How to modernize legacy codebases using AI coding agents (Step by step guide)

JK Gunnink demonstrates a practical step-by-step workflow for using AI coding agents to safely modernize legacy codebases. The approach includes preparing a documented sandbox environment, implementing a model split where pro-tier models write strict acceptance tests and faster models handle implementation, and preventing context drift through task decomposition with running markdown checklists. Targeted at developers working with poorly documented legacy systems who want to safely introduce AI-assisted modifications.