vercel/next.js / errors / improper-devtool.mdx

---

title: 'Improper webpack devtool used in development mode'

Why This Error Occurred

Next.js chooses the most optimal devtool for use with webpack. Changing the devtool in development mode will cause severe performance regressions with your application.

Possible Ways to Fix It

Please remove the custom devtool override or only apply it to production builds in your next.config.js.

module.exports = {
  webpack: (config, options) => {
    if (!options.dev) {
      config.devtool = options.isServer ? false : 'your-custom-devtool'
    }
    return config
  },
}