Limits here come from Shopify’s Cart and Checkout Validation Function API, which every app in this category is built on. They are not specific to Nex Validator.
Where validation runs
| Surface | Validated |
|---|---|
| Online store cart | Yes |
| Custom storefronts (Storefront API cart) | Yes |
| Checkout | Yes |
| Express checkouts — Shop Pay, PayPal, Google Pay, Apple Pay | Yes |
| Shopify POS | No |
It cannot be bypassed
Validation functions run on Shopify’s servers, not in the browser. Shopify’s own description is that they provide reliable enforcement of business logic that cannot be bypassed by customers.
That is the difference between this and a cart-page script: a script can be skipped by going straight to checkout, and this cannot. It is also why the app’s storefront quantity lock is presented as convenience rather than protection — that layer is browser-side.
The 25-function ceiling
Shopify allows a maximum of 25 active validation functions per store, counted across every installed app. Detail and how to get headroom in limits.
Where error messages surface
Shopify exposes validation errors to the Storefront API’s Cart object, in themes that use the cart template, and during checkout.
That caveat explains a common report: the message appears at checkout but not in the cart drawer. Many themes render drawers without the cart template, so the error is not surfaced natively there. The app’s cart display settings exist to bridge that — see showing the message on the cart page.
Not supported in POS
Shopify Functions do not run in Shopify POS, so order limits apply to online checkout only. An in-person sale is not validated.
Sources
Shopify revises these APIs periodically. Verified against the above in August 2026.