vercel/next.js · error

Pass either `webpack` or `turbopack`, not both.

Error message

Pass either `webpack` or `turbopack`, not both.

What it means

Option conflict in createServer for custom servers: bundler-selection inputs resolved to both Turbopack and webpack simultaneously (options.turbo/turbopack or IS_TURBOPACK_TEST together with options.webpack or IS_WEBPACK_TEST). Exactly one bundler can be forced, so the combination is rejected up front.

Solutions

1. Pass only one of webpack or turbopack to the Next.js server options.

When it happens

Trigger: Both webpack and turbopack options are passed to the server.

Common situations: A custom server passes both bundler options to next(); pass only one.

AI-assisted analysis of vercel/next.js@0eb3775416 (2026-08-19). Data as JSON: /api/errors/e9cf2cbf8236fba9. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/next.ts:630

}

// This file is used for when users run `require('next')`
function createServer(
  options: NextServerOptions & NextBundlerOptions
): NextWrapperServer {
  // next sets customServer to false when calling this function, in that case we don't want to modify the environment variables
  const isCustomServer = options?.customServer ?? true
  if (isCustomServer) {
    const selectTurbopack =
      options &&
      (options.turbo || options.turbopack || process.env.IS_TURBOPACK_TEST)
    const selectWebpack =
      options && (options.webpack || process.env.IS_WEBPACK_TEST)
    // Rspack is selected through env/config side effects instead of a custom
    // server option, so don't fall back to the default Turbopack auto mode.
    const selectRspack = !!process.env.NEXT_RSPACK
    if (selectTurbopack && selectWebpack && selectRspack) {
      throw new Error('Pass either `webpack` or `turbopack`, not both.')
    }
    if (selectTurbopack) {
      process.env.TURBOPACK ??= '1'
    } else if (!selectWebpack && !selectRspack) {
      process.env.TURBOPACK ??= 'auto'
    }
  } else {
    if (options && (options.webpack || options.turbo || options.turbopack)) {
      throw new Error(
        'Only custom servers can pass `webpack`, `turbo`, or `turbopack`.'
      )
    }
  }

  // The package is used as a TypeScript plugin.
  if (
    options &&
    'typescript' in options &&

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