---
title: "Improper webpack `devtool` used in development mode"
url: "https://nextjs.org/docs/messages/improper-devtool"
docs_index: /docs/llms.txt
---



## 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`.

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