diff --git a/app/Domains/Catalog/Controllers/ProductController.php b/app/Domains/Catalog/Controllers/ProductController.php
index b1a1c9f..6b74fb8 100644
--- a/app/Domains/Catalog/Controllers/ProductController.php
+++ b/app/Domains/Catalog/Controllers/ProductController.php
@@ -3,6 +3,7 @@
 namespace App\Domains\Catalog\Controllers;
 
 use App\Domains\Catalog\Models\Product;
+use App\Domains\Catalog\Requests\ProductDetailRequest;
 use App\Domains\Catalog\Requests\StoreProductRequest;
 use App\Domains\Catalog\Requests\UpdateProductRequest;
 use App\Domains\Catalog\Resources\ProductResource;
@@ -30,11 +31,11 @@ class ProductController extends Controller
         return ProductResource::make($product)->response()->setStatusCode(201);
     }
 
-    public function show(Request $request, Tenant $tenant, Product $producto, ProductService $productService): ProductResource
+    public function show(ProductDetailRequest $request, Tenant $tenant, Product $producto, ProductService $productService): ProductResource
     {
         $producto = $this->resolveScopedProduct($tenant, $producto);
         $variantId = $request->query('variant_id');
-        $variantId = is_numeric($variantId) ? (int) $variantId : null;
+        $variantId = $variantId !== null ? (int) $variantId : null;
         $producto = $productService->getProductDetail($tenant, $producto, $variantId);
 
         return ProductResource::make($producto);
diff --git a/app/Domains/Catalog/Requests/ProductDetailRequest.php b/app/Domains/Catalog/Requests/ProductDetailRequest.php
new file mode 100644
index 0000000..05f8962
--- /dev/null
+++ b/app/Domains/Catalog/Requests/ProductDetailRequest.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Domains\Catalog\Requests;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class ProductDetailRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    public function rules(): array
+    {
+        return [
+            'variant_id' => ['sometimes', 'integer'],
+        ];
+    }
+}
diff --git a/app/Domains/Catalog/Services/ProductService.php b/app/Domains/Catalog/Services/ProductService.php
index 122a8a9..41e82e9 100644
--- a/app/Domains/Catalog/Services/ProductService.php
+++ b/app/Domains/Catalog/Services/ProductService.php
@@ -10,6 +10,7 @@ use App\Domains\Tenant\Models\Tenant;
 use Illuminate\Contracts\Pagination\LengthAwarePaginator;
 use Illuminate\Http\UploadedFile;
 use Illuminate\Support\Facades\DB;
+use Illuminate\Validation\ValidationException;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 class ProductService
@@ -332,6 +333,12 @@ class ProductService
             throw new NotFoundHttpException('Product variant not found for product.');
         }
 
+        if ($variantId !== null && $selectedVariant->stock <= 0) {
+            throw ValidationException::withMessages([
+                'variant_id' => 'La variante seleccionada no tiene stock.',
+            ]);
+        }
+
         $selectedVariant ??= $product->variants->first();
 
         if ($selectedVariant !== null) {
diff --git a/tests/Feature/Catalog/ProductControllerTest.php b/tests/Feature/Catalog/ProductControllerTest.php
index 08fb365..1a82ff6 100644
--- a/tests/Feature/Catalog/ProductControllerTest.php
+++ b/tests/Feature/Catalog/ProductControllerTest.php
@@ -722,6 +722,52 @@ class ProductControllerTest extends TestCase
         $this->assertEquals(0, $defaultVariant->definitions()->count());
     }
 
+    public function test_it_rejects_product_detail_when_requested_variant_has_no_stock(): void
+    {
+        $product = Product::create([
+            'tenant_codigo' => $this->tenant->codigo,
+            'categoria_id' => 1,
+            'brand_id' => $this->brand->id,
+            'slug' => 'test-product-no-stock',
+            'nombre' => 'Test Product No Stock',
+            'precio' => 100.00,
+        ]);
+        $productAttributes = $this->syncVariantAttributes($product);
+
+        $outOfStockVariant = $product->createVariant([
+            'stock' => 0,
+            'definitions' => [
+                [
+                    'products_attribute_id' => $productAttributes['size']->id,
+                    'value' => 'S',
+                ],
+            ],
+        ]);
+
+        $response = $this->getJson("/api/tenants/{$this->tenant->codigo}/productos/{$product->id}?variant_id={$outOfStockVariant->id}");
+
+        $response->assertUnprocessable();
+        $response->assertJsonValidationErrors(['variant_id']);
+        $response->assertJsonPath('errors.variant_id.0', 'La variante seleccionada no tiene stock.');
+    }
+
+    public function test_it_rejects_product_detail_when_requested_variant_id_is_invalid_format(): void
+    {
+        $product = Product::create([
+            'tenant_codigo' => $this->tenant->codigo,
+            'categoria_id' => 1,
+            'brand_id' => $this->brand->id,
+            'slug' => 'test-product-invalid-format',
+            'nombre' => 'Test Product Invalid Format',
+            'precio' => 100.00,
+        ]);
+
+        $response = $this->getJson("/api/tenants/{$this->tenant->codigo}/productos/{$product->id}?variant_id=abc");
+
+        $response->assertUnprocessable();
+        $response->assertJsonValidationErrors(['variant_id']);
+    }
+
     private function createAttachment(string $path): Attachment
     {
         return Attachment::create([