Hey Shopify devs,
We just published a deep dive into how we upgraded Checkout Blocks’ UI extensions from the soon-to-be-sunset 2025-07 React and Remote UI version to Polaris web components, Preact and API version 2026-01.
The result: transferred bundle sizes dropped 40% to 85%, and every extension loaded 7-8% faster. 
We cover:
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migrating incrementally with legacy and new shared cores running side by side
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getting each extensions bundle size as small as possible
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how AI assisted heavily and informed our new AI toolkit for aiding in this upgrade
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some platform level changes we made along the way
It’s time to upgrade your ui extensions to avoid being blocked from deploying your app after October 1, 2026. After that date, you’ll be able to deploy your app only once all of its extensions have upgraded to version 2026-01 or later. We hope this case study and lessons shared will help!
Read the full engineering blog post here
Questions?
We would like to hear from you on anything you’re blocked on, what may be missing, or if you run into any issues. Drop your thoughts below.
Justin Henricks
Software Developer, Checkout