Fix The requested qty is not available" should be received instead of Internal server error

app/code/Magento/QuoteGraphQl/Model/Cart/AddSimpleProductToCart.php

@@ -75,7 +75,16 @@ public function execute(Quote $cart, array $cartItemData): void
             throw new GraphQlNoSuchEntityException(__('Could not find a product with SKU "%sku"', ['sku' => $sku]));
         }
 
-        $result = $cart->addProduct($product, $this->createBuyRequest($qty, $customizableOptions));
+        try {
+            $result = $cart->addProduct($product, $this->createBuyRequest($qty, $customizableOptions));
+        } catch (\Exception $e) {
+            throw new GraphQlInputException(
+                __(
+                    'Could not add the product with SKU %sku to the shopping cart: %message',
+                    ['sku' => $sku, 'message' => $e->getMessage()]
+                )
+            );
+        }
 
         if (is_string($result)) {
             throw new GraphQlInputException(__($result));

dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/AddSimpleProductToCartTest.php

@@ -0,0 +1,86 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GraphQl\Quote;
+
+use Magento\TestFramework\Helper\Bootstrap;
+use Magento\TestFramework\TestCase\GraphQlAbstract;
+use Magento\Quote\Model\Quote;
+use Magento\Quote\Model\QuoteIdToMaskedQuoteIdInterface;
+use Magento\Quote\Model\ResourceModel\Quote as QuoteResource;
+
+class AddSimpleProductToCartTest extends GraphQlAbstract
+{
+    /**
+     * @var QuoteResource
+     */
+    private $quoteResource;
+
+    /**
+     * @var Quote
+     */
+    private $quote;
+
+    /**
+     * @var QuoteIdToMaskedQuoteIdInterface
+     */
+    private $quoteIdToMaskedId;
+
+    /**
+     * @inheritdoc
+     */
+    protected function setUp()
+    {
+        $objectManager = Bootstrap::getObjectManager();
+        $this->quoteResource = $objectManager->get(QuoteResource::class);
+        $this->quote = $objectManager->create(Quote::class);
+        $this->quoteIdToMaskedId = $objectManager->get(QuoteIdToMaskedQuoteIdInterface::class);
+    }
+
+    /**
+     * @magentoApiDataFixture Magento/Catalog/_files/products.php
+     * @magentoApiDataFixture Magento/Checkout/_files/active_quote.php
+     * @expectedException \Exception
+     * @expectedExceptionMessage The requested qty is not available
+     */
+    public function testAddProductIfQuantityIsNotAvailable()
+    {
+        $sku = 'simple';
+        $qty = 200;
+
+        $this->quoteResource->load(
+            $this->quote,
+            'test_order_1',
+            'reserved_order_id'
+        );
+        $maskedQuoteId = $this->quoteIdToMaskedId->execute((int)$this->quote->getId());
+
+        $query = <<<QUERY
+mutation {  
+  addSimpleProductsToCart(
+    input: {
+      cart_id: "{$maskedQuoteId}", 
+      cartItems: [
+        {
+          data: {
+            qty: $qty
+            sku: "$sku"
+          }
+        }
+      ]
+    }
+  ) {
+    cart {
+      cart_id
+    }
+  }
+}
+QUERY;
+
+        $this->graphQlQuery($query);
+    }
+}