vercel/next.js · error · Error

`experimental.cssChunking: "strict"` is only supported with…

Error message

`experimental.cssChunking: "strict"` is only supported with webpack. Please remove the option or run Next.js with webpack in ${configFileName}.

What it means

Config validation in assignDefaultsAndValidate: experimental.cssChunking resolved to the 'strict' mode, but that mode is only implemented for webpack while the current run uses Turbopack. The guard fires outside production-server phases (build/dev) and tells the user to drop the option or run with webpack in their config file.

Solutions

  1. Remove experimental.cssChunking: "strict", or run Next.js with webpack.
Defensive patterns

Strategy: validation

When it happens

Trigger: experimental.cssChunking 'strict' mode is used without webpack.

Common situations: cssChunking: 'strict' in next.config while running with Turbopack.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/08850e068eb45660. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:597

    phase !== PHASE_TEST
  ) {
    if (result.experimental.durableUseCacheEntries && !process.env.TURBOPACK) {
      throw new Error(
        `\`experimental.durableUseCacheEntries: true\` is only supported with Turbopack. ` +
          `Please remove the option or run Next.js with Turbopack in ${configFileName}.`
      )
    }

    const cssChunkingValue = result.experimental.cssChunking
    const cssChunkingMode = resolveCssChunkingMode(cssChunkingValue)
    if (cssChunkingMode === 'graph' && !process.env.TURBOPACK) {
      throw new Error(
        `\`experimental.cssChunking: "graph"\` is only supported with Turbopack. ` +
          `Please remove the option or run Next.js with Turbopack in ${configFileName}.`
      )
    }
    if (cssChunkingMode === 'strict' && process.env.TURBOPACK) {
      throw new Error(
        `\`experimental.cssChunking: "strict"\` is only supported with webpack. ` +
          `Please remove the option or run Next.js with webpack in ${configFileName}.`
      )
    }
    // Only error when `false` was set explicitly. `undefined` (the default) also resolves to
    // `'off'` but that's the implicit default and must not error on Turbopack.
    if (cssChunkingValue === false && process.env.TURBOPACK) {
      throw new Error(
        `\`experimental.cssChunking: false\` is only supported with webpack. ` +
          `Please remove the option or run Next.js with webpack in ${configFileName}.`
      )
    }

    if (
      result.experimental.turbopackRustReactCompiler &&
      !process.env.TURBOPACK
    ) {
      throw new Error(

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