Hey @sebastian.pisula, thanks for the detailed write-up - that reproduction code is really helpful.
This looks like the same issue I’m tracking in another thread where other developers started seeing the same 422 error from async_shipping_rates.json today, all without any code changes on their end. You’re not alone here it seems!
I’ve raised this with the Cart API team along with x-request-ids and reproduction details from the other reports. Will update both threads once I hear back.
@PeterParker The /cart/add issue might be separate - are you seeing the same 422 error with shipping rates, or something different with the add endpoint? If different, could you share the error response and an x-request-id from a failed request?