How I Cut a 3-Day Refactor Down to 4 Hours Using a Single AI Prompt Pattern Last quarter I inherited a 4,000-line Node.js service — no tests, inconsistent error handling, callbacks mixed with promises. My estimate to the team: 3 days minimum . I finished in 4 hours. Here's exactly what I did differently. The Setup The service was a payment webhook handler. Spaghetti, but well-defined inputs and outputs — I knew what "correct" looked like. That constraint matters for what came next. The Prompt Pattern That Changed the Work Instead of asking AI to rewrite the file (which produces confident garbage), I used a constraint-first decomposition prompt : Context: I have a Node.js webhook handler, ~400 lines, mixed callbacks/promises, no error boundaries. I cannot change the function signatures — external callers depend on them. Task: Identify the 5 highest-risk sections I should refactor first, ranked by: 1. Likelihood of silent failure 2.…