Spec-Driven Development is a way of working with AI coding agents where the specification drives the code, not the prompt. I use Spec-Driven Development according to the AI Unified Process (AIUP) in several customer projects, and in this deep dive, I show how it works in practice.

We will build a business application from scratch, and also learn how to work on brownfield projects. We write specifications, let an AI agent generate the code, run it, and add tests that check the code against the spec. When the requirements change, we update the spec and regenerate the code rather than patching the code by hand.

We will explore the harness: the rules, context, and tests around the agent that keep its output predictable enough for real work. By the end, you will know what Spec-Driven Development is, where it helps, where it does not, and how to try it on your own stack.