Next.js 15.5 and Turbopack: Rust-Based Builds Move Closer to Production

Next.js 15.5 ships with Turbopack production builds in beta, stable Node.js middleware, TypeScript improvements, and the deprecation of next lint. If you're planning a Next.js 16 upgrade path, these changes are worth understanding now.

Turbopack Builds: Beta for Production

Turbopack, Vercel's Rust-based bundler, has been available for the dev server since Next.js 14. With 15.5, production builds (next build) have finally reached beta.

Reported performance characteristics compared to Webpack:

The actual gain depends on your project's size and configuration. Smaller projects see less impact; projects with hundreds of components and complex dependency graphs see the most.

Enabling It

# Via CLI flag
next build --turbopack

Since this is beta, verify compatibility with your existing Webpack customizations:

• Custom loaders (e.g., SVGR for React SVG components)
• Webpack plugins like Sentry's
• Logic in the webpack callback in next.config.mjs

Before promoting to production, run both Webpack and Turbopack builds in CI and diff the outputs. Catch differences early rather than in a production incident.

Stable Node.js Middleware

Middleware now runs in a full Node.js environment, removing the Edge Runtime restrictions that prevented using Node.js built-in modules.

// middleware.ts
import { NextRequest, NextResponse } from "next/server";
import { createHash } from "crypto"; // Now available in Middleware
 
export function middleware(request: NextRequest) {
  const token = request.headers.get("x-api-token") ?? "";
  const hash = createHash("sha256").update(token).digest("hex");
 
  if (!isValidToken(hash)) {
    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
  }
 
  return NextResponse.next();
}
 
export const config = {
  matcher: "/api/:path*",
};
 
function isValidToken(hash: string): boolean {
  const validHashes = process.env.VALID_TOKEN_HASHES?.split(",") ?? [];
  return validHashes.includes(hash);
}

This aligns with Vercel's Fluid Compute environment, which runs standard Node.js. If you previously worked around Edge Runtime limitations with workarounds or separate API routes, you can now simplify that logic directly in middleware.

next lint Deprecated

The next lint command is deprecated and will be removed in Next.js 16. The recommendation is to call ESLint directly.

// package.json
{
  "scripts": {
    "lint": "eslint src --ext .ts,.tsx,.js,.jsx"
  }
}

To preserve the same rule set that next lint provided, explicitly configure the Next.js ESLint packages:

// .eslintrc.json
{
  "extends": ["next/core-web-vitals", "next/typescript"]
}

TypeScript Improvements

Improved type definitions for React 19 features — Server Components and the Activity API in particular — reduce the need for any casts and manual type annotations in server-side code.

// Async Server Components now get accurate type inference
interface UserProfileProps {
  userId: string;
}
 
async function UserProfile({ userId }: UserProfileProps) {
  const user = await fetchUser(userId); // Return type inferred correctly
  return <div>{user.name}</div>;
}

Practical Approach

For new projects: enable Turbopack from the start and validate build quality early.

For existing projects: enable Turbopack for the dev server first to improve the development loop. Run production Turbopack builds in a separate CI job alongside the existing Webpack build, and diff the outputs before switching over completely.

Addressing the next lint deprecation now costs almost nothing. Doing it during a rushed Next.js 16 upgrade costs more. Take care of it in the next sprint.