WASM Checkout (WebAssembly at Checkout)

WASM checkout refers to the use of WebAssembly to execute merchant-defined logic during the cart-to-confirmation flow on commerce platforms, most concretely Shopify Functions, which uses WebAssembly to run discount, validation, and cart-transform logic written in Rust, AssemblyScript, or any language that compiles to WASM. The pattern emerged because Shopify sunset its Ruby-based Scripts platform on June 30, 2026, and Functions is the designated replacement. WebAssembly was chosen over Ruby (the previous Scripts runtime) and JavaScript (the obvious alternative) for three reasons: (1) execution speed: WASM modules typically execute in sub-millisecond timeframes, well within the latency budget for real-time checkout; (2) sandboxing: WASM modules can't make network calls or access the file system, making them safe to run inside Shopify's checkout flow without security review; (3) language flexibility: merchants can use Rust, AssemblyScript, Go, or other compile-to-WASM languages, which is essential for an ecosystem where one runtime needs to serve thousands of merchant-specific implementations. The trade-offs vs Scripts: Functions can do more (cart transforms, payment-method filtering, delivery-method customization), is faster, and is officially supported long-term. The cost is engineering effort: Scripts could be written by anyone comfortable with Ruby; Functions requires Rust or AssemblyScript expertise plus the WASM toolchain. For most mid-market merchants this is a meaningful jump. Margin-governance implications: Functions does NOT have built-in access to live COGS, freight zone costs, or FX rates; those data flows must be implemented separately. So a Functions-based margin enforcement requires both the WASM module AND the data pipeline that feeds it cost data. Profit-firewall solutions like Agentis address this by providing the data layer pre-built: the merchant configures policies in a UI, Agentis enforces them via the Functions API and a managed cost-data pipeline. WASM checkout is the future of programmatic commerce logic; for stores that don't want to operate a WASM toolchain, an Agentis-like layer is the higher-leverage path.