Next.js 16 is the biggest release since the App Router landed, and the upgrade is not a one-line bump. The caching model changed shape, params and searchParams are now promises everywhere, Turbopack runs your builds by default, and middleware.ts is on its way out in favour of proxy.ts . None of that is hard on its own. The trouble is that the changes touch almost every dynamic route in a real app at once, so a rushed upgrade tends to fail in a dozen small places rather than one obvi...