Answer by Hamfri for Cakephp3: How can I return json data?

As of cakePHP 4.x.x the following should work assuming that your controller and routes are set as shown below:controller: <your_project_name>/src/Controller/StudentsController.php

public function index()    {        $students = $this->Students->find('all');        $this->set(compact('students'));        $this->viewBuilder()->setOption('serialize',['students']);    }

Routes: <your_project_name>/config/routes.php

<?phpuse Cake\Routing\Route\DashedRoute;use Cake\Routing\RouteBuilder;/** @var \Cake\Routing\RouteBuilder $routes */$routes->setRouteClass(DashedRoute::class);$routes->scope('/', function (RouteBuilder $builder) {    $builder->setExtensions(['json']);    $builder->resources('Students');    $builder->fallbacks();});

Run bin/cake server and visit http://localhost:8765/students.json using postman/insomnia or just the normal browser.See further documentation for setting up Restful controllers and Restful Routing

Don't forget to set the method to GET on postman and insomnia.