Easy Upgrades with @next/codemod CLI

Next.js 15 introduces a CLI to streamline upgrading:

npx @next/codemod@canary upgrade latest
npm install next@latest react@rc react-dom@rc

It handles most of the breaking changes automatically, making the transition smoother.

Async Request APIs (Breaking Changes)

APIs like cookies(), headers(), params, and searchParams are now async. Codemods can help convert them, but warnings remain until the next major version.

Caching Semantics Overhaul

Defaults for caching have changed significantly:

• GET Route Handlers and Client Router Cache now default to no-store.
• Cached behavior can be restored via config, like:

export const dynamic = 'force-static';

Full React 19 Support

Next.js 15 now supports React 19 RC for App Router while keeping React 18 for Pages Router. It also enhances hydration debugging and introduces a new compiler (experimental).

Turbopack Goes Stable

The new dev server using Turbopack is now stable, giving performance boosts:

• ~77% faster startup
• ~96% faster Fast Refresh
• ~46% faster route compilation

Static Route Indicator

You can now visually distinguish static vs dynamic routes in the console during development.

Experimental unstable_after API

Allows deferring tasks like logging after response streams. To enable:

experimental.after = true

Use it in server-side code:

import { unstable_after as after } from 'next/server';
after(() => logAnalytics());

Migration Path

Follow these steps to upgrade:

1. Upgrade CLI

npx @next/codemod@canary upgrade latest

2. Run Async Codemod

npx @next/codemod@canary next-async-request-api .

3. Enable Features

• Add experimental.after if needed
• Update config to .ts
• Tweak caching settings if preserving old behavior

4. Test Thoroughly

• Especially routing logic and caching
• Watch out for unstable_after and instrumentation

5. Adopt Turbopack

• Start with next dev --turbo
• For production, test next build --turbopack