validation function can trap cart in invalid state

[aaaaargZombies]

Which template or example

• cart_checkout_validation
• typescript

Describe the bug

trapped-in-invalid-state.mp4

Validation function can trap cart in invalid state. I believe this is the same issue "Issue with Cart Validation Blocking Product Deletion" discussed in community with no resolution, attributed to a race condition there. I could not alter existing items, delete exiting items or add new items to the cart.

My understanding is that the validation function checks the new state of a cart on modification, blocking the change if it fails the validation.

This does not account for external effects such as changes to metafields accessed by the function, time or the initial state of the cart at time the validation is activated. This means that a previously valid cart can fall into an invalid state and there is then not a way for a user to correct it.

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example usecase: a product is reserved for a period of time when added to a cart, if the product is still in the cart after the reservation expires then it cannot be purchased. If they were to remove the item from the cart it would be valid again.

Steps to reproduce

based on this tutorial / documentation

This is the simplest way to produce the invalid state, using the default template.

• run shopify app generate extension --template cart_checkout_validation --name cart-checkout-validation
• select typescript
• run npm run dev
• add an item to the cart
• increase quantity to more than one
• enable validation in store settings, uncheck "Allow all customers to complete checkout"
• attempt to alter cart. changing quantity, deleting item and adding a new item to the cart are blocked.

Expected behavior

Once a cart has reached an invalid state, modifications that could bring the cart into a valid state should be allowed. Perhaps allowing all modification and displaying warnings but not allowing creation of an order if state is invalid.

Environment

• Linux
• Shopify CLI version @shopify/cli/3.70.0 linux-x64 node-v20.13.1

Additional context

I hope this is the correct place to file an issue, I apologize if this is not actually a bug.

[alex-saas]

I encountered the exact same issue.

Here’s an example of the user flow that leads to a non-modifiable cart:

1. The customer adds 15 units of product A and 15 units of product B to the cart.
2. The customer abandons the cart and leaves the store.
3. Later, the merchant or developer configures a validation function that prevents checkout if the total quantity of line items in the cart exceeds 10.
4. When the customer returns to the store, their cart still contains products A and B. However, here’s the trap: the validation rule blocks any attempt to remove a product or decrease its quantity. As a result, the customer is stuck—they can neither clean up the cart nor proceed to checkout.

This happens because the function executes on every cart action, using the current cart state as input. Even if the customer attempts to remove product A, the cart still contains 15 units of product B, which fails validation.
Of course, this might be a rare use case, but still realistic.

It would be helpful if the Shopify Functions team could provide one of the following solutions:

1. Skip function execution for the "remove product from the cart" action.
2. Force removal of the product from the cart.
3. Introduce a flag in the buyerJourney object within the function input that indicates the specific cart action—e.g., { "cart_action": "LINE_REMOVAL" }. This would allow developers to adjust function logic and prevent validation errors when products are being removed during the CART_INTERACTION step.