This talk introduces a crucial but often overlooked aspect of AI application development: evaluation-driven development (EDD). Using a simple LangGraph agent as a practical example, we'll demonstrate why and how to build a robust evaluation framework that goes beyond simple unit tests. We'll explore the importance of continuous evaluation during the development cycle and how this practice directly translates to the need for comprehensive observability in production, ensuring your AI application remains accurate, reliable, and effective in the real world.