Use Shopify Functions to build a custom discount on subscriptions during checkout

In-depth guide

Steps

1. Generate a Function extension: shopify app generate extension --type product_discounts; choose Rust or JavaScript/TypeScript as the runtime
2. Define the function's input query in input.graphql to fetch cart.lines with sellingPlanAllocation.sellingPlan.id to detect subscription line items
3. In the function logic, check if a line has a sellingPlanAllocation and apply a percentage discount using the FunctionResult type with discounts array containing targets (all or specific line GIDs) and value (percentage or fixed)
4. Configure the function in shopify.extension.toml under [extensions.ui.handle] and deploy with shopify app deploy
5. Create a discount in the Shopify Admin that links to your deployed Function using the discountAutomaticAppCreate Admin GraphQL mutation with functionId
6. Test locally using shopify function run which allows you to provide a JSON input and inspect the FunctionResult output before deploying