Confusion about --turbopack vs. experimental.turbo

[klimacka]

Summary

So after updating to Next 15.4.6 (latest stable) I have read about turbopack being almost ready (on Next blog) and went in to try it out.

When I add the CLI flag (I build via Docker, so in my Dockerfile I used line:
RUN ./node_modules/.bin/next build --turbopack
instead of previous:
RUN ./node_modules/.bin/next build

What confuses me is this build output:

 > [nextjs_build 2/2] RUN ./node_modules/.bin/next build --turbopack:                                                                                                                                                                                                                          
0.686  ⚠ The config property `experimental.turbo` is deprecated. Move this setting to `config.turbopack` as Turbopack is now stable.                                                                                                                                                           
0.735    ▲ Next.js 15.4.6 (Turbopack)                                                                                                                                                                                                                                                          
0.735    - Environments: .env                                                                                                                                                                                                                                                                  
0.735    - Experiments (use with caution):                                                                                                                                                                                                                                                     
0.735      · turbo
0.735      · clientTraceMetadata
0.735 
0.744    Creating an optimized production build ...
14.78  ✓ Finished writing to disk in 31ms
14.95  ⚠ Support for Turbopack builds is experimental. We don't recommend deploying mission-critical applications to production.
14.95 

I have no experimental.turbo in my next.config.js . Is the CLI flag adding it to the configuration on runtime?

My next.config.js:

const {
  version: playerVersion,
} = require("./projects/player/package.json");
const webpack = require("webpack");

/** @type {import("next").NextConfig} */
const nextConfig = {
  reactStrictMode: true,
  transpilePackages: ["react-daisyui"],
  images: {
    remotePatterns: [
      {
        protocol: "https",
        hostname: "**/*",
      },
    ],
  },
  turbopack: {
    root: __dirname,
  },
  output: "standalone",
  eslint: {
    ignoreDuringBuilds: true,
  },
  async headers() {
    return [
      {
        source: "/static/test/:path*",
        headers: [
          {
            key: "Access-Control-Allow-Origin",
            value: "*",
          },
        ],
      },
    ];
  },
  webpack(config) {
    config.plugins.push(
      new webpack.DefinePlugin({
        __PLAYER_VERSION__: JSON.stringify(playerVersion),
        __PLAYER_TAILWIND_PREFIX__: '""',
      }),
    );
    return config;
  },
};

const createNextIntlPlugin = require("next-intl/plugin");
const withNextIntl = createNextIntlPlugin();

const { withSentryConfig } = require("@sentry/nextjs");
const finalConfig = withSentryConfig(nextConfig, {
  org: "<REDACTED>",
  project: "<REDACTED>",
  authToken: process.env.SENTRY_AUTH_TOKEN,
  silent: !process.env.CI,
  disableLogger: true,
  sourcemaps: {
    disable: true,
  },
});

module.exports = withNextIntl(finalConfig);

🤔 Should I ignore the warning about config property experimental.turbo being deprecated? Or do I enable turbo in some other way than CLI flag --turbopack ?

When I don't use the CLI flag, it seems to me that the old Webpack build is being used.

Additional information

No response

Example

No response

[melodyxpot]

In Next.js ≤15.4.x, the --turbopack CLI flag is basically a shortcut for “turn on Turbopack mode” at runtime.
So that flag sets experimental.turbo: true under the hood. So even you didn't set it in your next.config.js file, that warning will be appeared.
So instead of relying on experimental.turbo (old key), you should explicitly enable Turbopack in your config under the new stable key.

// next.config.js
module.exports = {
  turbopack: {
    root: __dirname, // optional
  },
};

And about second warning

⚠ Support for Turbopack builds is experimental.

Turbopack still isn’t fully stable.
That’s not about your config — that’s a general disclaimer from the Next.js team.
They still don’t recommend Turbopack for mission-critical production apps, but it’s getting closer.

To avoid this warning while keeping turbopack testing,

1. Remove the CLI flag from Docker.

2. Add this to your next.config.js:

   turbopack: {
     root: __dirname,
   },

3. Run next build normally.

But as Next.js team doesn't recommend this one, removing turbopack in building will be better I guess.

Try it out.

[klimacka]

When I don't use the CLI tag and only keep config option:

turbopack: {
  root: __dirname,
},

And when I get any error, I can see in the dev error helper, that Webpack is still being used.

Also, this hints that Turbopack is not enabled:

[icyJoseph]

What's your withSentryConfig version?

[icyJoseph]

Mm nope, it is likely coming from: https://github.com/amannn/next-intl/blob/ce300a4a2f0fa63e396f01242340ce8e51fc8cdd/packages/next-intl/src/plugin/getNextConfig.tsx#L87-L98

[icyJoseph]

We can debug this with: https://nextjs.org/docs/app/api-reference/config/next-config-js#configuration-as-a-function

And see what next-intl plugin injects into the config, we'll likely see the experimental.turbo there.

[klimacka]

I am using next-intl might be it. I am planning to upgrade to v4 from v3.25

[icyJoseph]

Even v4 does that:

import type { NextConfig } from 'next'

import createNextIntlPlugin from "next-intl/plugin"

const withNextIntl = createNextIntlPlugin();

const nextConfig = (phase: unknown, { defaultConfig }: { defaultConfig: NextConfig }) => {
  const config = withNextIntl()
  console.log(config.experimental)
  return config
}

export default nextConfig;

And you'll see this when building - so config.experimental.turbo is added by next-intl.

{
  turbo: { resolveAlias: { 'next-intl/config': './i18n/request.ts' } }
}

[klimacka]

Thanks for analysis! I am no longer confused, so I will mark it as answered. You have been very helpful 🙂

Btw, turns out turbopack does not work for me still so I will not be enabling it. It chokes on some CSS in the nested player project and I have no idea what the issue is there.