Shopify Functions & Checkout Extensibility in 2025 — What You Need to Know

By nuwudorder | September 22, 2025

The checkout is the most critical part of any e-commerce funnel. It’s where customers commit—or abandon. As Shopify has evolved, so too have its capabilities for customizing the checkout flow. In 2025, Shopify Functions and Checkout Extensibility aren’t just optional features for high-end or enterprise stores—they’re central to scaling conversion, security, and brand consistency.

This long-form guide covers what these tools are, what’s changed, concrete use cases, how to migrate, and what to watch out for.

What Are Shopify Functions & Checkout Extensibility?

  • Shopify Functions are custom backend logic: you can write functions to modify behavior (discounts, shipping, payment options, delivery methods, validation rules) in secure, upgrade-safe environments. These replace many use cases formerly handled by scripts, apps with script tags, or the legacy theme files.carthook-web
    Shopify
    ithelps Digital
  • Checkout Extensibility is Shopify’s framework for making safe, maintainable, and powerful customizations to checkout and order status / thank you pages, without relying on deprecated code. It includes:
    1. UI Extensions (placing custom UI blocks, fields, banners, upsells, etc.).Shopify
    2. Checkout Branding API / Editor (colors, layout, fonts, logos).ithelps Digital
    3. Web pixel extensions and app blocks for injecting scripts/tracking in a controlled way.Shopify Help Center
      Shopify
      ithelps Digital

What’s New or Critical in 2025

Here are the updates as of 2025 you absolutely need to know:

  1. Deprecation of checkout.liquid, script tags, additional scripts
    • The old system (checkout.liquid, script tags / raw JS injections) is being phased out. For Plus merchants, the deadline for migrating Thank You and Order Status pages is August 28, 2025. After that, script tags / additional scripts / customizations via checkout.liquid for these pages will no longer work.Digital Position
      Shopify Help Center 
      meetdomaine.com
    • For other pages (Information, Shipping, Payment), earlier deadlines already passed.carthook-web
      Digital Position
  2. New capability & performance improvements
    • Functions can now modify many checkout-adjacent behaviors, including custom discount logic, altering payment options or shipping, enforcing validation (e.g. line-limit, quantity constraints) securely.ithelps Digital
    • UI Extensions and App Blocks allow visually inserting content (trust badges, upsells, custom fields) without impacting checkout performance as badly as previous script injections.ithelps Digital
  3. Upgrade safety and maintenance
    • Shopify’s system ensures that customizations built via these newer extension APIs and blocks are upgrade-compatible — meaning fewer breakages when Shopify itself releases new features.ithelps Digital
      Rootsyntax Technologies
    • Security improvements: sandboxing, limiting raw code execution, reducing exposure to cross-site vulnerabilities. 

      Flatline Agency

  4. Migration required now for many merchants
    • If you're using checkout.liquid or leveraging script tags / raw additional scripts, you must migrate to Checkout Extensibility. Missing August 28, 2025 means automatic updates that may break customizations.Shopify Help Center
      carthook-web
    • Shopify provides a “Upgrade Guide” and a “Customizations Report” in admin to help see what customizations you have and which are incompatible.Shopify Help Center

Use Cases & Examples

Here are concrete examples of what you can do (or should migrate to) with Functions + Checkout Extensibility:

Use Case Before (legacy) New / With Extensibility
Custom discount type (e.g. volume + bundle) App + script or hacky code in checkout.liquid Shopify Function for discounts + UI block to let merchant configure
Custom shipping options or pickup logic Hard-coded or app with scripts, potential breakage on theme changes Function to compute/determine shipping rules + UI extension to show shipping choice / cost
Adding custom fields at checkout (e.g. PO number) Script injection or theme edit + risk of breaking when Shopify updates Checkout UI extension that adds the field in supported extension point, safe and upgradeable
Branding / visual customization of checkout UI checkout.liquid tweaks, adding CSS/scripts manually Branding API, theme compatible UI customizations, app blocks

Trade-Offs: What You Might Lose / What to Watch Out For

  • Limitations on raw HTML/JS injections: Some legacy custom code will not translate directly; if you’ve relied on raw code hacks, you’ll need to rebuild those as extensions or UI blocks.
  • Plan requirements: Some extensibility features are available only on Shopify Plus (or enterprise-level tiers).ithelps Digital
  • Learning curve / dev effort: Moving from old systems to Functions & UI Extensions will require developer time, especially for complex or legacy customizations.
  • Testing & compatibility: Ensuring third-party apps or custom code are compatible with the new checkout system; app vendors may need updates.
  • Feature gaps: There may still be very niche customizations that the new APIs don’t support exactly as you had, so planning is essential.

Best Practices & Migration Checklist

To make the transition smoother, here are best practices and a checklist:

  1. Audit existing customizations
    • Identify all usages of checkout.liquid.
    • Find all “additional scripts” / script tags used in checkout / order status / thank you pages.
    • Use Shopify’s Customizations Report (in admin). 

      carthook-web

  2. Map functionalities to new APIs
    • For each old customization, decide: can this be done via Function, UI extension, App Block, or Branding API?
    • Prioritize customer-facing / conversion-sensitive customizations first (e.g. trust badges, payment options, fields), then secondary ones (tracking scripts, visuals).
  3. Develop & test in staging/dev preview
    • Test performance impact.
    • Verify tracking / analytics works via web pixels and app-compatible means.
    • Check that required behaviors (e.g. validation, shipping/pickup logic) work under edge cases.
  4. Plan the rollout & communicate
    • Merchant notice: some customization may look slightly different.
    • Provide fallback or ensure UI consistency.
    • Monitor customer behavior (drop offs, conversion, errors) post-migration.
  5. Maintain & iterate
    • Monitor Shopify’s updates to extensibility APIs; new extension points get added over time.
    • Ensure third-party apps you use are compatible.
    • Capture feedback from UX / support to refine checkout flow continuously.

Conclusion & Recommendation

In 2025, Shopify Functions and Checkout Extensibility are no longer optional enhancements—they are necessary. If your store relies on checkout.liquid, raw scripts, or any deprecated customizations, you’re exposed to risks: broken functionality, poor performance, security issues, and loss of customizations after Shopify’s auto-upgrade deadlines.

For both SMBs and enterprises, the recommendation is:

  • If you’re still on legacy checkout customization: audit immediately and begin migration to functions/extensions.
  • If you’re already using the new framework: ensure you're using all relevant extension points (UI blocks, branding) and keep an eye on new Shopify releases.
  • If you have very custom or complex requirements: plan for dev time, test carefully, and ensure your third-party ecosystem is aligned.

A well-migrated checkout is not just secure and maintainable—it converts better, loads faster, and gives your brand more control.

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