Cart & Checkout Validation Function

The Cart and Checkout Validation Function is the Shopify Function type whose job is to say no. Where most Functions change something (apply a discount, reorder payment methods, transform line items), a validation function inspects the cart or checkout and either passes silently or returns one or more errors. Each error carries a message, and can target a specific field so the storefront shows it in context; if the list is empty, checkout proceeds. While errors remain, the buyer cannot complete the purchase. This makes it the sanctioned way for an app to enforce hard rules in Shopify checkout, such as minimum or maximum quantities, B2B order minimums, address or region restrictions, or product combinations that must not be sold together. Like other Functions, it is compiled to WebAssembly and run by Shopify under strict resource limits, and for most uses it cannot call external systems during execution. Any data a rule needs beyond the cart, such as per-SKU thresholds, is staged ahead of time, typically in metafields, and kept current by a separate process. Its limits follow from where it runs. Every error is shown to a buyer who is trying to pay, so an overly strict or mis-staged rule costs conversion directly. Decisions can only use data available inside checkout, which makes rules based on true margin, after current ERP cost, freight, fees, and FX, difficult to keep accurate. And it has to be built and maintained by a developer or an app. Agentis does not use the Cart and Checkout Validation Function and cannot stop a checkout. It works after the order is placed: within 60 seconds it computes the order's true net margin from live store data, NetSuite COGS, freight, fees, and FX, and flags the order or automatically holds it before fulfillment under merchant-approved rules, logging each decision to the Evidence Ledger. A validation function is the right tool for rules the cart can answer on its own; a post-order margin check covers the orders that pass checkout but would ship below floor.