Shopify Storefront API — Headless Commerce

Procedural guide for building headless storefronts against the Shopify Storefront API (GraphQL). Covers cart, checkout, and customer account operations. For product data, collections, and pages use the Frontic skill instead.

Architecture

Frontic Client    →  Product data, collections, pages, search (read)
Storefront API    →  Cart, checkout, customer accounts (write/session)

The Storefront API is a GraphQL API. All operations are mutations or queries sent to a single endpoint.

API Endpoint & Authentication

POST https://{store}.myshopify.com/api/{version}/graphql.json

Headers

API versions follow YYYY-MM format (e.g., 2025-01). New versions release quarterly.

Core Workflows

Add to Cart → Checkout

1. cartCreate                         → Create cart (optionally with lines)
2. cartLinesAdd / cartLinesUpdate     → Manage items
3. cartDiscountCodesUpdate            → Apply discount codes
4. cartBuyerIdentityUpdate            → Associate logged-in customer
5. Query cart { checkoutUrl }         → Get hosted checkout URL
6. Redirect to checkoutUrl            → Shopify handles payment & order

Customer Login → Cart Association

1. customerAccessTokenCreate          → Login, get accessToken
2. cartBuyerIdentityUpdate            → Attach customerAccessToken to cart
3. Query cart { checkoutUrl }         → Checkout pre-fills address & payment

Checkout Model

Shopify uses hosted checkout only. There is no custom checkout API — the old Checkout mutations are deprecated (removed April 2025). The flow:

1. Build cart with mutations
2. Optionally associate customer via cartBuyerIdentityUpdate
3. Query cart.checkoutUrl
4. Redirect buyer to that URL — Shopify handles payment, shipping, order creation
5. Shopify redirects back to your configurable thank-you URL

Reference Files

• Cart operations: All cart mutations (create, lines, discounts, gift cards, notes, attributes, delivery), cart query, response shapes
• Checkout: Hosted checkout flow, checkoutUrl usage, limitations
• Customer account: Registration, login/logout (access tokens), profile updates, addresses, password recovery/reset, order history query

Common Patterns

GraphQL Client Wrapper

async function storefrontQuery<T>(
  query: string,
  variables?: Record<string, unknown>
): Promise<T> {
  const response = await fetch(
    `https://${SHOP_DOMAIN}/api/${API_VERSION}/graphql.json`,
    {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'X-Shopify-Storefront-Access-Token': STOREFRONT_TOKEN,
      },
      body: JSON.stringify({ query, variables }),
    }
  )

  const { data, errors } = await response.json()
  if (errors?.length) throw new StorefrontError(errors)
  return data
}

Error Handling

All mutations return userErrors (or customerUserErrors):

mutation {
  cartLinesAdd(cartId: $cartId, lines: $lines) {
    cart { id }
    userErrors { field message code }
  }
}

Always check userErrors — the mutation may "succeed" (HTTP 200) but contain business logic errors.

MoneyV2 Type

All prices use MoneyV2:

{
  amount    # String — decimal like "49.95"
  currencyCode  # CurrencyCode enum — "USD", "EUR", etc.
}

Cart ID Persistence

Cart IDs contain a secret key component (e.g., gid://shopify/Cart/Z2NwLX...?key=abc123). Store the full ID in a cookie or server-side session.

Rate Limiting

The Storefront API uses calculated query costs. Default: 50 points/second, 1000-point bucket. Keep queries lean by selecting only needed fields.