Next.js 15 App Router: What Actually Changed and Should You Upgrade Your MVP?

Next.js 15 changes the contract on dynamic APIs. If your MVP still assumes sync params and cookies(), upgrades will break builds | not runtime flakiness, hard TypeScript failures.

Breaking: Async params

// Next.js 14
export default function Page({ params }: { params: { slug: string } }) {
  const { slug } = params;
}

// Next.js 15
export default async function Page({
  params,
}: { params: Promise<{ slug: string }> }) {
  const { slug } = await params;
}

Breaking: cookies() and headers()

const cookieStore = await cookies();
const hdrs = await headers();

Fetch caching

Default fetch caching behavior changed. Audit everyfetch() | add explicit cache ornext: { revalidate: 3600 } for marketing content;no-store for user-specific data.

Migration Checklist

1. Upgrade Next + React together per release notes
2. Grep for params. in pages and fix async
3. Await cookies/headers in Server Components and route handlers
4. Run npm run build on CI before merging
5. Smoke test Stripe webhooks and Supabase auth callbacks

Need this shipped in production, not just in a blog post? Start your MVP.