This is the second post in a series about spec-driven development from a practitioner who has been maintaining a spec across nine SDKs for three years. The first post covered what a spec actually is and how to keep it useful over time. This post is about what happens when you start feeding that spec to an LLM.
There is a bold idea gaining traction in the SDD community right now: spec is the new high-level programming language, implementation details do not matter anymore. Write a precise enough specification, hand it to a frontier model, and good software comes out the other side.
It is an appealing idea. And it is not entirely wrong. But after a year of seriously using our spec to drive LLM-assisted development at Ably, I think it is only half the story. The missing half matters quite a lot in practice.
The Problem With "Spec Is the New Code"
Anthropic's engineering team recently built a working C compiler using Claude, driven largely by documentation and specification. It compiled real programs. It passed the tests. You can read about it on the Anthropic engineering blog.
