Incident Summary:
On October 27, 2025, some merchants experienced an issue where their Smart Cart failed to open. This occurred shortly after the release of a new feature designed to support nested cart items in Smart Cart V2. The feature enables merchants to display parent-child product relationships (such as bundles or configurable products) directly within the Smart Cart, improving consistency between the cart and checkout experience.
Root Cause:
The issue stemmed from a missing optionality check in the new nested cart item functionality. The shared cart file (Cart.js) is used by both Smart Cart V1 and V2. The new code called a function, nestedCartItemsEnabled(), that only exists in V2. Because this function does not exist in V1, the call caused a JavaScript error that prevented the Smart Cart from loading for merchants still using the legacy version.
Resolution:
A fix was implemented by adding an optionality check to ensure the function is only executed when present. This change restored functionality for all merchants using Smart Cart V1, while maintaining the new nested cart item capability for V2. The fix was verified locally and across multiple staging environments using different Smart Cart configurations prior to implementation.
Preventive Measures: