Quite some work done here.

src/Game/Controller/GameApiController.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace App\Game\Controller;
 
+use App\Game\Service\GameResponseService;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpFoundation\Request;
@@ -12,6 +13,12 @@ use Symfony\Component\Routing\Annotation\Route;
 #[Route('/game/api', name: 'game_api_')]
 final class GameApiController extends AbstractController
 {
+
+    public function __construct(
+            protected GameResponseService $gameResponseService) {
+
+    }
+
     #[Route('/ping', name: 'ping', methods: ['GET'])]
     public function ping(): JsonResponse
     {
@@ -22,27 +29,18 @@ final class GameApiController extends AbstractController
         ]);
     }
 
-    #[Route('/echo', name: 'echo', methods: ['POST'])]
-    public function echo(Request $request): JsonResponse
+    #[Route('/message', name: 'message', methods: ['POST'])]
+    public function message(Request $request): JsonResponse
     {
         $raw = (string) $request->getContent();
         $data = null;
         if ($raw !== '') {
-            try {
-                /** @var array<string,mixed>|null $decoded */
-                $decoded = json_decode($raw, true, 512, JSON_THROW_ON_ERROR);
-                $data = $decoded;
-            } catch (\Throwable $e) {
-                return $this->json([
-                    'ok' => false,
-                    'error' => 'Invalid JSON: ' . $e->getMessage(),
-                ], Response::HTTP_BAD_REQUEST);
-            }
+            $data = $this->gameResponseService->getGameResponse($raw);
         }
 
         return $this->json([
             'ok' => true,
-            'received' => $data,
+            'result' => $data,
             'ts' => date('c'),
         ]);
     }

src/Game/Controller/GameController.php

@@ -12,6 +12,8 @@ final class GameController extends AbstractController
     #[Route(path: '', name: 'game')]
     public function index(): Response
     {
-        return $this->render('game/index.html.twig');
+        return $this->render('game/index.html.twig', [
+            'user_id' => $this->getUser()?->getId(),
+        ]);
     }
 }