Specifications doesn't tell you what to do, they say what the end state should be. In between that you need a codebase analysis step and an implementation plan.

My recursive-mode workflow handles all of that and more and gives you full traceability: https://recursive-mode.dev/introduction

I do (1) the same but (2) differently. In my workflow, (2) are AI generated specs using human written (1) as the input. It's an intermediate stage between (1) and the codebase, allowing for a gradual token expansion from 30k to 250k to the final code which is 2-3M. The benefit I've found with this approach is it gives the AI a way to iterate on the details of whole system in one context window, whereas fitting the whole codebase into one prompt is impossible. The code is then nothing more than a style transfer from (2).