healthy, dedicated UI package. We will cover: - Crafting widget APIs that are flexible, consistent, and easy to adopt across teams. - Structuring tokens for colors, typography, and spacing that provide a cohesive foundation for your Design System. - Implementing practical strategies to protect the brand, including widget and Golden tests for visual regression, and the use of component galleries or tools like Widgetbook to document, visualize and validate the UI in isolation. Whether you’re starting a design system from scratch or trying to regain control of an existing one, joining us in this talk will give you practical tools to ensure your code remains a source of truth rather than a source of frustration.