How can overuse of a service container, sometimes called the service locator anti pattern, actually harm code quality despite the container itself being a useful tool?

While a service container is valuable for automatically wiring together a class's declared dependencies, directly calling the container from within business logic code to fetch a dependency on demand, rather than having that dependency properly injected through the constructor, hides a class's true dependencies, makes the class harder to test since its real requirements are no longer visible from its constructor signature, and creates a hidden, implicit coupling to the container itself throughout the codebase, which is why most experienced developers recommend using the container purely for object construction and always relying on proper constructor injection within actual business logic.

// Anti pattern: hides real dependencies, harder to test
class OrderService {
    public function process() {
        $mailer = app(Mailer::class);
    }
}

// Preferred: explicit constructor injection
class OrderService {
    public function __construct(private Mailer $mailer) {}
}