Part II: The Specification Spectrum
Three Maturity Levels of Specification-Driven Work
Not all teams implement Spec-Driven Development at the same depth. Like any engineering practice, there is a spectrum of maturity — from teams that write specifications before coding (spec-first) to teams that maintain specifications alongside code (spec-anchored) to teams where specifications are the only thing humans edit and code is entirely generated (spec-as-source).
Understanding where you are on this spectrum — and where you want to be — is the first step toward adopting SDD effectively. Trying to jump straight to spec-as-source from no specifications at all is like trying to run a marathon before you can jog a mile. Each maturity level builds skills and infrastructure that the next level requires.
This part maps the spectrum and helps you assess your current position, plan your progression, and understand the tradeoffs at each level.
What You Will Learn
Chapter 4: Spec-First Development
The entry point to SDD. You will learn to write specifications before writing code — a deceptively simple practice that immediately improves AI output quality, reduces debugging time, and creates a lightweight documentation layer. Includes a hands-on tutorial comparing spec-first vs. code-first development for the same feature.
Chapter 5: Spec-Anchored Development
The intermediate level. Specifications don't just exist before code — they stay aligned with code through automated validation. You will learn to use OpenAPI contracts, BDD scenarios, and database schema definitions as living specifications that fail the build when code diverges. Includes a tutorial on building a spec-anchored API with automated contract validation.
Chapter 6: Spec-as-Source Development
The advanced level. Humans edit specifications; machines generate all code. You will explore where this model already exists (automotive, hardware design, API generation), understand what makes it viable, and practice a constrained version with a tutorial on generating a complete API from an OpenAPI specification plus behavioral specs.
The Maturity Assessment
By the end of this part, you will be able to score your team or project on the specification maturity spectrum and create a concrete plan for progressing to the next level.
| Level | Human Edits | AI Generates | Validation |
|---|---|---|---|
| Spec-First | Specs and code | Suggestions and drafts | Manual review |
| Spec-Anchored | Specs and code | Features from specs | Automated spec-code alignment |
| Spec-as-Source | Specs only | All code | Spec-driven test suites |