Persistent 422 (Unprocessable Content) error when adding to cart

Functions
1

We’ve got a merchant store that is returning persistent 422 (Unprocessable Content) when adding our customisable bundle product to cart.

{
  "status": 422,
  "message": "Cart Error",
  "description": "The bundle product 'product_name' cannot be added to the cart."
}

The Cart Transform function is returning a success with a valid output showing all expected expandedCartItems. We’re using the expand operation on a custom bundle (not a fixed bundle).

This merchant’s store was previously working without issue and now no custom bundles can be added to cart without the 422 error.

Shopify Bundles app adds bundles to cart as expected, but this uses the bundle relationships, rather than the custom Cart Transform.

Things we’ve troubleshooted so far:

  1. Component Publication and Availability
  • All component variants referenced in the bundle are published and available for sale.
  • None are unpublished, archived, or deleted.
  1. Component Inventory
  • All component variants have sufficient inventory.
  • No out-of-stock errors are being returned ie. “This product is already sold out” or “All products are in the cart”
  1. Locations, Shipping, Markets settings
  • The store has only one location. Bundle product variant, and component variants are all available and active at the same location.
  • Store only has the default shipping profile, which the bundle product, and the component items belong too
  • Store is not eligible for Markets, so is only published to the default.
  1. Bundle Selection Data
  • The line item property in the payload that has the id for expanding the bundle matches exactly what the function expects (evident in the successful output)
  • The function logs show the correct expandedCartItems output.
  1. Bundle Configuration Metafield
  • The bundle product has the correct metafield configuration.
  • The metafield is valid and accessible, as evidenced by successful expansion.
  1. Function Output
  • The function runs successfully and outputs valid expand operations.
  • No errors are shown in the function logs.
  1. App Ownership, Permissions, and Function Activation
  • The app that created the bundle is the one managing it.
  • The cart transform function that the same app created is still deployed and active on the store when querying via admin API.
  1. Store and API Eligibility
  • The store is eligible for bundles when querying via admin API
  • The bundle product and all components are published to the correct sales channel.
  1. Payload Format
  • The payload sent to /cart/add.js is well-formed and includes all required fields for Functions to correctly process the expand operation.
  1. Theme and App Conflicts
  • No known theme or third-party app conflicts are interfering with the add-to-cart process. The store is very vanilla. We’re the only app running checkout rules.
  1. Error Specificity
  • The error returned is a generic 422 “unprocessable content” error, not a specific error about inventory or available as you would expect with sold out variants, or not enough inventory available for component items.
  1. Function Expansion
  • The function expands the bundle as expected, and the expanded items are valid.
  1. Testing Scope
  • The issue is isolated to this store; other stores with similar setup do not experience the error.

What else could we have missed for troubleshooting this one?

Any additional pointers would be super helpful.