I Use Development-Driven Specs, Not Spec-Driven Development

Ensemble is both an AI coding agent and an executable teaching book about building one. Its central claim is that useful specifications should be earned through development, rather than written in exhaustive detail before coding or reconstructed after the fact. The developer and AI first discuss a design, then draft only the next chapter’s summary and exercises. They pair-program the implementation, recording uncertainties, corrections, decisions, and tradeoffs. Once the code passes exercise graders, those notes become a chapter that explains how and why to build the system. A fresh agent then attempts the exercises from the chapter, revealing gaps that can be corrected. The resulting specification aims to transfer the judgment needed to reproduce the implementation, not merely list features.

The approach is embodied in successive editions: Edition 2 used the book to build Edition 3, whose application is written in C# 14 on .NET 10 and developed within Ensemble. The repository preserves chapter solutions, graders, implementation evidence, and earlier editions; it also distinguishes current application code from Go-based grading and publishing tools. Chapters document their own verification limits, and the README gives explicit build, test, and grading commands, including the requirement for a .NET 10 SDK. The project’s broader lesson is that developers gain expertise through guided implementation, so documentation should teach the construction process and reasoning rather than serve as a blueprint assumed to be complete in advance.