Shopify Bug of the Ajax API after upgrade the new checkout extension

Shopify Plus

Topic summary

A merchant reports critical Ajax API bugs after upgrading to Shopify’s new checkout extensibility:

Issue 1 - Partial Inventory Addition:

  • When attempting to add 10 units of a product with only 5 available, the API returns a 422 error but silently adds the 5 available units to cart
  • Previous behavior: 422 error with nothing added (expected)
  • New behavior: 422 error but partial quantity added without notification

Issue 2 - Cart Update Failure:

  • Customer successfully adds last available unit (1 qty) to cart
  • Attempting to add same product again correctly shows 422 “sold out” error
  • When trying to remove the product using cart/update.js with line_item.qty = 0, the operation fails with 422 error and displays “no more stock can be added”
  • Expected: Product should be removed from cart successfully

Current Status:
The merchant notes that while Shopify recommends upgrading to checkout extensibility, support appears unaware of these API inconsistencies. They’re requesting help to investigate these cases.

Summarized with AI on November 14. AI used: claude-sonnet-4-5-20250929.
1

After I tried to upgrade the checkout extensibility , below 2 scenarios will trigger bug when I using Ajax API for adding product to cart.

  1. customer tried to add 10 qty product which has only 5 inventory left into his/her cart

in original version , the cart/add.js , will reject & response 422 error code , then nothing added into cart .

in the new version , the cart/add.js , will also reject & response 422 error code , then , added 5 into cart without any reponse for how many did added.

  1. customer tried to add 1 qty product which has only 1 inventory left into his/her cart. – success

customer tried to add 1 more qty into his/her cart. – failed with 422 thru cart/add.js , it is normal , as no more stock can added .

but at this time , we call cart/update.js , to remove the product from cart , use line_item.key to make ti qty = 0 , expected success , but it failed with 422 error , & showed no more stock can added .

after above action , when customer , tried to add the same product again into cart, then will be promopted error : 422 , sold out .

Could support help to check with above case ? as shopify recommand to upgrade the checkout extensibility , but the surrounding support is not keeping up with the recommended upgrade .