When running shopify app dev with a checkout-ui-extension, I get 500 error

This was working yesterday, but today I get 500 from the server. I’ve reverted to the state from yesterday, but it is still 500.

Is there server down today?

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Just FYI… this issue is caused by putting something of the format

"extensions": {
    "foo": "bar"
  }

if the locales/en.default.json of a checkout UI extension. As far as I can tell, it is the word “extensions” that is the problem. It will cause a deadlock in the initial log. After you kill the cli and run shopify app dev again, you get the 500.

To recover, you need to remove the “extensions” property and run shopify app dev --reset choosing the options to make a new app.

Note, this is only an issue if “extensions” is an object.

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Glad you figured what was causing this issue and thanks for coming back to post the solution!