Build a cart with the Shopify Storefront API cartCreate mutation including country and language context for a localized headless checkout experience

In-depth guide

Steps

1. Call `cartCreate` with a `CartInput` that includes `lines` (array of `CartLineInput` with `merchandiseId` and `quantity`), `buyerIdentity` with `countryCode` set to the buyer's country (e.g., `DE`), and optionally `email`
2. Apply the `@inContext(country: DE, language: DE)` directive to the cartCreate mutation for language-localized field responses (titles, descriptions), but be aware that cart pricing resolution comes from `buyerIdentity.countryCode`, not the directive
3. In the response, read `cart.cost.totalAmount { amount currencyCode }` to confirm the cart is priced in the correct local currency, and check `cart.buyerIdentity.countryCode` to verify country was accepted
4. To apply discount codes, follow up with `cartDiscountCodesUpdate(cartId: $cartId, discountCodes: ["CODE"])` and inspect `cart.discountCodes { code applicable }` to confirm the code resolved
5. Retrieve `cart.checkoutUrl` and redirect the buyer to it to complete purchase in Shopify-hosted checkout; the checkout will inherit the cart's buyer identity country for tax and duty calculation
6. Handle the `cart.userErrors` array on every mutation — errors include invalid variant IDs, out-of-stock items, and mismatched currencies