War Story: We Migrated From Next.js 14 to Next.js 15 and Cut Bundle Size by 55% – But Broke 10% of Our User Base’s Browsers Our team had been running Next.js 14 for 8 months on our e-commerce platform, serving 2.4M monthly active users. We’d heard the hype around Next.js 15: 30% faster build times, native support for React 19, and aggressive tree-shaking that promised to slash our 1.2MB initial bundle size. We decided to migrate in a two-week sprint, with full regression testing on Chrome, Firefox, Safari, and Edge latest versions. We thought we were covered. The Migration: Smooth Sailing (At First) The actual migration took 3 days. We updated next, react, react-dom to their latest 15.x/19.x versions, fixed a handful of breaking changes (mostly around the new App Router caching defaults), and ran our test suite. All 1,200 unit tests passed.…