Quick answer
If a Shopify app suddenly refuses to deploy because of an old checkout UI extension, check every UI extension in the app, not just the extension you were editing. Shopify’s 2025-07 checkout UI extension API was the last version to support the older React-based UI components. From October 1, 2026, deployments can be blocked until affected UI extensions are upgraded to API version 2026-01 or newer and migrated to Polaris web components.
The app can still have working extensions in production while new deployments are blocked. That is why this can look like a deployment problem even when the code you just changed has nothing to do with checkout.
On this page
Why this appears out of nowhere
Shopify versions app extensions independently through configuration files, but an app deployment packages the app version as a whole. One stale UI extension can therefore stop an otherwise unrelated deployment.
This is especially confusing in apps that have accumulated multiple checkout, customer-account, or admin UI extensions over time. The extension you recognize in the Partner Dashboard may not be the extension causing the block.
Start by inventorying the extensions
Before changing code, identify every extension inside the app repository. Look for each extension’s shopify.extension.toml and note its API version and target.
Do not assume “we only use one checkout extension” because the app has one visible feature. Old experiments, retired blocks, thank-you-page extensions, or duplicated extension folders can still exist in the app and be part of deployment.
Why changing the API version is not always enough
API version 2025-07 was the last checkout UI extension version supporting the older React component model. Newer versions use Polaris web components. An extension that imports older component APIs may fail type generation or build after you change the version.
That failure is useful. It tells you the extension needs an actual migration, not just a TOML edit.
A safer upgrade sequence
- List every UI extension in the app.
- Record each extension target and API version.
- Upgrade one extension at a time to a supported version.
- Run Shopify type generation where the extension uses generated API types.
- Fix deprecated component, hook, or API usage.
- Build the app locally.
- Run extension-specific tests.
- Repeat until every blocking extension is on a supported API.
- Deploy only after the entire app builds cleanly.
Watch for checkout blocking code
If the extension uses buyer-journey interception or block_progress, the migration is not only cosmetic. Shopify deprecated that blocking path starting in API version 2026-07 and recommends Cart and Checkout Validation Functions for business-rule enforcement.
That can turn what looks like a version bump into a small architecture decision: keep the UI in the extension, but move authoritative checkout blocking into a Validation Function.
Do not delete an extension just to make deploy pass
If an extension appears unused, verify that before removing it. Check the app configuration, checkout editor placements, current production behavior, and the extension’s purpose.
A stale folder is clutter. A still-installed extension that nobody remembered is production functionality.
What to check when the build still fails
- Old React component imports.
- Deprecated checkout hooks.
- Generated types that no longer match the selected API version.
- Extension targets that changed between API versions.
- Configuration keys that are no longer valid.
- One forgotten extension still pinned to 2025-07 or older.
Common misunderstanding
A blocked Shopify app deployment does not mean the feature you just edited is broken. The blocker can be another UI extension elsewhere in the same app. Treat the app as a deployment unit and audit every extension before chasing the last file you touched.
How to test this
- Search the repository for every
shopify.extension.toml. - Confirm the API version for every UI extension.
- Run type generation after version changes.
- Run the local build before attempting deploy.
- Test checkout extensions in the checkout editor and a real checkout path.
- Verify any blocking logic separately if it was moved to a Validation Function.
- Only remove an old extension after confirming it is not installed or used.