Commerce Architecture
WASM Checkout (WebAssembly at Checkout)
By Herzel MishelFounder, AgentisLast reviewed
Definition
The use of WebAssembly modules to run merchant logic at checkout, replacing legacy Ruby-based Shopify Scripts. Shopify Functions is the canonical implementation; sub-millisecond execution and sandboxed runtime.
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 is sunsetting 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.
Related Terms
Profit Governance
Checkout Governance
The application of margin governance specifically to the checkout layer — defining and enforcing rules about what discount combinations, freight scenarios, and promo stacks are allowed to confirm.
Profit Governance
Policy Engine
The configurable rules layer of a profit firewall — where finance teams declaratively define margin floors, discount limits, MAP rules, and other enforcement criteria.
Profit Governance
Profit Firewall
A real-time decision layer at checkout that blocks, modifies, or redirects any order failing margin policy — analogous to how a network firewall blocks traffic that violates security rules.
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