Spec-driven development is a methodology that prioritizes creating detailed specifications as the foundational step in software development, aimed at improving the reliability and quality of code generated by AI coding agents. This process involves treating specifications as dynamic, executable documents that evolve alongside the project, ensuring they serve as the primary source of truth throughout development. Spec Kit, an open-source toolkit, facilitates this approach by structuring the workflow into four phases—specify, plan, tasks, and implement—each with specific roles and checkpoints to ensure clarity and alignment with project goals. The methodology is particularly effective in contexts like greenfield projects, feature development in existing systems, and legacy modernization, as it separates the stable intent of the project from flexible implementation methods. Spec-driven development shifts the focus from code being the source of truth to intent, allowing AI agents to generate, test, and validate code more effectively by following clear, well-defined specifications and plans.