Spec-driven development is having a moment. Write a detailed spec, hand it to the agent, get working code. It's a clean story and it maps neatly onto how we already think about requirements.

I think it's the wrong shape, and the failure mode isn't obvious until you're six months in.

Specs rot at four different speeds

Here's the actual problem with a spec: it isn't one document. It's four documents wearing a trench coat.

A typical spec contains, all interleaved: