SDD vs TDD: Why Spec Driven Development Changes the Game for AI-Assisted Coding

SDD vs TDD: Why Spec Driven Development Changes the Game for AI-Assisted Coding Most developers are familiar with TDD — Test Driven Development. Write a failing test, make it pass, refactor. It's been a cornerstone of software craft for over two decades. But as AI coding agents enter the picture, a different kind of discipline is emerging: Spec Driven Development (SDD) . This article breaks down what each approach does, where they differ, and why SDD has become essential for teams working with AI tools like Claude, Cursor, or Windsurf. What is TDD? Test Driven Development is a micro-cycle practice. The loop is tight: Write a failing test that describes desired behavior Write the minimum code to make the test pass Refactor without breaking the test TDD operates at the code level . Its primary job is to keep the implementation honest: the test expresses what a function or module should do, and the code must satisfy that contract. The benefits are real. TDD forces you to think about inter