I’ve worked on similar hybrid setups during migrations — especially between Symfony versions — and this kind of session management conflict is pretty common.

Is there a proper way to tell Symfony 6 to not start a session but use the existing one?

// In a Symfony 6 controller or service
$session = $request->getSession();
if ($session->isStarted()) {
    // Access session safely without triggering session_start()
    $userId = $session->get('user_id');
}

But to avoid starting the session, you must:

• Avoid using $session->start() (Symfony calls it internally when needed).
• Avoid calling anything that implicitly starts the session like $session->set(...) if it wasn’t started yet. You can inject the session instead of initializing it too early.