Configure Shopify cartLinesAdd with custom attributes and validate those attributes survive through to the order using a checkout validation Function

Steps

1. Call the cartLinesAdd Storefront API mutation, including an attributes array on each CartLineInput with the custom key-value pairs you want to preserve (for example gift message, engraving text, or configuration data)
2. After adding lines, query the cart to confirm attributes are present on each cartLine by selecting the attributes field in the query response
3. Build a cart-checkout-validation Shopify Function whose run.graphql input query fetches cartLines with their attributes field to inspect the values at checkout time
4. In the function run logic, validate the attribute values (for example, check that an engraving text does not exceed a character limit) and return errors for invalid lines
5. After order creation, query the order's lineItems via the Admin API and confirm that the line item properties (which map from cart line attributes) are present on the order record
6. In a downstream integration such as a fulfillment system, read the line item properties from order webhooks or the Admin API to act on the custom data