vercel/next.js · error

Configuring Next.js via '${basename(unsupportedConfig)}' is

Error message

Configuring Next.js via '${basename(unsupportedConfig)}' is not supported. Please replace the file with 'next.config.js', 'next.config.mjs', or 'next.config.ts'.

What it means

Thrown at config.ts:2212 when Next.js cannot find a supported config file but does find one with an unsupported extension. The `findUp` search looks for `next.config.cjs`, `next.config.cts`, `next.config.mts` (only when Node's TypeScript feature is disabled), `next.config.json`, `next.config.jsx`, and `next.config.tsx`. Only `.js`, `.mjs`, and `.ts` are supported.

Source

Thrown at packages/next/src/server/config.ts:2213

    }

    return [finalConfig, meta]
  } else {
    const configBaseName = basename(CONFIG_FILES[0], extname(CONFIG_FILES[0]))
    const unsupportedConfig = findUp.sync(
      [
        `${configBaseName}.cjs`,
        `${configBaseName}.cts`,
        // TODO: Remove `as any` once we bump @types/node to v22.10.0+
        ...((process.features as any).typescript ? [] : ['next.config.mts']),
        `${configBaseName}.json`,
        `${configBaseName}.jsx`,
        `${configBaseName}.tsx`,
      ],
      { cwd: dir }
    )
    if (unsupportedConfig?.length) {
      throw new Error(
        `Configuring Next.js via '${basename(
          unsupportedConfig
        )}' is not supported. Please replace the file with 'next.config.js', 'next.config.mjs', or 'next.config.ts'.`
      )
    }
  }

  const clonedDefaultConfig = cloneObject(defaultConfig) as NextConfig

  enforceExperimentalFeatures(clonedDefaultConfig, {
    isDefaultConfig: true,
    configuredExperimentalFeatures,
    debugPrerender,
    phase,
  })

  // always call assignDefaults to ensure settings like
  // reactRoot can be updated correctly even with no next.config.js

View on GitHub (pinned to 0ae8c72462)

Solutions

1. Rename the config file to `next.config.js`, `next.config.mjs`, or `next.config.ts`.
2. If you have multiple config files, delete the unsupported one so only the supported name remains.
3. For a JSON-style config, convert it to a `.js`/`.mjs` module that exports the object.

Example fix

// before: file named next.config.json
// after: rename to next.config.js and use module.exports = { ... }

import { existsSync } from 'fs';
const supported = ['next.config.js','next.config.mjs','next.config.ts'];
if (!supported.some(existsSync)) {
  throw new Error('Rename config to a supported extension');
}

function isSupportedConfigName(name: string): boolean {
  return ['next.config.js','next.config.mjs','next.config.ts'].includes(name);
}

When it happens

Trigger: Naming the config `next.config.json`, `next.config.cjs`, `next.config.tsx`, `next.config.jsx`, or `next.config.mts` (on a non-TypeScript Node).

Common situations: Renaming the file during a tooling change; committing a JSON copy of the config; scaffolding from a template that used a non-standard extension; migrating to TypeScript and using `.mts` instead of `.ts`.

Related errors

• Specified images.loaderFile does not exist at "${absolutePat
• Invalid handler fields configured for "cacheHandlers": ${inv
• You can not have a '_next' folder inside of your public fold
• No artifact run found in ${requested}. Expected a results.js
• `src` and `dest` are required

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/43bf09a2eb6f5c95. Report an issue: GitHub.