Upgrading Checkout Blocks app to Polaris web components

Checkout Blocks’ UI extensions render on a third of all customized checkouts and were built on React and Shopify's legacy Remote UI bridge, running on the soon-to-be-sunset 2025-07 API version. Shopify is moving its entire UI extension surface to remote-dom with Polaris web components, so staying on the old path was no longer an option.

We upgraded all five extensions to 2026-01 running on remote-dom, with Preact and Polaris web components, and rewrote the codebase to TypeScript. This is how we did it, and what we learned along the way.

Why we took this on

Checkout is an extremely performance-sensitive surface. Every extension adds buyer-facing latency: code that downloads, parses, and executes in the buyer's browser at the moment they're deciding whether or not to buy. Polaris web components and remote-dom cut that overhead directly: smaller bundles, faster page loads, and ultimately a faster checkout experience for the buyer.