Cart transform Extension giving InvalidOutputError error

Retired Boards Appdev Extensions

Topic summary

Issue: Cart Transform function’s update operation returns InvalidOutputError when including a price field, despite documentation indicating price updates are supported.

What was attempted:

  • Environment: Tested on both a Dev store and a Plus store.
  • Payload: Update operations included cartLineId, title, and price.adjustment.fixedPricePerUnit.amount.
  • Result: Validation errors for each operation: path [operations, n, update, price], explanation “Field is not defined on UpdateOperation.”

Observed behavior:

  • Updating non-price fields (e.g., title) succeeds.
  • Adding price causes the error. Image updates were suggested but not confirmed; title was verified to work.

Context/technical notes:

  • Cart Transform is a Shopify Functions extension to modify cart lines; UpdateOperation is expected (per docs) to accept fields like price/title/image.
  • The attached JSON payloads and error messages are central to the issue.

Status and next steps:

  • No resolution yet. The discrepancy suggests either a documentation mismatch or current API limitation.
  • Awaiting confirmation from Shopify on whether price is currently supported in UpdateOperation and any required conditions or flags.
Summarized with AI on January 5. AI used: gpt-5.
1

Following the cart transform documentation https://shopify.dev/docs/api/functions/reference/cart-transform
I was trying to have update operation get going, but I am getting error for adding “price” parameter in Input
As per documentation, update operation does accept price,title,image etc kind of data https://shopify.dev/docs/api/functions/reference/cart-transform#example-update-operation
But to me its giving me error

Any idea why that is ?

{
    "path": [
      "operations",
      0,
      "update",
      "price"
    ],
    "explanation": "Field is not defined on UpdateOperation"
  }
2

Hi Siddhant,

Just to confirm - are you attempting this of a dev store or Plus store? Is your operation the same as the one in this example?

3

Hi @Liam , Yes I can confirm that I have tried to do this on Dev store and Plus store

posting the output details and error

{
  "operations": [
    {
      "update": {
        "cartLineId": "gid://shopify/CartLine/af289e2f-bf8f-4313-a83e-2edc0876ef69",
        "title": "blue sunset from extension",
        "price": {
          "adjustment": {
            "fixedPricePerUnit": {
              "amount": "0"
            }
          }
        }
      }
    },
    {
      "update": {
        "cartLineId": "gid://shopify/CartLine/d0f5bffd-5911-4c11-b399-adacbe5331ed",
        "title": "The Summer And Frost Bundle from extension",
        "price": {
          "adjustment": {
            "fixedPricePerUnit": {
              "amount": "0"
            }
          }
        }
      }
    }
  ]
}

Error

[
  {
    "path": [
      "operations",
      0,
      "update",
      "price"
    ],
    "explanation": "Field is not defined on UpdateOperation"
  },
  {
    "path": [
      "operations",
      1,
      "update",
      "price"
    ],
    "explanation": "Field is not defined on UpdateOperation"
  }
]
4

Hi again Siddhant,

If you try to update something else other than the price (eg: image) can you complete this update operation?

5

Hey @Liam , yes I did try to run update operation with just title , it seems to be working with only title, but when I add price, it gives error

6

Hey @Liam , any updates on this ??