Context windows, Plan agent, and TDD: What I learned building a countdown app with GitHub Copilot

In the final Rubber Duck Thursdays stream of 2025, Chris Reddington aimed to create a celebratory project that encapsulated the spirit of collaboration and learning from mistakes, which the streams are known for. He developed a New Year countdown app using GitHub Copilot, Vite, TypeScript, and Tailwind CSS, embracing a spec-driven development approach to ensure high-quality code. The project included various features such as a time zone selector, interactive world map, and dynamic fireworks display, all while demonstrating iterative and test-driven development practices. Through the use of custom AI agents like the Plan agent, he identified and addressed potential edge cases, highlighting the importance of context management and specialized AI tools in software development. The stream emphasized the joy of building and learning with a global community, culminating in the creation of a unified open-source countdown app called Timestamp, encouraging developers to pursue projects that bring them personal satisfaction.