This is not a rewrite. It is a replacement.
Shopify Scripts are ending on June 30, 2026. What replaces them is not new code — it is a new execution model.
Scripts allowed you to inject logic into checkout. Shopify Functions require you to declare behavior through apps, using defined extension points.
This article exists to answer one question only:
What exactly replaces my Scripts — and how do I choose correctly?
No hype. No roadmap speculation. Just the migration map.
What Shopify Functions are (operator definition)
Shopify Functions are server-side checkout behaviors delivered through apps.
Key properties that matter operationally:
- They run on Shopify's infrastructure (not your theme, not your server)
- They execute deterministically and quickly
- They are upgrade-safe (Shopify updates do not overwrite them)
- They replace behavior, not UI
What they are not:
- theme code
- frontend scripts
- ad-hoc logic you paste and forget
Think of Functions as policy enforcement hooks built into checkout.
The replacement matrix (this is the core)
Most Script migrations fall cleanly into one of four buckets.
| Script Type | What It Usually Did | Replacement Mechanism |
|---|---|---|
| Line-item Script | Tiered discounts, bundles, exclusions | Discount Function |
| Shipping Script | Hide or modify rates, conditional shipping | Delivery Customization Function |
| Payment Script | Hide or reorder payment methods | Payment Customization Function |
| Guardrail Script | Min order value, blocking logic | Checkout Validation Function |
If you cannot map a Script to one of these, pause. You may be relying on undocumented behavior that cannot be replicated.
Replacement paths: App vs Custom Function
Every Function is delivered via an app. The real choice is who owns the logic.
Use a Functions-based app if:
- the rule is common (discount caps, shipping conditions, payment rules)
- the logic changes often
- Ops or Finance needs visibility and control
- you want vendor maintenance and updates
This is the default for most Plus teams.
Build a custom Function if:
- the logic is unique to your business
- it depends on proprietary data or workflows
- you have strong testing discipline
- you accept long-term ownership
Custom Functions are powerful — but expensive to maintain.
Bias toward apps unless you have a clear reason not to.
What does not translate 1:1 from Scripts
This is where migrations go wrong.
Order of execution may differ
Scripts sometimes relied on implicit ordering. Functions are explicit — you must re-validate outcomes.
Some Script tricks were unofficial
If a Script depended on side effects or quirks, expect it to break.
Testing is mandatory
You cannot assume parity. You must prove behavior, especially during promos.
This is not Shopify being difficult. It is Shopify enforcing determinism.
A safe migration sequence (how disciplined teams do it)
- Inventory
- Pull the Scripts report
- Write a plain-language policy summary for each Script
- Classify
- Map each Script to a Function category
- Decide App vs Custom
- Replace
- Install or configure apps, or build Functions
- Document intended behavior
- Parallel test
- Where possible, test new logic alongside old
- Validate promo, shipping, and payment edge cases
- Cut over deliberately
- Choose a low-risk window
- Disable Scripts only after proof
- Observe
- Monitor checkout outcomes
- Log anomalies
- Adjust policies, not code
Rushing any step usually costs more than waiting a week.
Where CartMoat fits (one specific category)
Some Shopify Scripts existed solely to prevent unprofitable checkouts: excessive discount stacking, free shipping erasing margin, payment fees tipping orders negative, and small carts that never should have shipped.
CartMoat replaces that guardrail category using Checkout Validation Functions, contribution-margin policy enforcement, clear shopper messaging, and finance-grade audit trails.
If you had a Script protecting margin, this is the moment to replace it with an explicit, reviewable policy.
What good looks like after migration
- checkout behavior is explainable
- policies live in apps, not forgotten code
- Finance understands the rules
- Ops can change constraints without redeploying logic
- Shopify updates no longer threaten your checkout
That is the real upgrade.
Final note
This migration is not about compliance. It is about control.
Scripts were powerful, but fragile. Functions are constrained, but reliable.
The teams that treat this as a policy migration — not a dev chore — will finish calmly and not think about it again.
That is the goal.