Next.js 15 Complete Guide: New Features, Breaking Changes, and Best Practices

Next.js 15 is the biggest release since App Router. With React 19 support, stable Turbopack, async params, and redesigned caching — here is everything you need to know to upgrade or start a new project.

What is New in Next.js 15?

Released in late 2024, Next.js 15 brings React 19 support, Turbopack stability for development builds, redesigned caching behavior that is more opt-in than automatic, and the new after() API for running code after a response is sent. The upgrade from Next.js 14 has a few breaking changes that catch developers off guard.

Turbopack is now stable in next dev and delivers dramatically faster hot module replacement — typically 5-10x faster startup than webpack, with even bigger gains in large codebases. This alone makes upgrading worthwhile for development speed, even if production builds still use webpack.

The Most Important Breaking Change: Async Params

In Next.js 14, route params were synchronous objects. In Next.js 15, params and searchParams are Promises that must be awaited. This breaks every dynamic route on upgrade if you do not address it. The fix is straightforward: change your page function to async, then await the params before accessing properties.

The same change applies to headers(), cookies(), and draftMode() from next/headers — all now return Promises. The Next.js upgrade codemod handles most of these automatically. Run npx @next/codemod@canary upgrade latest in your project directory to apply automated fixes.

App Router vs Pages Router in 2025

For all new projects in 2025, use App Router. Pages Router is maintained for backwards compatibility but receives no new features. App Router enables React Server Components, which render on the server and send zero JavaScript to the client by default. This dramatically improves performance and Core Web Vitals.

Server Components allow direct database queries inside components without API routes, streaming with Suspense for progressive loading, and parallel data fetching. The learning curve is steeper than Pages Router, but the performance and developer experience benefits are significant for any project beyond a simple prototype.

Caching Changes and the New after() API

Next.js 15 makes caching more explicit. fetch() requests are no longer cached by default — you must opt in with cache: force-cache or use revalidate. This is a more predictable model that reduces surprise stale data bugs, but requires updating existing fetch calls on upgrade.

The after() API is a genuinely new capability: it lets you run cleanup, logging, or analytics code after the response has been sent to the user. This is perfect for recording page views, sending webhooks, or triggering background jobs without delaying the response the user receives.

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Next.js 15 Complete Guide: New Features, Breaking Changes, and Best Practices