Fix 400 Bad Request on guest virtual checkout when setting payment

[dmytrokaplin]

Description (*)

Fixes an intermittent HTTP 400 Bad Request error on guest checkout for virtual products (or checkouts without a shipping step) when only a single payment method is available.

Root Cause

On a guest virtual checkout (where the shipping step is absent), the payment step renders immediately upon page load. If there is only one payment method available, Magento_Checkout/js/model/payment-service::setPaymentMethods() automatically selects it via selectPaymentMethod(filteredMethods[0]):
https://github.com/magento/magento2/blob/2.4-develop/app/code/Magento/Checkout/view/frontend/web/js/model/payment-service.js#L54

This triggers a call to save payment information before the guest user has typed an email address into the checkout form. At this point, quote.guestEmail is still undefined or null.

When sending the REST API request to /V1/guest-carts/:cartId/set-payment-information, JSON.stringify() drops keys with undefined values (or omits non-string types).

Since \Magento\Quote\Api\GuestPaymentInformationManagementInterface::savePaymentInformation() defines $email as a required scalar parameter without a default value, Magento's Webapi ServiceInputProcessor fails validation and throws an InputException: "email" is required. Enter and try again.

Passing quote.guestEmail || '' ensures that "email": "" is present in the payload, satisfying Webapi requirements when quote.guestEmail is still undefined or null, until the user actually inputs their email address.

Related Issue (#)

N/A

Fixed Issues (if relevant)

1. Fixes HTTP 400 error on guest virtual checkout when a single payment method is auto-selected upon initial page load.

Manual testing scenarios (*)

1. Ensure the store configuration has only one active payment method available for guest checkout.
2. Add a virtual product to the shopping cart as a guest user.
3. Open a clean/incognito session and proceed to Checkout (/checkout/#payment).
4. Open Browser DevTools -> Console and Network tabs.
5. Observe the initial POST request to /rest/default/V1/guest-carts/{cart_id}/set-payment-information.

Actual Result (Before fix)

The request returns HTTP 400 Bad Request with response body:
{"message":"\"%fieldName\" is required. Enter and try again.","parameters":{"fieldName":"email"}}

Expected Result (After fix)

The request completes successfully without Webapi payload schema validation failure.

Questions or comments

N/A

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests pass locally

Resolved issues:

1. resolves [Issue] Fix 400 Bad Request on guest virtual checkout when setting payment #41112: Fix 400 Bad Request on guest virtual checkout when setting payment

[m2-assistant]

Hi @dmytrokaplin. Thank you for your contribution!
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[engcom-Bravo]

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