Test Shopify checkout extensions in layers: first verify the target and plan prerequisites, then use shopify app dev for rapid feedback, confirm merchant placement in the checkout editor, exercise Shopify’s checkout scenarios, and finally add unit, browser-functional, and screenshot checks. A screenshot diff can show that pixels changed, but it cannot prove an extension is active in every merchant placement or checkout context.
Start with the platform constraints
Identify which checkout surface you are testing
Checkout UI extensions do not all have the same eligibility rules. Extensions rendered on the information, shipping, and payment steps require a Shopify Plus store. For those surfaces, Shopify’s getting-started guidance recommends a development store with Shopify Plus enabled. Shopify announced on March 13, 2026 that new Plus development shops could use checkout UI extensions after the developer preview ended.
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Do not apply the Plus requirement to every extension target. Record the specific target, checkout surface, and merchant plan needed for the behavior under test. A test that passes on a target available to ordinary development stores does not establish that an information-, shipping-, or payment-step extension will work on a non-Plus shop.
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Before running tests, write down each extension target and module from shopify.extension.toml, along with the configured API version. The current Shopify API reference lists unit-test support through @shopify/ui-extensions-tester as available for API version 2026-04. Treat that as version-specific: confirm the extension’s configured version before adding the package or depending on its behavior.
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Shopify’s stable API versions receive at least 12 months of support, and the CLI prevents deployment to API versions older than 12 months. During maintenance, recheck the version rather than assuming a test setup remains valid after an API change.
Use Shopify’s native feedback loop first
Start the development preview
- Sign in to the Shopify organization and select the development store that matches the surface being tested.
- From the app project directory, run
shopify app dev - Open the preview URL for that development store and change the extension code.
- Confirm that the preview reloads and inspect the changed behavior before moving to a different test layer.
Shopify’s development command creates a preview on the selected development store and reloads extension changes. If the app has a backend, the same development command can serve it locally through a Cloudflare tunnel. This loop is the fastest way to catch rendering and interaction mistakes, but it is not a substitute for merchant configuration and scenario coverage.
Separate static targets from block targets
A static target renders automatically at its configured location. A block target is different: its location is selected by the merchant, so one successful preview at the preferred position does not cover the other positions the app supports.
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?placement-reference={name}Replace {name} with the placement reference you want to preview. Shopify documents placement references for wallet, information, delivery, and payment areas, as well as separate placements on Thank you and Order Status surfaces. Test only the references supported by the extension, but include every supported position that a merchant could select.
Verify the merchant-facing editor
Add and place the app block
An extension does not appear in checkout merely because the code is available in the local preview. Shopify’s editor workflow requires a merchant to add the app block and place it through the checkout editor.
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- Open the checkout editor for the development store.
- Add the app block to the relevant checkout surface.
- Move it among the supported targets and save the configuration.
- Open the resulting checkout and verify that the saved placement matches what the editor showed.
This step catches configuration failures that a direct local render can miss: a block that is not added, a target that is unavailable in a particular checkout, or a placement that changes when the buyer uses an accelerated flow.
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Check one-page, three-page, and Shop Pay contexts
Shopify’s editor testing instructions call out one-page checkout, three-page checkout, and Shop Pay. Exercise the contexts that the extension claims to support rather than treating one checkout layout as representative of all of them.
Shop Pay testing has an additional prerequisite. Shopify requires Shopify Payments to be configured in test mode on the development store. Shop Pay Installments requires completed Shopify Payments setup as well. If those prerequisites are absent, record the context as unavailable instead of interpreting the missing flow as an extension failure.
Build a scenario matrix before automating
Shopify’s checkout UX guidance is a useful starting matrix. Select cases according to the extension’s data dependencies; for example, a delivery customization needs delivery choices, while a block that reads buyer identity needs both guest and accelerated-checkout coverage.
| Axis | Scenarios | What to observe |
|---|---|---|
| Device | Mobile and desktop | Responsive layout, wrapping, touch targets, and whether content remains visible at the intended width. |
| Buyer flow | Guest checkout and Shop Pay | Identity availability, target location, authentication transitions, and any accelerated-checkout differences. |
| Cart | Physical, digital, and discounted products | Whether product, pricing, discount, and fulfillment data used by the extension are present and correctly handled. |
| Fulfillment | Shipping and pickup | Delivery-state changes, pickup-specific content, and behavior when a delivery option is unavailable. |
| Layout | One-page and three-page checkout | Step transitions, placement, state persistence, and responsive spacing. |
| Placement | Every supported block reference and each configured static target | Rendering, spacing, interaction, and whether the extension remains usable at each merchant-selected location. |
Shopify notes that target locations can change between guest checkout and Shop Pay. Therefore, a block’s position must be asserted in each relevant buyer flow, not inferred from the guest checkout result.
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Test functional behavior at the right layer
Unit-test extension logic when the API version supports it
For a checkout UI extension configured for API version 2026-04, Shopify currently points developers to @shopify/ui-extensions-tester for unit tests. Confirm the API-version requirement before installing or upgrading the tester. If the extension uses another configured version, follow the support information for that version instead of assuming the same test APIs are available.
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Use unit tests for deterministic decisions that do not require a real checkout: input interpretation, conditional visibility, formatting, validation, and event-handling branches. Keep fixtures explicit about the buyer, cart, discount, and fulfillment state so a passing test identifies the state it actually covers. A unit test should fail when the expected output or interaction contract changes; it should not be treated as proof that Shopify will place the extension correctly in a merchant’s checkout.
Add browser-level functional checks
Use a browser test for behavior that depends on the rendered checkout and its navigation: opening or dismissing a panel, changing an option, showing validation, moving between checkout steps, and confirming that state survives a supported transition. Run the browser test against the same development-store configuration used for editor verification.
Keep each scenario focused. A test that combines a discount, pickup, Shop Pay, and several unrelated interactions can fail without revealing which condition changed. Prefer separate cases whose names identify the device, buyer flow, cart type, fulfillment mode, and placement.
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For each behavior that depends on checkout data, include the absence or change of that data where it is meaningful. Examples include a digital cart with no shipping step, a pickup order without shipping options, a guest buyer without an authenticated identity, and a discount that changes the displayed total. The goal is to verify that the extension remains valid when a supported context removes information it normally receives.
Add visual regression checks without overclaiming what they prove
Use deterministic screenshot assertions
Playwright Test supports screenshot assertions with await expect(page).toHaveScreenshot(). The first run creates reference images; later runs compare the current capture with those references.
await expect(page).toHaveScreenshot('checkout-extension.png');Capture the extension in the placements and scenarios that matter to its layout. Give each baseline a name that identifies the flow and viewport, such as a desktop guest checkout or a mobile pickup checkout, rather than overwriting one image for every context.
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Keep capture conditions stable
Playwright warns that browser rendering can vary with the host operating system, browser version, browser settings, hardware, power source, and headless mode. Generate and compare baselines in the same environment whenever possible.
- Pin the browser version used by the visual project.
- Use a consistent operating-system image and viewport size.
- Keep zoom, font availability, color scheme, locale, and relevant checkout settings consistent.
- Use the same headless or headed mode for baseline creation and comparison.
- Separate projects or baseline sets when browser or platform differences are intentional.
A screenshot diff is evidence that pixels changed under a defined environment. It is not evidence that the extension is active in every merchant placement, buyer flow, or checkout configuration. Conversely, a green screenshot run does not prove that an unsupported placement or an untested context is correct.
Review diffs instead of accepting a green run blindly
Inspect every changed region and classify it as an intended design change, an environment drift, or a regression. Update a baseline only after confirming that the new appearance is correct in the editor and the relevant checkout scenario. A baseline update without that review can turn a real regression into the new expected image.
Use hosted comparison only when it fits the workflow
Percy can route existing Playwright toHaveScreenshot() calls to cloud comparison. Its documented workflow compares captures against a baseline and reports visual changes for review. A Percy difference can be reported while the test run still passes, so reviewers must inspect the Percy build or configure a pipeline wait step that fails on unapproved differences. Hosted comparison is an option for teams that need centralized review or cross-browser snapshots; it is not required by Shopify.
Organize a regression run
Use a layered order
- Validate the target, module, API version, and required Shopify plan or development-store capability.
- Run
shopify app devand check the changed code in the local preview. - Verify static rendering and preview each supported block placement with
placement-reference. - Add and move the block in the checkout editor.
- Run the selected one-page, three-page, and Shop Pay contexts.
- Exercise the device, buyer, cart, and fulfillment cases that affect the extension.
- Run supported unit tests and browser-functional checks.
- Capture screenshots in the controlled visual-test environment and review every diff.
This order keeps failures diagnosable. There is little value in investigating a screenshot mismatch if the block was never added in the editor, or in debugging a unit test when the extension is configured for an API version the tester does not support.
Keep evidence with each result
For every failure, retain the extension version or commit, API version, target, development-store plan, checkout context, cart and fulfillment setup, viewport, browser, and whether the capture was local or hosted. These details distinguish an actual regression from a changed merchant configuration or a different rendering environment.
Troubleshoot common failures
The extension is missing from checkout
- Confirm that the target is eligible for the store plan, especially for information, shipping, and payment steps.
- Check that the merchant added the app block in the checkout editor.
- Verify that the selected placement is supported by the extension.
- For Shop Pay, confirm Shopify Payments is configured in test mode; for Shop Pay Installments, confirm the required Shopify Payments setup is complete.
The block appears in one position but not another
Use the documented placement-reference value for the missing position and verify that the extension declares or supports that target. A successful static-target render does not demonstrate block-target support, and a preferred block position does not cover every merchant-selectable position.
A screenshot changes on every run
- Compare the browser and operating-system versions used for baseline and test runs.
- Check viewport, device scale, font availability, zoom, color scheme, locale, and headless mode.
- Look for dynamic checkout content and wait for the intended state before capturing.
- If platforms must differ, create separate projects or baseline sets instead of mixing images.
Percy reports a difference but CI is green
That behavior is expected when Percy is configured to report changes for review without blocking the test process. Review the Percy build, or add a pipeline wait step that fails when a difference has not been approved.
The unit-test package does not behave as expected
Check the extension’s configured API version first. Shopify’s current reference ties @shopify/ui-extensions-tester support to API version 2026-04; an extension on another version may require a different supported setup. Also separate unit-test failures from browser or editor failures: they validate different layers.
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What a trustworthy result looks like
A trustworthy regression result identifies the exact target and API version, confirms that the required development-store capability was present, shows that the extension was added and positioned in the checkout editor, names the checkout and customer scenario, and records the browser and screenshot environment. Functional automation can demonstrate that a defined interaction works. Visual automation can demonstrate that the rendered pixels match an approved baseline under controlled conditions. Neither alone proves universal coverage.
Shopify describes the purpose of scenario testing this way: “Testing checkout UI extensions in different scenarios is a UX guideline. The purpose is to recreate merchant and customer experiences and ensure that extensions behave as desired.” Use that principle to decide which matrix cases are material for the extension, then keep the native preview, editor verification, functional tests, and reviewed screenshots as complementary—not interchangeable—evidence.
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