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To migrate a Shopify checkout UI extension, you must do more than rename React components. Choose a supported API version, inventory each target and capability, replace legacy components with Polaris web components, update the runtime and TypeScript setup, move API and metafield calls to the current model, and test every checkout path before publishing.
Shopify says API versions earlier than 2025-10 need to adopt Polaris web components, while 2025-10 and later use them by default. Version 2025-07 was the last version that supported React-based UI components. Because Shopify’s announced October 1, 2026 update restriction has already passed, verify the behavior of your current CLI and app workflow immediately rather than relying on an old deadline.
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What changes in this migration
A checkout UI extension runs inside Shopify’s hosted checkout, so its UI, data access, and allowed behavior are controlled by the extension API version and target. The migration therefore has several independent workstreams:
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- Runtime: Shopify recommends Preact for UI extensions on API versions
2025-10and later. - API access: calls that previously arrived through callback parameters move to the global
shopifyobject, with Preact hooks still available as convenience wrappers. - Data: newer versions change how cart metafields are read and written.
- Configuration: the extension’s
shopify.extension.toml, dependencies, TypeScript settings, and requested metafields may all need edits. - Placement: the target determines where the extension renders, which components are available, and which capabilities can be used.
Shopify’s migration guide is the authoritative starting point: Upgrade to the latest API version and Polaris web components. Component-specific guidance is also available in more than 60 migration pages, including pages for Button, Checkbox, TextField, Banner, and View, as announced on May 13, 2026: Polaris web components migration guides.
1. Inventory the extension before changing code
Start with a written inventory for every checkout extension in the app. Do not begin by globally replacing JSX tags; first record what each extension is allowed to do and where it appears.
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Record the API and target
Open each extension’s shopify.extension.toml and record its api_version, target, requested capabilities, and metafield declarations. Shopify’s guide uses 2026-04 as an example target version, but that example is not a permanent statement of the latest supported version. Check Shopify’s current supported versions when you make the change.
Then classify the target. A block target is merchant-positionable in the checkout or accounts editor. A runnable target performs event-driven work without rendering a visible interface. A static target renders at a fixed location. The target is not merely a label: it controls placement, available APIs, and the component set that can be used.
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- List every imported UI component and the file in which it is used.
- Record each prop, event handler, validation rule, loading state, and conditional branch.
- List calls made through callback parameters, hooks, or extension APIs.
- Record every cart, customer, product, and app metafield read or write.
- Note whether a cart metafield must be copied to the eventual order.
- Capture the checkout paths that must be tested: express checkout where applicable, address entry, shipping selection, payment, error states, and an extension-disabled state.
This inventory gives you a migration checklist and makes it possible to distinguish a rendering failure from an API or target-availability failure.
2. Select an API version deliberately
Shopify’s versioned Checkout UI extensions documentation identifies 2025-07 as the final API version supporting React-based UI components: Checkout UI extensions, API version 2025-07. Versions 2025-10 and later use Polaris web components by default. Select a currently supported version rather than copying a date from an older tutorial.
| Starting point | Version implication | Required action |
|---|---|---|
React-based extension on 2025-07 or earlier |
Legacy React components are being left behind. | Raise api_version, replace components, update runtime and API calls, then test behavior. |
| JavaScript extension using legacy extension components | The component model changes even if there is no React tree. | Map each element to its Polaris web-component equivalent and verify events and properties. |
| Extension already using Preact and web components | The main work may be version, configuration, API, and metafield compatibility. | Compare the current code with the selected version’s reference and retest target support. |
Update the api_version entry only after you have identified the code and data changes it triggers. A version bump can expose deprecated metafield properties, unavailable components, changed event names, or stricter TypeScript errors before the extension can build.
3. Map legacy components to Polaris web components
Polaris web components are custom HTML elements supplied by Shopify. The mapping is component-specific, not a universal search-and-replace operation. Shopify’s migration pages document the new element name, properties, events, and behavioral differences for each component.
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| Legacy concern | Polaris migration task | What to verify |
|---|---|---|
| Button or action control | Use the documented button web component and its current event model. | Variant, disabled and loading behavior, click handling, and whether the target permits the control. |
| Checkbox or selection control | Use the documented checkbox element. | Checked value type, change event payload, label content, and validation display. |
| Text input | Use the documented text-field element. | Value updates, input versus change events, autocomplete behavior, and error messaging. |
| Banner or status message | Use the documented banner element. | Status tone, dismiss behavior, accessibility text, and whether the target exposes it. |
| Layout container | Use the documented stack, view, or other layout element. | Spacing, direction, alignment, and the target’s component availability. |
In Shopify’s Preact example, the code imports Preact rendering support from @shopify/ui-extensions/preact, calls Preact’s render, and composes elements such as <s-stack>, <s-text-field>, and <s-button>. Treat that shape as a pattern, not a promise that every old property has an identically named replacement. For each component, read the matching migration page and rewrite its props, events, and state handling.
Preserve behavior while changing markup
Keep business rules separate from the view during the conversion. Move validation, discount logic, and persistence into functions that can be called by the new event handlers. Then reproduce loading, success, and failure states with the components available to the target. This approach prevents a visual conversion from silently dropping an API call or allowing a second submission while the first is pending.
4. Update the runtime, dependencies, and TypeScript
For API versions 2025-10 and later, Shopify recommends Preact for UI extensions. The migration guide illustrates dependencies on preact, @preact/signals, and @shopify/ui-extensions. The exact package versions in that example are tied to the guide and must be checked against the API version you select.
Dependency checklist
- Install the Preact packages required by your extension template.
- Use the version of
@shopify/ui-extensionscompatible with the chosen API version and current CLI. - Remove legacy React-only packages after the converted code no longer imports them.
- Regenerate the lockfile and run a clean install in CI so that local and deployment dependency trees match.
Extension-level TypeScript configuration
Shopify recommends an extension-level tsconfig.json configured for Preact JSX and including shopify.d.ts. A typical configuration must tell TypeScript which JSX runtime to use and must include the generated declarations; do not copy a root-project setting that points at React’s JSX runtime.
After changing the configuration, run the extension’s type checker independently from the main app. Errors about intrinsic elements, event types, or missing properties usually indicate that the Preact JSX settings, generated declarations, or package versions do not agree.
5. Upgrade Shopify CLI and move to the global API object
Use a current Shopify CLI that supports building extensions with web components. Running shopify app dev generates a shopify.d.ts file in the extension directory for the global shopify object. Commit the configuration that belongs in source control, but treat generated declarations as build artifacts if your repository policy does not commit them; the important requirement is that local and CI builds generate or receive the same declarations.
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Replace callback-based access
Older extension code often receives APIs through a callback argument or initialization object. The current model exposes APIs on the global shopify object. Shopify recommends that object for straightforward reads and writes; Preact hooks remain useful convenience wrappers when a component needs reactive state.
- Find every callback parameter that exposes checkout, cart, buyer, or extension APIs.
- Identify the equivalent property on the global
shopifyobject in the selected API version. - Update the call site and its TypeScript type, preserving error handling and loading state.
- Remove the old callback plumbing only after all consumers use the new access path.
- Run the type checker and a local checkout session to catch APIs that exist in one target but not another.
Do not infer that an API is available everywhere because it exists on the global object. Target restrictions and API-version support still apply.
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6. Migrate cart metafields and order persistence
Metafield changes are often the most consequential part of the migration because they affect data written during checkout, not just the interface. For API version 2026-04, Shopify says the metafields property is deprecated.
Read existing values
Replace reads through the deprecated property with shopify.appMetafields. Update the code that handles an absent value, a malformed value, and a value belonging to a different namespace or key. Keep parsing and validation explicit; changing the source property should not change the business meaning of the stored data.
Write and remove cart values
Use the updateCartMetafield change type to write a cart metafield and removeCartMetafield to delete one. Shopify also documents deprecations affecting useApplyMetafieldsChange() and older metafield change inputs, so inspect every hook call and input object rather than changing only the property name.
- List each namespace and key the extension reads or writes.
- Declare the required metafields in the extension TOML configuration.
- Change reads to
shopify.appMetafieldsand verify the returned shape in the selected API version. - Change writes and removals to the current cart metafield change types.
- Test duplicate writes, clearing a value, checkout reload, and an API error.
Copy cart metafields to orders
If a value must survive as an order metafield, Shopify requires the write_orders scope and an order metafield definition with the cart_to_order_copyable capability. Request the required metafields in the extension TOML configuration and verify the value on the resulting order, not only in the checkout UI. A successful checkout render does not prove that order persistence is configured.
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7. Check targets, capabilities, and Polaris constraints
Checkout UI extensions combine three pieces: the target chooses where the extension appears, the target API supplies checkout data and operations, and the components render the interface. Changing one does not automatically make the others compatible.
Confirm placement and availability
- Confirm that the selected target still exists in the chosen API version.
- Verify that every component used by the converted tree is available at that target.
- Check that the target’s capability allows the API operation you need.
- Test the extension in the merchant editor if it is a block target and in the live event path if it is runnable.
Checkout extensions for the information, shipping, and payment steps are available only to Shopify Plus stores. That restriction applies to those steps; do not generalize it to every checkout target without checking the current target reference.
Stay inside the provided design system
Shopify’s web-component documentation states: “Checkout UI extensions don’t have access to the real checkout DOM and can’t render arbitrary HTML.” See Web components, API version 2026-01. You also cannot override the CSS of Shopify-provided UI components.
Plan the interface around the supplied elements, properties, and design tokens. Do not depend on selectors aimed at checkout’s host DOM, inject arbitrary markup to recreate a missing component, or ship CSS that assumes control of Shopify’s internal layout. If a visual requirement cannot be expressed with the target’s components, revisit the requirement or choose a target that supports the needed element.
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Shopify’s June 11, 2026 changelog describes an AI Toolkit for upgrading extensions to Polaris web components: Shopify AI Toolkit for upgrading extensions to Polaris web components. Shopify says the toolkit can assist with React-to-Preact conversion, legacy-component replacement, and API updates, and that Shopify used the skills to migrate first-party extensions internally.
That announcement does not establish a success rate, migration time, or compatibility guarantee for your codebase. If you use the toolkit, treat its output as a code-change proposal:
- Give it the selected API version, target, and the component and API inventory.
- Review every generated component mapping against the relevant migration page.
- Inspect event payloads, controlled values, asynchronous operations, and error paths manually.
- Check metafield reads, writes, scopes, and order-copy behavior separately.
- Run type checking, local development, and checkout-path tests before accepting the change.
9. Run a local review and test matrix
Shopify explicitly recommends running the extension locally to find gaps before publishing. Use shopify app dev with the upgraded CLI and verify that the generated declarations, preview session, and selected API version are the ones you intend to deploy.
Build and type checks
- The extension builds with no unresolved legacy component imports.
- Preact JSX and
shopify.d.tsare included in the extension TypeScript project. - No deprecated metafield property or old change input remains unintentionally.
- Global
shopifycalls have valid types and target-compatible APIs. - Production and CI installs use the same lockfile and package versions.
Interactive checkout checks
- Every converted control renders at its intended target.
- Typing, selection, focus, validation, disabled, and loading states behave as before.
- Success and failure responses are visible without relying on host-DOM selectors.
- Cart data loads after a checkout reload and after changing the relevant input.
- Metafield writes, removals, and malformed-value handling work.
- Values intended for orders appear on the resulting order under the correct definition.
- Targets restricted to Shopify Plus are tested in an eligible store.
- Unsupported targets or components fail clearly in logs rather than silently disappearing.
Merchant and customer review
For block targets, place the block through the checkout or accounts editor and verify the merchant’s saved placement. For runnable targets, trigger the event that starts the extension and confirm that the operation runs once, handles retries safely, and does not require a rendered component. Test a checkout where the extension is disabled or unavailable so the purchase flow still completes.
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10. Troubleshoot the common migration failures
The build still expects React
Check the extension-level JSX configuration, remove React-specific JSX imports, and confirm that the Preact package and UI-extension package versions match. A root TypeScript configuration can mask a missing extension setting, so inspect the effective configuration used by the extension build.
A component renders nowhere
Verify the target first. Shopify determines component availability by target, and a component that works in one checkout location may not be exposed in another. Confirm the API version and the exact element name in the component migration page before debugging CSS.
An event handler receives the wrong value
Legacy and Polaris components may use different event names or payload shapes. Read the component-specific migration page, type the handler against the current event, and log the value during local testing. Do not assume that a DOM-style event contains the same property as the legacy callback.
Metafield data is missing
Check the namespace and key, requested metafields in TOML, the selected API version, and whether the code still reads the deprecated metafields property. For writes, confirm the current change type and inspect the returned error rather than treating a rendered success state as proof of persistence.
The extension works locally but cannot update
Shopify announced that apps with extensions on API versions 2025-07 or earlier would be unable to update after October 1, 2026. That date is now past. The exact enforcement behavior can depend on the current CLI and app workflow, so run the update command in the same environment used for deployment, inspect its version warning or block, and move to a supported version if required.
Migration decision framework
| Situation | Best next step | Main risk to control |
|---|---|---|
| Small React extension with standard controls | Manual component mapping with a focused Preact conversion. | Dropping event semantics or validation while changing markup. |
| Large extension with many legacy components | Use the AI Toolkit or an automated draft, then review each diff and test every target. | Assuming generated code is complete or behaviorally equivalent. |
| Extension with complex cart metafields | Plan the data migration before UI conversion and validate order persistence separately. | Breaking reads, writes, scopes, or cart-to-order copying. |
| Extension spanning several targets | Validate each target’s component and capability matrix independently. | Using a component or API that is valid only in one target. |
| Already on Preact and web components | Audit version, CLI, TypeScript declarations, APIs, and deprecations. | Missing a configuration or data change because the UI already looks modern. |
Release checklist
- Choose a currently supported API version and update
shopify.extension.toml. - Confirm each target, capability, store eligibility rule, and available component.
- Map every legacy component using Shopify’s component-specific migration guidance.
- Update Preact, UI-extension dependencies, JSX settings, and
shopify.d.tsgeneration. - Move callback-based API access to the global
shopifyobject where appropriate. - Replace deprecated metafield reads and writes, and configure any order-copy requirements.
- Run type checks, a clean build, and
shopify app dev. - Test block, runnable, or static behavior according to the extension’s actual target.
- Exercise success, validation, loading, error, reload, and disabled-extension paths.
- Run the same update and deployment workflow used by CI or production and resolve version enforcement before release.
A successful migration is one in which the extension’s rendered behavior, checkout APIs, target placement, and persisted data all work under the new version. Polaris web components are the visible part of the change; the dependable upgrade comes from validating the runtime, configuration, target contract, and metafield lifecycle together.
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