Write a Shopify cart/checkout validation Function and handle its input query constraints

In-depth guide

Steps

1. Create a cart transform or order validation Function extension with the Shopify CLI and define the target API type in your extension TOML.
2. Write your input query in the `input.graphql` file requesting only the fields your function needs; the maximum input query size (excluding comments) is 3,000 bytes.
3. Keep calculated query cost at or below 30 and ensure no list-type argument or variable exceeds 100 elements.
4. Return a `FunctionRunResult` with errors array; each error object includes a `localizedMessage` and optionally targets specific cart line items by ID.
5. Store any merchant-configurable thresholds as metafields on the Function owner and read them via the input query; metafield values over 10,000 bytes are not returned.
6. Test locally with `shopify app function run` feeding a sample JSON input; validate the instruction count stays within the limit before deploying.