Checkout UI extensions now default to non-blocking behavior

As of January 26, 2026, Checkout UI extensions that support blocking now default to non-blocking behavior. To enable blocking, merchants must explicitly activate the Allow app to block checkout setting in the checkout and accounts editor.

If your Checkout UI extension requires blocking, display a warning in the editor using the useExtensionEditor() hook, and include this step in your activation process. Inform merchants that they must enable Allow app to block checkout for the UI extension to function as intended.

We now recommend building custom checkout validation using Cart and Checkout Validation Functions instead of Checkout UI extensions, as they are more secure, performant, and guaranteed to run across supported checkouts. Learn more about Cart and Checkout Validation Functions.