Webpack removed SizeFormatHelpers in v5.76.0. Your project probably pulled in the newer webpack automatically, which breaks the BuildOutputPlugin in older Mix releases.

Just bump laravel-mix to ^6.0.49 in your package.json. Jeffrey patched this exact issue a while back so you don't need to downgrade Webpack like the AI suggested.

Wipe your node_modules and package-lock.json, run npm install, and you should be good to go.

Thanks, I tried deleting package-lock.json and node_modules but it was still breaking. I ended up having to use my old package-lock.json in production to get it to build so there's some other updated dependency breaking it.

I have an older project still using Laravel-mix for building out the front-end. Just ran into this issue:

[webpack-cli] Error: Cannot find module 'webpack/lib/SizeFormatHelpers'
Require stack:
- /var/www/node_modules/laravel-mix/src/webpackPlugins/BuildOutputPlugin.js
- /var/www/node_modules/laravel-mix/src/builder/webpack-plugins.js
- /var/www/node_modules/laravel-mix/src/builder/WebpackConfig.js
- /var/www/node_modules/laravel-mix/src/Mix.js
- /var/www/node_modules/laravel-mix/setup/webpack.config.js
- /var/www/node_modules/webpack-cli/lib/webpack-cli.js
- /var/www/node_modules/webpack-cli/lib/bootstrap.js
- /var/www/node_modules/webpack-cli/bin/cli.js
- /var/www/node_modules/webpack/bin/webpack.js
    at Function.Module._resolveFilename (node:internal/modules/cjs/loader:985:15)
    at Function.Module._load (node:internal/modules/cjs/loader:833:27)
    at Module.require (node:internal/modules/cjs/loader:1057:19)
    at require (node:internal/modules/cjs/helpers:103:18)
    at Object.<anonymous> (/var/www/node_modules/laravel-mix/src/webpackPlugins/BuildOutputPlugin.js:6:24)
    at Module._compile (node:internal/modules/cjs/loader:1155:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1209:10)
    at Module.load (node:internal/modules/cjs/loader:1033:32)
    at Function.Module._load (node:internal/modules/cjs/loader:868:12)
    at Module.require (node:internal/modules/cjs/loader:1057:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/var/www/node_modules/laravel-mix/src/webpackPlugins/BuildOutputPlugin.js',
    '/var/www/node_modules/laravel-mix/src/builder/webpack-plugins.js',
    '/var/www/node_modules/laravel-mix/src/builder/WebpackConfig.js',
    '/var/www/node_modules/laravel-mix/src/Mix.js',
    '/var/www/node_modules/laravel-mix/setup/webpack.config.js',
    '/var/www/node_modules/webpack-cli/lib/webpack-cli.js',
    '/var/www/node_modules/webpack-cli/lib/bootstrap.js',
    '/var/www/node_modules/webpack-cli/bin/cli.js',
    '/var/www/node_modules/webpack/bin/webpack.js'
  ]
}

Removing node_modules and package-lock.json and rebuilding the front-end does not resolve this missing module.