The Shopify ecosystem is shifting rapidly toward extensions as the primary way to customize and enhance merchant stores. Since checkout.liquid being has been deprecated, extensions are now the only way to customize the checkout, and understanding how to build with extensions isn't just helpful anymore — it's essential for any developer working in the Shopify ecosystem.

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The current state of Shopify extensions

Extensions represent Shopify's vision for how developers should build customizations into their platform. Rather than allowing unrestricted modifications that can impact performance and reliability, extensions provide controlled ways to add functionality while maintaining platform stability.

With extensions, Shopify has more control over how customizations can be implemented. By keeping the experience as consistent as possible while still allowing for some amount of flexibility, they’re able to help merchants by optimizing their core platform to be a familiar, high-conversion experience for shoppers.

Shopify Functions are extension targets that inject code to handle backend business logic in Shopify, like discount calculations, shipping rules, and payment methods. (We have an in-depth post about Shopify Functions here)

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Building Shopify extensions

Setting up your development environment

Before you start building extensions, you'll need to set up your local environment with the Shopify CLI. This tool is required for generating, developing, and deploying extensions. Unlike older development approaches like checkout.liquid and Shopify Scripts, you can't build extensions without using the CLI.

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The Shopify CLI lets you manage extension development in several ways:

- Generates extension scaffolding with the correct structure and files 

- Provides a local development server for testing

- Manages deployment to Shopify's infrastructure

- Handles version management and updates

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(Pro tip — you can build Shopify extensions with Gadget to handle the scaffolding and simplify maintenance)

Your extension development environment also needs Node.js and Git. While Shopify provides JavaScript templates for most extension types, you can also use languages that compile to WebAssembly for Shopify Functions.

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Understanding extension architecture

Every extension type has its own architecture and constraints, but they share some common patterns.

Extensions can't run arbitrary code – they're limited to specific APIs and capabilities defined by Shopify. This is intentional, ensuring extensions can't negatively impact performance or security. For example, checkout UI extensions must use Shopify's component library rather than custom components, and Functions must complete execution within 5ms.

Extensions run on Shopify's infrastructure, not your own servers. This means you don't have to worry about scaling or maintaining infrastructure, but it also means you need to work within Shopify's performance requirements. Your extension code gets compiled and deployed to Shopify's infrastructure, where it runs in a controlled environment with specific limitations on memory usage, execution time, and API access.

Each extension type has specific mount points where it can integrate with Shopify's platform. For checkout UI extensions, these are predefined locations in the checkout flow where you can add custom interfaces. For admin UI extensions, they're specific points in the admin interface where you can add functionality. Theme extensions use app blocks that merchants can place within their theme sections.

There are several types of extensions you'll encounter when building for Shopify, and the architecture of each has been specially designed for a specific use case.

Admin UI extensions focus on merchant workflows and integrate deeply with Shopify's App Bridge. They can access shop data and modify admin behavior but must maintain the platform's look and feel, letting you add custom interfaces and actions directly into the Shopify admin. These are perfect for adding merchant-facing controls and settings that integrate naturally with Shopify's existing admin interface. These types of extensions are typically only used by merchants — not shoppers.

Checkout UI extensions are heavily optimized to drive conversions and allow you to modify the checkout experience without compromising performance or security. Unlike checkout.liquid customizations, these extensions run on Shopify's infrastructure with strict performance requirements. They use a specialized component library designed for fast rendering and minimal impact on checkout completion rates. These extensions can access checkout data but have limited ability to modify core checkout behavior.

Theme extensions provide a way to add functionality to storefronts without requiring merchants to modify theme code. They need to work across different theme structures while maintaining compatibility with Shopify's theme editor. They use a block-based architecture that allows merchants to add, remove, and configure functionality without ever touching code. This allows merchants to customize their stores in a low-risk setting where changes are much less likely to break.

‍Customer account extensions are the latest addition to Shopify’s extensibility options, and they allow developers to create custom experiences for shoppers using the newest iteration of Customer Accounts. They’re available on the Order Status, Order Index, and Profile pages, with the option to create full-page extensions. Most merchants and developers have chosen to use them to introduce customizations for recurring customers, such as loyalty programs, wish lists, and subscriptions.

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The shift toward extensions represents more than just a technical change – it's a fundamental transformation in how developers build for Shopify, and understanding how to build and combine different extension types effectively is crucial for any Shopify developer.

By focusing on extensions, Shopify is creating a more stable, performant platform where customizations can scale reliably. While this means adjusting how we approach development, it also means spending less time on infrastructure and maintenance, and more time building features that matter to merchants.

If you're starting to build with Shopify extensions and want to connect with other developers in the Shopify ecosystem, join our Discord community! The future of Shopify development is in extensions, and the tools to build them effectively are already here.