Unable to get Line Item app in Checkout to display anything

Extensions
1

Hi, I’m new to developing apps. I’m currently trying to create an app that displays a “final sale” banner under line items that are tagged with “final sale”.

For some reasons no matter what I do to debug, nothing is displaying and I’m wondering if I’m missing something. I did do both shopify app dev and shopify app deploy on my dev store, and I even have the app setup as a block in Checkout customizer.

Test “./src/Checkout.jsx” code

import { reactExtension, Text } from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.cart-line-item.render-after',
  () => <Text appearance="critical">EXTENSION LOADED</Text>
);

And the important bit from shopify.extension.toml

api_version = "2025-01"

[[extensions]]
name = "Line Item Flags"
handle = "line-item-flags"
type = "ui_extension"
uid = "xxxxxredactedxxxxxx"

[[extensions.targeting]]
target = "purchase.checkout.cart-line-item.render-after"
module = "./src/Checkout.jsx"

[edited to add:]

When I try to set api_version to 2025-10, I get an error in Powershell

Type reference for purchase.checkout.cart-line-item.render-after could not  │
│   be found. You might be using the wrong @shopify/ui-extensions version.     │
│                                                                              │
│  Fix the error by ensuring you have the correct version of                   │
│  @shopify/ui-extensions, for example ~2025.10.0, in your dependencies.

This is even after I set package JSON to the following

{
  "name": "line-item-flags",
  "private": true,
  "version": "1.0.0",
  "license": "UNLICENSED",
  "dependencies": {
    "@shopify/ui-extensions": "2025.10.0",
    "@shopify/ui-extensions-react": "2025.10.0"
    "preact": "^10.19.3",
    "react-reconciler": "^0.29.0"
  }
}

Not sure what I’m doing wrong

2

If you are using version 2025.10.0 in your package.json, that is not compatible with version 2025-01 that you have in your shopify.extension.toml. They should match. If this is a new extension, I would recommend using the latest version 2025-10, which uses preact and not react like you have here.