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To build a Shopify Checkout UI extension, scaffold an app with Shopify CLI, choose a supported extension target, configure that target in shopify.extension.toml, implement the feature with the APIs and UI components available there, then preview, test, and deploy it. First check where the feature belongs and whether the merchant’s plan supports that checkout location: extensions for the information, shipping, and payment steps require Shopify Plus, while Thank you and Order status page extensions are available on plans other than Shopify Starter.
1. Choose where the feature should run
Start with the job the extension must do. A delivery-instructions field or address autocomplete belongs near the relevant checkout step; a promotion may need a suitable checkout placement; a post-purchase survey may belong on the Thank you page. Shopify’s extension targets determine both where code runs and which APIs it can use, so select the target before designing the implementation.
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Shopify groups targets into three broad types:
| Target type | Placement and behavior | Use it when |
|---|---|---|
| Block | A UI block that a merchant can place in the editor at a supported location. | The feature needs visible UI and a configurable placement. |
| Static | UI rendered at a fixed supported location. | The feature needs visible UI, but not an editor-positioned block. |
| Runnable | Provides data or behavior without rendering UI. | The feature needs to perform supported work without displaying a component. |
These categories do not make every target interchangeable. Check the target list and target-specific API reference for the checkout stage and behavior you need. If the feature requires a custom field, for example, confirm that the chosen target exposes the data and interaction APIs needed to collect and handle its value.
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Checkout-step availability and post-purchase availability differ by plan. Check the restriction before committing to a target:
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| Extension location | Plan requirement |
|---|---|
| Information, shipping, or payment step | Shopify Plus required. |
| Thank you or Order status page | Available on plans except Shopify Starter. |
Shopify’s getting-started guidance calls for app development permissions and a development store; for checkout UI work and testing, use a Plus-enabled development store. If you do not have access to a qualifying store, choose a location and testing plan that match the app’s intended merchant setup before building around a restricted checkout step.
3. Scaffold the app and extension
Shopify CLI initializes the app and generates the extension files. In a terminal, run the app setup flow and follow the prompts:
shopify app initChange into the app directory, then generate the extension. The quickstart flow prompts you to choose Checkout UI and a language:
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteshopify app generate extension --name my-extensionThe Checkout UI reference also documents the extension-specific template command:
shopify app generate extension --template checkout_uiCLI prompts and generated files can vary with the installed CLI version, so follow the choices presented by your local command. Once generation is complete, start the development server from the app directory:
shopify app dev4. Configure the API version and target
The extension’s shopify.extension.toml file defines its API version, metadata, target and module path, along with any required capabilities. Each targeting entry pairs one target identifier with a JavaScript or TypeScript module. Multiple targets can be declared, but each target needs its own module.
Shopify’s current reference example uses API version 2026-07. Treat that as the reference’s example, not a version every app must use. Before publishing, confirm Shopify’s latest supported stable version and the target’s current configuration requirements. Shopify states that stable API versions are supported for at least 12 months and that CLI prevents deployment to versions more than 12 months old.
api_version = "2026-07[0m"
[[extensions]]
type = "ui_extension"
name = "My checkout UI extension"
[[extensions.targeting]]
target = "purchase.checkout.block.render"
module = "./src/Checkout.tsx"Adapt the target identifier and module path to the feature you are actually building. The example’s block-render target is not a universal default.
5. Implement with the APIs and UI supported by the target
Shopify exposes target APIs through the shopify global. Depending on the selected target, these APIs can expose checkout data or functions related to buyer identity, cart contents, costs, delivery, discounts or address data. Use only the APIs documented for that target; do not assume that data available at one checkout location is available at another.
For rendered UI, use Shopify’s provided Polaris web components and follow the platform’s accessibility and design guidance. The current development path is important for teams maintaining older extensions: Shopify’s documentation says, “Version 2025-07 is the last API version to support React-based UI components.” Later API versions use Polaris web components. Shopify gave October 1, 2026 as the date after which an upgrade would be needed to avoid being blocked from updating the extension; that date has passed. The stated consequence concerns updates, not an assertion that every existing extension stopped running automatically. Extensions still using the legacy React-based approach should consult Shopify’s upgrade guidance before making further changes.
Enable capabilities only when the feature needs them. The extension configuration reference lists options including Storefront API access, external network access, buyer consent collection and blocking buyer progress. They are not automatically granted by choosing a target: declare the needed capability and check its specific behavior and any merchant permission requirement.
6. Design around checkout’s sandbox
A Checkout UI extension runs in an isolated sandbox. It cannot inspect or manipulate checkout page HTML or assets, and it cannot access sensitive payment information. Its interface and behavior are limited to Shopify’s exposed components, APIs and supported global web APIs.
That boundary changes the implementation approach: build through the extension’s supported APIs and UI rather than treating checkout like a page where arbitrary scripts, styles or DOM access are available. If a feature depends on data or page behavior that Shopify does not expose to its target, redesign it around supported extension mechanisms instead of assuming a capability that is not documented.
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Run shopify app dev and use the preview it provides on the development store. Shopify says the development server reloads extension changes automatically. If the app includes a backend, the command can expose it locally through a Cloudflare tunnel while you develop; this local setup does not replace deploying the backend for production.
Test the extension at its actual target and include the cases most likely to change its behavior:
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- Try the feature with optional merchant settings unset, as well as with their intended values.
- Exercise validation failures, unavailable data and other error states relevant to the feature.
- Check the checkout configurations the app supports rather than relying on a single preview path.
For automated tests, Shopify documents @shopify/ui-extensions-tester beginning with API version 2026-04. Confirm the tester’s current setup instructions for your extension and API version before adding it to the project.
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8. Deploy and maintain the extension
When the extension is ready, deploy the app with Shopify CLI:
shopify app deployShopify’s deployment process builds and uploads the extension. The compiled UI extension bundle must not exceed 64 KB. If your app has a backend, deploy that service separately to your chosen hosting environment; Shopify hosts the extension code, not your app’s backend.
For future releases, review Shopify’s supported API versions, target-specific requirements and upgrade guidance before changing the extension configuration. A version that was valid when the extension was scaffolded may not be the right version for a later update.
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