Override Laravel config during Dusk tests

DuskConfigOverride.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;

class DuskConfigOverride
{
    /**
     * Set config, serverside on dusk tests
     *
     * @param \Illuminate\Http\Request $request
     * @param \Closure $next
     * @return mixed
     */
    public function handle(Request $request, Closure $next)
    {
        //Only handle the DuskRuntimeConfig when debug=TRUE and environment=testing
        if (!config('app.debug') || !app()->environment() == 'testing') {
            return $next($request);
        }

        if ($request->has('duskconfig') || $request->session()->has('duskconfig')) {
            $configJson = $request->get('duskconfig') ?? $request->session()->get('duskconfig');
            $configValues = json_decode($configJson, true);

            if (sizeof($configValues) > 0) {
                foreach ($configValues as $key => $value) {
                    config([$key => $value]);
                }
            }

            \Session::put('duskconfig', $configJson);
            \Session::save();
        }

        return $next($request);
    }
}

DuskTestCase.php

// override newBrowser to return an instance of your Browser.

    protected function newBrowser($driver)
    {
        return new Browser($driver);
    }

kernel.php

<?php

namespace App\Http;

use App\Http\Middleware\DuskConfigOverride;
use Illuminate\Contracts\Foundation\Application;
use Illuminate\Foundation\Http\Kernel as HttpKernel;
use Illuminate\Routing\Router;

class Kernel extends HttpKernel
{
    // ..
  
    protected $middlewareGroups = [
        'web' => [
            // ..
            \Illuminate\Session\Middleware\StartSession::class,
            DuskConfigOverride::class, // after StartSession
            // ..
        ],
    // ..
}

override-config-in-dusk-tests.php

<?php

namespace Tests\Browser;

class Browser extends \Laravel\Dusk\Browser
{
    protected $modifiedConfig = [];

    public function withConfig($config = [])
    {
        $this->modifiedConfig = $config;
    }

    public function visit($url)
    {
        if (!empty($this->modifiedConfig))
            $url .= (parse_url($url, PHP_URL_QUERY) ? '&' : '?') . 'duskconfig=' . json_encode($this->modifiedConfig);

        return parent::visit($url);
    }
}

SomeDuskTest.php

<?php

namespace Tests\Browser;

use App\Components\Currency\Currencies\GBP;
use App\Models\User;
use Illuminate\Foundation\Testing\DatabaseMigrations;
use Tests\DuskTestCase;

class SomeDuskTest extends DuskTestCase

    public function test_something()
    {
        $this->browse(function(Browser $browser) {
            $browser->withConfig(['currency.default' => 'GBP'])
                ->visit('/login')
                ->waitFor('@login.email')
                // ...
        });
    }

public function withConfig($config = []) { $this->modifiedConfig = $config; }

should return $this;

`public function withConfig($config = [])
{
$this->modifiedConfig = $config;

    return $this;
}`

Also is better to register the middleware using AppServiceProvider boot method, like this:
public function boot(Kernel $kernel) { if (app()->environment() == 'testing' || app()->environment() == 'local') { $kernel->appendMiddlewareToGroup('web', DuskConfigOverrideMiddleware::class); } }