> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# tools.less

- **Type:** `Object | Function`
- **Default:**

```js
const defaultOptions = {
  lessLoaderOptions: {
    lessOptions: {
      javascriptEnabled: true,
    },
    // CSS Source Map enabled by default in development environment
    sourceMap: isDev,
  },
};
```

`tools.less` modifies the options of [@rsbuild/plugin-less](https://rsbuild.rs/plugins/list/plugin-less). It accepts the full plugin options:

| Option              | Description                                                                                         |
| ------------------- | --------------------------------------------------------------------------------------------------- |
| `lessLoaderOptions` | Options passed to [less-loader](https://github.com/webpack-contrib/less-loader), object or function |
| `include`           | Files handled by less-loader, defaults to `/\.less$/`                                               |
| `exclude`           | Files that less-loader should skip                                                                  |
| `parallel`          | Whether to compile Less modules in worker threads, defaults to `false`                              |

### Modifying less-loader options

When `lessLoaderOptions` is an `Object`, it is merged with the default config through Object.assign in a shallow way. It should be noted that `lessOptions` is merged through deepMerge in a deep way. For example:

```js
export default {
  tools: {
    less: {
      lessLoaderOptions: {
        lessOptions: {
          javascriptEnabled: false,
        },
      },
    },
  },
};
```

When `lessLoaderOptions` is a `Function`, the default config is passed as the first parameter, which can be directly modified or returned as the final result. The second parameter provides some utility functions that can be called directly. For example:

```js
export default {
  tools: {
    less: {
      lessLoaderOptions(config) {
        // Modify the config of lessOptions
        config.lessOptions = {
          javascriptEnabled: false,
        };
      },
    },
  },
};
```

### Parallel compilation

Less compilation is pure JavaScript and runs on the Node.js main thread by default. With `parallel` enabled, Less modules are compiled in a pool of worker threads, which shortens the build when a project has many Less files.

```js
export default {
  tools: {
    less: {
      parallel: true,
    },
  },
};
```

:::tip
Options sent to worker threads must satisfy the [structured clone algorithm](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm). With `parallel` enabled, `lessLoaderOptions` therefore cannot contain functions, such as an `additionalData` function or a custom `implementation`.
:::

### Modifying Less Version

In some scenarios, if you need to use a specific version of Less instead of the built-in Less v4 in Modern.js, you can install the desired Less version in your project and set it up using the `implementation` option of the `less-loader`.

```js
export default {
  tools: {
    less: {
      lessLoaderOptions: {
        implementation: require('less'),
      },
    },
  },
};
```

### Util Function

#### addExcludes

- **Type:** `(excludes: RegExp | RegExp[]) => void`

Used to specify which files `less-loader` does not compile, You can pass in one or more regular expressions to match the path of less files, for example:

```js
export default {
  tools: {
    less: {
      lessLoaderOptions(config, { addExcludes }) {
        addExcludes(/node_modules/);
      },
    },
  },
};
```

The plugin's `exclude` option is the recommended equivalent:

```js
export default {
  tools: {
    less: {
      exclude: /node_modules/,
    },
  },
};
```

### Legacy form

:::warning Do not mix the two layers
As soon as an object contains a plugin-level key (`lessLoaderOptions`, `include`, `exclude`, ...), the whole object is parsed as plugin options and any loader option in it has no effect. For example, `lessOptions` in `tools.less: { parallel: true, lessOptions: { ... } }` is ignored and must be moved under `lessLoaderOptions`; a hint is printed in development.
:::

In earlier versions, `tools.less` took the less-loader options directly, e.g. `tools.less: { lessOptions: {} }` or `tools.less(config, { addExcludes }) {}`. This form still works: Modern.js wraps it into `lessLoaderOptions` and produces exactly the same config as before. A migration hint is printed in development, and the legacy form will be removed in the next major version.

```js
// legacy
export default {
  tools: {
    less: {
      lessOptions: { javascriptEnabled: false },
    },
  },
};

// current
export default {
  tools: {
    less: {
      lessLoaderOptions: {
        lessOptions: { javascriptEnabled: false },
      },
    },
  },
};
```