Shopify Checkout Extensibility: The 2026 Guide to UI Extensions, Functions and What Replaced Scripts

What is Shopify checkout extensibility?

Checkout extensibility is Shopify's framework for customizing checkout without editing its code. It has four main parts: checkout UI extensions, Shopify Functions, web pixels and the Checkout Branding API. Together they replaced checkout.liquid, additional scripts and Shopify Scripts.

The old approach let Plus merchants edit checkout templates directly. That was flexible, but it broke during upgrades, slowed checkout down, and kept stores from getting new features like Shop Pay improvements. Extensions run in a controlled environment, so Shopify can update checkout without breaking your customizations.

• Checkout UI extensions add custom interface elements, such as banners, fields, upsells and delivery notes, at defined spots in checkout.
• Shopify Functions run custom backend logic for discounts, shipping, payments, validation and bundles.
• Web pixels handle tracking and analytics events in a sandbox, replacing scripts pasted into checkout.
• Checkout Branding API controls detailed styling beyond the checkout editor's settings.

What happened to checkout.liquid?

checkout.liquid is no longer supported. Plus stores had to upgrade the information, shipping and payment steps by August 13, 2024, and Shopify started auto-upgrading the remaining stores in January 2025. For the Thank you and Order status pages, the deadline was August 28, 2025 for Plus and August 26, 2026 for all other plans.

Sources: Shopify's checkout upgrade guide, Plus upgrade guide, Thank you and Order status upgrade page and the Scripts deprecation changelog.

Auto-upgrades bring pages back to Shopify's default layout. Brand settings from the checkout editor still apply, but custom Liquid, JavaScript and CSS don't. Shopify moves some analytics scripts to custom pixels on a best-effort basis. If your conversion tracking dropped around one of these dates, check it first.

What happened to Shopify Scripts?

Shopify Scripts, the Ruby-based Plus feature for line item, shipping and payment logic, have been retired. Since April 15, 2026 they can't be edited or published, and on June 30, 2026 they stopped running. Shopify Functions replace them.

Typical Scripts and what replaces them:

• Line item scripts (tiered discounts, buy X get Y, B2B price tweaks) become Discount Functions or Cart Transform for bundles.
• Shipping scripts (hiding, renaming or reordering rates) become Delivery Customization Functions.
• Payment scripts (hiding payment methods for certain carts or customers) become Payment Customization Functions.

Shopify's Scripts customizations report in the admin shows which of your Scripts map to Functions or to existing public apps. If you see unexpected prices or shipping options since July 2026, an unmigrated Script is a likely cause.

What are checkout UI extensions?

Checkout UI extensions are app components that render in specific places, called targets, in checkout, the Thank you page and the Order status page. Merchants place and configure them in the checkout and accounts editor, with no code editing.

Common uses include:

• Upsell and cross-sell offers before payment.
• Gift messages, delivery instructions or custom fields such as a VAT number or PO details.
• Trust badges, shipping or returns notes, and loyalty point balances.
• Post-purchase surveys and reorder prompts on the Thank you page.

Shopify's documentation says extensions on the information, shipping and payment steps are available only to Plus stores. Other plans can use app blocks on the Thank you and Order status pages, which became the only option once the August 26, 2026 upgrade deadline passed.

What are Shopify Functions?

Shopify Functions are small pieces of custom backend logic that Shopify runs inside its own infrastructure during cart and checkout. They're fast and they keep working when Shopify updates checkout.

The current Function APIs are:

• Discount for custom discount logic, including product, order and shipping discounts.
• Cart Transform for bundles and line item changes.
• Delivery Customization to hide, rename or reorder shipping options.
• Payment Customization to hide, rename or reorder payment methods.
• Cart and Checkout Validation to block checkout when rules aren't met, such as B2B minimums or restricted regions.
• Fulfillment Constraints and Order Routing Location Rule to control which locations fulfill orders.

Plan rules matter here. Any plan can use public apps from the Shopify App Store that include Functions, unless an individual API's documentation says otherwise. Custom apps containing Function APIs work only on Plus.

What's Plus-only in checkout?

Plus gets full checkout customization. Every other plan gets branding settings, Thank you and Order status blocks, web pixels and public Function apps.

Sources: Checkout UI extensions, checkout styling and Function API availability on shopify.dev.

How do you audit and migrate your checkout?

1. Inventory everything. List every custom checkout element, tracking tag, Script and checkout-related app. Use Shopify's upgrade guide and Scripts customizations report.
2. Use built-in settings first. Many old customizations now exist as native settings or checkout editor options.
3. Look for public apps. App Store apps with UI extensions or Functions often replace custom code at a lower cost.
4. Build custom where it pays off. On Plus, write custom UI extensions and Functions for logic that's specific to your business.
5. Move tracking to pixels. Recreate purchase events for GA4, Meta and other platforms with app pixels or custom pixels, then check them against your order data.
6. Test the full flow. Run real orders across discounts, shipping methods, payment methods, markets and B2B buyers before you publish.

Our take

The move to extensibility was painful, but it was the right call. Checkout is faster and more stable, and it no longer breaks when Shopify ships updates. If you haven't reviewed your tracking and discount logic since the 2026 deadlines, do it now. Silent tracking gaps and missing discounts are the most common issues we see after auto-upgrades.

How we can help

We build checkout UI extensions and Shopify Functions, migrate legacy Scripts and fix post-upgrade tracking. See our Shopify development services or, for Plus checkout projects, our Shopify Plus services.