Import the `applyAttributeChange` function (or the `useApplyAttributeChange` hook, depending on the current extension SDK version — verify in current Shopify Checkout UI Extensions docs) from `@shopify/ui-extensions-react/checkout`.
Invoke `applyAttributeChange` with an object containing the attribute `key`, `type` (typically `'updateAttribute'`), and `value` string to write a custom attribute to the cart/order.
Trigger the apply call from a user interaction (button click, toggle, field change) or from a `useEffect` when a condition is met — avoid calling it unconditionally on every render.
Read back the applied attribute using `useAttributes()` to confirm the write succeeded before proceeding with dependent logic.
For order-level note updates, use the equivalent `applyNoteChange` API — verify the current function name in the extension docs.
Validate that the attribute key and value appear on the resulting order object via the Admin GraphQL API after checkout completion.
Known gotchas
`applyAttributeChange` is asynchronous and returns a result object indicating success or failure. Always await and check the result before assuming the attribute was written — silent failures occur if the checkout session has moved to a terminal state.
Cart attributes set by a Checkout UI Extension appear on the Order object in the Admin API under `customAttributes`. They are not the same as metafields and are not queryable as structured metafield data.
The extension sandbox limits how frequently you can call mutation APIs. Debounce `applyAttributeChange` calls triggered by user input (e.g., text field changes) to avoid hitting rate limits or degraded checkout performance.
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