vercel/next.js · error · Error

The "sassOptions.functions" option is not supported when…

Error message

The "sassOptions.functions" option is not supported when using Turbopack. Custom Sass functions are only available with webpack. Please remove the "functions" property from your sassOptions in ${configFileName}.

What it means

Error "The "sassOptions.functions" option is not supported when using Turbopack. Custom Sass functions are only available with webpack. Please remove the "functions" property from your sassOptions in ${configFileName}." thrown in vercel/next.js.

Solutions

  1. Remove sassOptions.functions, or switch to webpack (next dev/build --webpack).
Defensive patterns

Strategy: validation

When it happens

Trigger: sassOptions.functions is configured while using Turbopack.

Common situations: Custom Sass functions in next.config with the default Turbopack bundler; only webpack supports them.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/0e68058bc6ae380c. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:565

    }
  }

  if (
    result.experimental?.allowDevelopmentBuild &&
    process.env.NODE_ENV !== 'development'
  ) {
    throw new Error(
      `The experimental.allowDevelopmentBuild option requires NODE_ENV to be explicitly set to 'development'.`
    )
  }

  // Validate sassOptions.functions is not used with Turbopack
  if (
    process.env.TURBOPACK &&
    result.sassOptions &&
    'functions' in result.sassOptions
  ) {
    throw new Error(
      `The "sassOptions.functions" option is not supported when using Turbopack. ` +
        `Custom Sass functions are only available with webpack. ` +
        `Please remove the "functions" property from your sassOptions in ${configFileName}.`
    )
  }

  // Validate experimental.cssChunking compatibility with the active bundler. Graph mode is
  // Turbopack-only; strict mode and `false` (single-chunk-per-module) are webpack-only.
  // Only validate during build/dev — `next start` doesn't pick a bundler and would otherwise
  // see `process.env.TURBOPACK` unset and reject a valid `cssChunking: "graph"` config.
  if (
    phase !== PHASE_PRODUCTION_SERVER &&
    phase !== PHASE_INFO &&
    phase !== PHASE_TEST
  ) {
    if (result.experimental.durableUseCacheEntries && !process.env.TURBOPACK) {
      throw new Error(
        `\`experimental.durableUseCacheEntries: true\` is only supported with Turbopack. ` +

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