September 30, 2026 · Ahmed · More by Ahmed
Shopify's Fulfillment Constraints Fail Open, Not Closed
Shopify's Fulfillment Constraints Function API blocks split shipments at checkout — but its own reference code fails open, not closed.
Two Operations, One Store-Wide Limit
The Fulfillment Constraints Function API gives developers exactly two operations to work with: deliverableLinesMustFulfillFromAddOperation, which forces specific cart line items to ship from one named location, and deliverableLinesMustFulfillFromSameLocationAddOperation, which forces a set of items to ship from whichever single location happens to carry all of them, without your function naming which one. A store can have at most 25 fulfillment constraint functions active at once. Both operations run at the Checkout function target, meaning they're evaluated before a buyer sees shipping options — not after an order is already placed, which is what distinguishes this API from Order Routing Location Rules.
When the Constraint Can't Be Satisfied
If the constrained items aren't available together at any qualifying location, checkout doesn't fall back to a partial shipment or a workaround — it shows no shipping options at all, with a message identifying which items are out of stock, and blocks the buyer from completing the purchase. That's a hard stop, not a soft warning, so a badly scoped constraint can silently take a whole product combination off the shelf at checkout without anyone noticing until conversion drops.
Where This Actually Runs
Registering a rule needs the write_fulfillment_constraint_rules access scope and the fulfillmentConstraintRuleCreate mutation, and each rule can be scoped to specific delivery method types — SHIPPING only, for example, to leave local pickup unconstrained. Support is uneven across surfaces: it's fully supported on Cart, Checkout, B2B, the Create Order API, Draft Order on both Admin and Checkout, Storefront Accelerated Checkout, and Subscription; partially supported on Order Edit (Admin), where it only applies to new items, and on POS, only when shipping to a customer; and unsupported on Order Edit (Checkout) and on Pre-order and Try Before You Buy.
The Fail-Open Default Hiding in the Reference Code
The name "constraint" implies something that blocks by default, but Shopify's own reference implementation does the opposite when it can't resolve what it needs. In both the official Rust and JavaScript samples, if the named location isn't found in the function's input data, the function returns zero operations — no_fulfillment_constraints_result() in Rust, a NO_CHANGES constant in JavaScript — rather than erroring or blocking checkout. An unresolvable location reference doesn't stop a sale; it just silently skips the constraint, which means a typo'd location name or a deleted location won't surface as a checkout failure. It'll surface later, as an order that shipped from the wrong place.