Beyond Vibe Coding: An AI Builder’s Guide to Migrating 10k+ React Components

Ads Manager powers 84% of Meta’s Ads Revenue, but its decade-old codebase was stuck on React class components — blocking modern patterns like hooks, Relay, and React Compiler. Manual migration of 10k+ components class to functional component was impractical. In this talk, I’ll show how we built an AI codemod pipeline using LLMs that achieved a 5x speedup, migrating 3k+ components with a 99.6% success rate. I’ll cover the tradeoffs between AST and AI codemods, how to classify components by complexity for targeted prompts, and the grading/eval strategies that made non-deterministic AI agents production-ready.