diff --git a/.changeset/gift-card-add-mutation.md b/.changeset/gift-card-add-mutation.md
new file mode 100644
index 0000000000..9dc0f8fff1
--- /dev/null
+++ b/.changeset/gift-card-add-mutation.md
@@ -0,0 +1,70 @@
+---
+'@shopify/hydrogen': minor
+'@shopify/cli-hydrogen': patch
+'@shopify/create-hydrogen': patch
+---
+
+Add `cartGiftCardCodesAdd` mutation
+
+## New Feature: cartGiftCardCodesAdd
+
+Adds gift card codes without replacing existing ones.
+
+**Before (2025-07):**
+```typescript
+const codes = ['EXISTING1', 'EXISTING2'];
+await cart.updateGiftCardCodes(['EXISTING1', 'EXISTING2', 'NEW_CODE']);
+```
+
+**After (2025-10):**
+```typescript
+await cart.addGiftCardCodes(['NEW_CODE']);
+```
+
+## Verified API Behavior
+
+| Scenario | Behavior |
+|----------|----------|
+| Valid gift card code | Applied successfully |
+| UPPERCASE code | Works (API is case-insensitive) |
+| Duplicate code in same call | Idempotent - applied once, no error |
+| Re-applying already applied code | Idempotent - no error, no duplicate |
+| Multiple different codes | All applied successfully |
+| Invalid code | Silently rejected (no error surfaced) |
+| Code with whitespace | Rejected (API does not trim whitespace) |
+| Empty input | Graceful no-op |
+
+**Note:** The API handles duplicate gift card codes gracefully - submitting an already-applied code results in silent success (idempotent behavior), not an error. No `DUPLICATE_GIFT_CARD` error code exists.
+
+**Note on whitespace:** The API does NOT trim whitespace from codes. Ensure codes are trimmed before submission if accepting user input.
+
+## API Reference
+
+**New method:**
+- `cart.addGiftCardCodes(codes)` - Appends codes to cart
+- `CartForm.ACTIONS.GiftCardCodesAdd` - Form action
+
+## Skeleton Template Changes
+
+The skeleton template has been updated to use the new `cartGiftCardCodesAdd` mutation:
+- Removed `UpdateGiftCardForm` component from `CartSummary.tsx`
+- Added `AddGiftCardForm` component using `CartForm.ACTIONS.GiftCardCodesAdd`
+
+If you customized the gift card form in your project, you may want to migrate to the new `Add` action for simpler code.
+
+## Usage
+
+```typescript
+import {CartForm} from '@shopify/hydrogen';
+
+<CartForm action={CartForm.ACTIONS.GiftCardCodesAdd} inputs={{giftCardCodes: ['CODE1', 'CODE2']}}>
+  <button>Add Gift Cards</button>
+</CartForm>
+```
+
+Or with createCartHandler:
+
+```typescript
+const cart = createCartHandler({storefront, getCartId, setCartId});
+await cart.addGiftCardCodes(['SUMMER2025', 'WELCOME10']);
+```
diff --git a/packages/hydrogen/src/cart/CartForm.test.tsx b/packages/hydrogen/src/cart/CartForm.test.tsx
index e581ce0114..c35831e885 100644
--- a/packages/hydrogen/src/cart/CartForm.test.tsx
+++ b/packages/hydrogen/src/cart/CartForm.test.tsx
@@ -87,6 +87,7 @@ describe('<CartForm />', () => {
       DeliveryAddressesRemove: 'DeliveryAddressesRemove',
       DiscountCodesUpdate: 'DiscountCodesUpdate',
       GiftCardCodesUpdate: 'GiftCardCodesUpdate',
+      GiftCardCodesAdd: 'GiftCardCodesAdd',
       GiftCardCodesRemove: 'GiftCardCodesRemove',
       LinesAdd: 'LinesAdd',
       LinesUpdate: 'LinesUpdate',
diff --git a/packages/hydrogen/src/cart/CartForm.tsx b/packages/hydrogen/src/cart/CartForm.tsx
index 5e5d92c985..6b03147fd6 100644
--- a/packages/hydrogen/src/cart/CartForm.tsx
+++ b/packages/hydrogen/src/cart/CartForm.tsx
@@ -87,6 +87,20 @@ type CartGiftCardCodesUpdateRequire = {
   } & OtherFormData;
 };
 
+type CartGiftCardCodesAddProps = {
+  action: 'GiftCardCodesAdd';
+  inputs?: {
+    giftCardCodes: string[];
+  } & OtherFormData;
+};
+
+type CartGiftCardCodesAddRequire = {
+  action: 'GiftCardCodesAdd';
+  inputs: {
+    giftCardCodes: string[];
+  } & OtherFormData;
+};
+
 type CartGiftCardCodesRemoveProps = {
   action: 'GiftCardCodesRemove';
   inputs?: {
@@ -278,6 +292,7 @@ type CartActionInputProps =
   | CartCreateProps
   | CartDiscountCodesUpdateProps
   | CartGiftCardCodesUpdateProps
+  | CartGiftCardCodesAddProps
   | CartGiftCardCodesRemoveProps
   | CartLinesAddProps
   | CartLinesUpdateProps
@@ -297,6 +312,7 @@ export type CartActionInput =
   | CartCreateRequire
   | CartDiscountCodesUpdateRequire
   | CartGiftCardCodesUpdateRequire
+  | CartGiftCardCodesAddRequire
   | CartGiftCardCodesRemoveRequire
   | CartLinesAddRequire
   | CartLinesUpdateRequire
@@ -345,6 +361,7 @@ CartForm.ACTIONS = {
   Create: 'Create',
   DiscountCodesUpdate: 'DiscountCodesUpdate',
   GiftCardCodesUpdate: 'GiftCardCodesUpdate',
+  GiftCardCodesAdd: 'GiftCardCodesAdd',
   GiftCardCodesRemove: 'GiftCardCodesRemove',
   LinesAdd: 'LinesAdd',
   LinesRemove: 'LinesRemove',
diff --git a/packages/hydrogen/src/cart/createCartHandler.test.ts b/packages/hydrogen/src/cart/createCartHandler.test.ts
index 1d3a0318bb..f0f932c1a7 100644
--- a/packages/hydrogen/src/cart/createCartHandler.test.ts
+++ b/packages/hydrogen/src/cart/createCartHandler.test.ts
@@ -37,7 +37,7 @@ describe('createCartHandler', () => {
     const cart = getCartHandler();
 
     expectTypeOf(cart).toEqualTypeOf<HydrogenCart>;
-    expect(Object.keys(cart)).toHaveLength(19);
+    expect(Object.keys(cart)).toHaveLength(20);
     expect(cart).toHaveProperty('get');
     expect(cart).toHaveProperty('getCartId');
     expect(cart).toHaveProperty('setCartId');
@@ -47,6 +47,7 @@ describe('createCartHandler', () => {
     expect(cart).toHaveProperty('removeLines');
     expect(cart).toHaveProperty('updateDiscountCodes');
     expect(cart).toHaveProperty('updateGiftCardCodes');
+    expect(cart).toHaveProperty('addGiftCardCodes');
     expect(cart).toHaveProperty('removeGiftCardCodes');
     expect(cart).toHaveProperty('updateBuyerIdentity');
     expect(cart).toHaveProperty('updateNote');
@@ -72,7 +73,7 @@ describe('createCartHandler', () => {
     });
 
     expectTypeOf(cart).toEqualTypeOf<HydrogenCartCustom<{foo: () => 'bar'}>>;
-    expect(Object.keys(cart)).toHaveLength(20);
+    expect(Object.keys(cart)).toHaveLength(21);
     expect(cart.foo()).toBe('bar');
   });
 
@@ -86,7 +87,7 @@ describe('createCartHandler', () => {
     });
 
     expectTypeOf(cart).toEqualTypeOf<HydrogenCart>;
-    expect(Object.keys(cart)).toHaveLength(19);
+    expect(Object.keys(cart)).toHaveLength(20);
     expect(await cart.get()).toBe('bar');
   });
 
diff --git a/packages/hydrogen/src/cart/createCartHandler.ts b/packages/hydrogen/src/cart/createCartHandler.ts
index b3a3954765..4f653f4239 100644
--- a/packages/hydrogen/src/cart/createCartHandler.ts
+++ b/packages/hydrogen/src/cart/createCartHandler.ts
@@ -49,6 +49,10 @@ import {
   type CartGiftCardCodesUpdateFunction,
   cartGiftCardCodesUpdateDefault,
 } from './queries/cartGiftCardCodeUpdateDefault';
+import {
+  type CartGiftCardCodesAddFunction,
+  cartGiftCardCodesAddDefault,
+} from './queries/cartGiftCardCodesAddDefault';
 import {
   type CartGiftCardCodesRemoveFunction,
   cartGiftCardCodesRemoveDefault,
@@ -94,6 +98,7 @@ export type HydrogenCart = {
   removeLines: ReturnType<typeof cartLinesRemoveDefault>;
   updateDiscountCodes: ReturnType<typeof cartDiscountCodesUpdateDefault>;
   updateGiftCardCodes: ReturnType<typeof cartGiftCardCodesUpdateDefault>;
+  addGiftCardCodes: ReturnType<typeof cartGiftCardCodesAddDefault>;
   removeGiftCardCodes: ReturnType<typeof cartGiftCardCodesRemoveDefault>;
   updateBuyerIdentity: ReturnType<typeof cartBuyerIdentityUpdateDefault>;
   updateNote: ReturnType<typeof cartNoteUpdateDefault>;
@@ -274,6 +279,7 @@ export function createCartHandler<TCustomMethods extends CustomMethodsBase>(
           )
         : await cartCreate({giftCardCodes}, optionalParams);
     },
+    addGiftCardCodes: cartGiftCardCodesAddDefault(mutateOptions),
     removeGiftCardCodes: cartGiftCardCodesRemoveDefault(mutateOptions),
     updateBuyerIdentity: async (buyerIdentity, optionalParams) => {
       return cartId || optionalParams?.cartId
@@ -425,6 +431,10 @@ export type HydrogenCartForDocs = {
    * Updates gift card codes in the cart.
    */
   updateGiftCardCodes?: CartGiftCardCodesUpdateFunction;
+  /**
+   * Adds gift card codes to the cart without replacing existing ones.
+   */
+  addGiftCardCodes?: CartGiftCardCodesAddFunction;
   /**
    * Removes gift card codes from the cart.
    */
diff --git a/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.doc.ts b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.doc.ts
new file mode 100644
index 0000000000..aa13f98609
--- /dev/null
+++ b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.doc.ts
@@ -0,0 +1,56 @@
+import {ReferenceEntityTemplateSchema} from '@shopify/generate-docs';
+
+const data: ReferenceEntityTemplateSchema = {
+  name: 'cartGiftCardCodesAddDefault',
+  category: 'utilities',
+  subCategory: 'cart',
+  isVisualComponent: false,
+  related: [
+    {
+      name: 'cartGiftCardCodesUpdateDefault',
+      type: 'utilities',
+      url: '/docs/api/hydrogen/utilities/cartgiftcardcodesupdatedefault',
+    },
+    {
+      name: 'cartGiftCardCodesRemoveDefault',
+      type: 'utilities',
+      url: '/docs/api/hydrogen/utilities/cartgiftcardcodesremovedefault',
+    },
+    {
+      name: 'createCartHandler',
+      type: 'utilities',
+      url: '/docs/api/hydrogen/utilities/createcarthandler',
+    },
+  ],
+  description:
+    'Creates a function that adds gift card codes to a cart without replacing existing ones',
+  type: 'utility',
+  defaultExample: {
+    description:
+      'Add gift card codes to a cart using the default cart fragment',
+    codeblock: {
+      tabs: [
+        {
+          title: 'JavaScript',
+          code: './cartGiftCardCodesAddDefault.example.js',
+          language: 'js',
+        },
+        {
+          title: 'TypeScript',
+          code: './cartGiftCardCodesAddDefault.example.ts',
+          language: 'ts',
+        },
+      ],
+      title: 'example',
+    },
+  },
+  definitions: [
+    {
+      title: 'cartGiftCardCodesAddDefault',
+      type: 'CartGiftCardCodesAddDefaultGeneratedType',
+      description: '',
+    },
+  ],
+};
+
+export default data;
diff --git a/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.example.js b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.example.js
new file mode 100644
index 0000000000..6a1bf78ebd
--- /dev/null
+++ b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.example.js
@@ -0,0 +1,11 @@
+import {cartGiftCardCodesAddDefault} from '@shopify/hydrogen';
+
+export async function action({context}) {
+  const cartAddGiftCardCodes = cartGiftCardCodesAddDefault({
+    storefront: context.storefront,
+    getCartId: () => context.cart.getCartId(),
+  });
+
+  const result = await cartAddGiftCardCodes(['SUMMER2025', 'WELCOME10']);
+  return result;
+}
diff --git a/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.example.ts b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.example.ts
new file mode 100644
index 0000000000..dfd16595f1
--- /dev/null
+++ b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.example.ts
@@ -0,0 +1,16 @@
+import {
+  cartGiftCardCodesAddDefault,
+  type HydrogenCart,
+  type CartQueryOptions,
+} from '@shopify/hydrogen';
+
+export async function action({context}: {context: CartQueryOptions}) {
+  const cartAddGiftCardCodes: HydrogenCart['addGiftCardCodes'] =
+    cartGiftCardCodesAddDefault({
+      storefront: context.storefront,
+      getCartId: context.getCartId,
+    });
+
+  const result = await cartAddGiftCardCodes(['SUMMER2025', 'WELCOME10']);
+  return result;
+}
diff --git a/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.test.ts b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.test.ts
new file mode 100644
index 0000000000..eaa02bbb51
--- /dev/null
+++ b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.test.ts
@@ -0,0 +1,102 @@
+/**
+ * TODO: These tests are placeholders that verify mock returns, not actual API behavior.
+ * They should be improved in a follow-up PR to test real integration scenarios.
+ * See PR #3284 for context.
+ */
+import {describe, it, expect} from 'vitest';
+import {CART_ID, mockCreateStorefrontClient} from '../cart-test-helper';
+import {
+  cartGiftCardCodesAddDefault,
+  CART_GIFT_CARD_CODES_ADD_MUTATION,
+} from './cartGiftCardCodesAddDefault';
+
+describe('cartGiftCardCodesAddDefault', () => {
+  describe('basic functionality', () => {
+    it('should add gift card codes to cart without replacing existing ones', async () => {
+      const addGiftCardCodes = cartGiftCardCodesAddDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+      });
+
+      const result = await addGiftCardCodes(['SUMMER2025']);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+    });
+
+    it('should handle multiple gift card codes in single call', async () => {
+      const addGiftCardCodes = cartGiftCardCodesAddDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+      });
+
+      const result = await addGiftCardCodes([
+        'GIFT123',
+        'GIFT456',
+        'WELCOME25',
+      ]);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+    });
+
+    it('should handle empty array', async () => {
+      const addGiftCardCodes = cartGiftCardCodesAddDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+      });
+
+      const result = await addGiftCardCodes([]);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+    });
+  });
+
+  describe('cartFragment override', () => {
+    it('can override cartFragment for custom query fields', async () => {
+      const cartFragment = 'cartFragmentOverride';
+      const addGiftCardCodes = cartGiftCardCodesAddDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+        cartFragment,
+      });
+
+      const result = await addGiftCardCodes(['TESTCODE']);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+      expect(result.userErrors?.[0]).toContain(cartFragment);
+    });
+  });
+
+  describe('mutation structure', () => {
+    it('should include required mutation fields for error and warning handling', () => {
+      const mutation = CART_GIFT_CARD_CODES_ADD_MUTATION();
+
+      expect(mutation).toContain('cartGiftCardCodesAdd');
+      expect(mutation).toContain('userErrors');
+      expect(mutation).toContain('warnings');
+      expect(mutation).toContain('CartApiError');
+      expect(mutation).toContain('CartApiWarning');
+    });
+
+    it('should include @inContext directive for internationalization', () => {
+      const mutation = CART_GIFT_CARD_CODES_ADD_MUTATION();
+
+      expect(mutation).toContain('@inContext');
+      expect(mutation).toContain('$country');
+      expect(mutation).toContain('$language');
+    });
+  });
+
+  describe('no duplicate filtering', () => {
+    it('should pass duplicate codes to API without filtering', async () => {
+      const addGiftCardCodes = cartGiftCardCodesAddDefault({
+        storefront: mockCreateStorefrontClient(),
+        getCartId: () => CART_ID,
+      });
+
+      const codesWithDuplicates = ['GIFT123', 'GIFT123', 'GIFT456'];
+      const result = await addGiftCardCodes(codesWithDuplicates);
+
+      expect(result.cart).toHaveProperty('id', CART_ID);
+    });
+  });
+});
diff --git a/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.ts b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.ts
new file mode 100644
index 0000000000..fecb9b85be
--- /dev/null
+++ b/packages/hydrogen/src/cart/queries/cartGiftCardCodesAddDefault.ts
@@ -0,0 +1,75 @@
+import {StorefrontApiErrors, formatAPIResult} from '../../storefront';
+import {
+  CART_WARNING_FRAGMENT,
+  MINIMAL_CART_FRAGMENT,
+  USER_ERROR_FRAGMENT,
+} from './cart-fragments';
+import type {
+  CartOptionalInput,
+  CartQueryData,
+  CartQueryDataReturn,
+  CartQueryOptions,
+} from './cart-types';
+
+export type CartGiftCardCodesAddFunction = (
+  giftCardCodes: string[],
+  optionalParams?: CartOptionalInput,
+) => Promise<CartQueryDataReturn>;
+
+/**
+ * Adds gift card codes to the cart without replacing existing ones.
+ *
+ * This function sends a mutation to the Storefront API to add one or more gift card codes to the cart.
+ * Unlike `cartGiftCardCodesUpdate` which replaces all codes, this mutation appends new codes to existing ones.
+ *
+ * @param {CartQueryOptions} options - The options for the cart query, including the storefront API client and cart fragment.
+ * @returns {CartGiftCardCodesAddFunction} - A function that takes an array of gift card codes and optional parameters, and returns the result of the API call.
+ *
+ * @example Add gift card codes
+ * const addGiftCardCodes = cartGiftCardCodesAddDefault({ storefront, getCartId });
+ * await addGiftCardCodes(['SUMMER2025', 'WELCOME10']);
+ */
+export function cartGiftCardCodesAddDefault(
+  options: CartQueryOptions,
+): CartGiftCardCodesAddFunction {
+  return async (giftCardCodes, optionalParams) => {
+    const {cartGiftCardCodesAdd, errors} = await options.storefront.mutate<{
+      cartGiftCardCodesAdd: CartQueryData;
+      errors: StorefrontApiErrors;
+    }>(CART_GIFT_CARD_CODES_ADD_MUTATION(options.cartFragment), {
+      variables: {
+        cartId: options.getCartId(),
+        giftCardCodes,
+        ...optionalParams,
+      },
+    });
+    return formatAPIResult(cartGiftCardCodesAdd, errors);
+  };
+}
+
+//! @see https://shopify.dev/docs/api/storefront/latest/mutations/cartGiftCardCodesAdd
+export const CART_GIFT_CARD_CODES_ADD_MUTATION = (
+  cartFragment = MINIMAL_CART_FRAGMENT,
+) => `#graphql
+  mutation cartGiftCardCodesAdd(
+    $cartId: ID!
+    $giftCardCodes: [String!]!
+    $language: LanguageCode
+    $country: CountryCode
+  ) @inContext(country: $country, language: $language) {
+    cartGiftCardCodesAdd(cartId: $cartId, giftCardCodes: $giftCardCodes) {
+      cart {
+        ...CartApiMutation
+      }
+      userErrors {
+        ...CartApiError
+      }
+      warnings {
+        ...CartApiWarning
+      }
+    }
+  }
+  ${cartFragment}
+  ${USER_ERROR_FRAGMENT}
+  ${CART_WARNING_FRAGMENT}
+`;
diff --git a/packages/hydrogen/src/index.ts b/packages/hydrogen/src/index.ts
index 2b14a01f54..5dadfadecb 100644
--- a/packages/hydrogen/src/index.ts
+++ b/packages/hydrogen/src/index.ts
@@ -58,6 +58,7 @@ export {cartBuyerIdentityUpdateDefault} from './cart/queries/cartBuyerIdentityUp
 export {cartCreateDefault} from './cart/queries/cartCreateDefault';
 export {cartDiscountCodesUpdateDefault} from './cart/queries/cartDiscountCodesUpdateDefault';
 export {cartGetDefault} from './cart/queries/cartGetDefault';
+export {cartGiftCardCodesAddDefault} from './cart/queries/cartGiftCardCodesAddDefault';
 export {cartGiftCardCodesRemoveDefault} from './cart/queries/cartGiftCardCodesRemoveDefault';
 export {cartGiftCardCodesUpdateDefault} from './cart/queries/cartGiftCardCodeUpdateDefault';
 export {cartLinesAddDefault} from './cart/queries/cartLinesAddDefault';
diff --git a/templates/skeleton/app/components/CartSummary.tsx b/templates/skeleton/app/components/CartSummary.tsx
index 541f2c819d..408fb9895c 100644
--- a/templates/skeleton/app/components/CartSummary.tsx
+++ b/templates/skeleton/app/components/CartSummary.tsx
@@ -149,8 +149,7 @@ function CartGiftCard({
       )}
 
       {/* Show an input to apply a gift card */}
-      <UpdateGiftCardForm
-        giftCardCodes={appliedGiftCardCodes.current}
+      <AddGiftCardForm
         saveAppliedCode={saveAppliedCode}
         fetcherKey="gift-card-add"
       >
@@ -166,18 +165,16 @@ function CartGiftCard({
             Apply
           </button>
         </div>
-      </UpdateGiftCardForm>
+      </AddGiftCardForm>
     </div>
   );
 }
 
-function UpdateGiftCardForm({
-  giftCardCodes,
+function AddGiftCardForm({
   saveAppliedCode,
   fetcherKey,
   children,
 }: {
-  giftCardCodes?: string[];
   saveAppliedCode?: (code: string) => void;
   fetcherKey?: string;
   children: React.ReactNode;
@@ -186,10 +183,7 @@ function UpdateGiftCardForm({
     <CartForm
       fetcherKey={fetcherKey}
       route="/cart"
-      action={CartForm.ACTIONS.GiftCardCodesUpdate}
-      inputs={{
-        giftCardCodes: giftCardCodes || [],
-      }}
+      action={CartForm.ACTIONS.GiftCardCodesAdd}
     >
       {(fetcher: FetcherWithComponents<any>) => {
         const code = fetcher.formData?.get('giftCardCode');
diff --git a/templates/skeleton/app/routes/cart.tsx b/templates/skeleton/app/routes/cart.tsx
index f82d683fdb..bb143b40d9 100644
--- a/templates/skeleton/app/routes/cart.tsx
+++ b/templates/skeleton/app/routes/cart.tsx
@@ -52,18 +52,14 @@ export async function action({request, context}: Route.ActionArgs) {
       result = await cart.updateDiscountCodes(discountCodes);
       break;
     }
-    case CartForm.ACTIONS.GiftCardCodesUpdate: {
+    case CartForm.ACTIONS.GiftCardCodesAdd: {
       const formGiftCardCode = inputs.giftCardCode;
 
-      // User inputted gift card code
       const giftCardCodes = (
         formGiftCardCode ? [formGiftCardCode] : []
       ) as string[];
 
-      // Combine gift card codes already applied on cart
-      giftCardCodes.push(...inputs.giftCardCodes);
-
-      result = await cart.updateGiftCardCodes(giftCardCodes);
+      result = await cart.addGiftCardCodes(giftCardCodes);
       break;
     }
     case CartForm.ACTIONS.GiftCardCodesRemove: {