Retrospective: Migrating to Next.js 15 App Router Broke 30% of Our User Routes

Our team decided to migrate from Next.js 14 Pages Router to15 App Router to leverage React Server Components, Server Actions, and improved performance optimizations. We followed the official incremental migration guide, tested locally, validated all routes in staging, and deployed with confidence. Within 15 minutes of production deployment, 30% of user-facing routes (including dynamic profile pages, dashboards, and settings flows) began returning 404 errors, 500 internal server errors, and broken hydration mismatches.

The Pre-Migration Plan

We adhered to Next.js recommended practices: enabled incremental adoption via the app directory alongside pages , ran the provided codemods to update deprecated APIs, and achieved 100% route coverage in our automated test suite.…