Cannot add items to shopping cart: find the rule

The shopper presses Add to cart and the cart refuses without moving. The refusal can come from a rule the shopper never saw: the size has two left, the store caps each order at a set number, or a subscription already in the cart blocks a one-time purchase. Some stores print that rule in small text under the button. Many show nothing but the same button, so the shopper assumes the site is broken. An owner who adds the item from an admin login, with an empty cart and no history, never meets the rule, so the owner's test passes while the shopper is stopped.

• Does the add fail on the first press, with the same quantity, in a private window with an empty cart?
• Does the refusal name a rule, such as stock or a limit, or only say the item is unavailable?
• Does the quantity selector allow a number above the store's per-order limit?
• Is a subscription, pre-order or item for another region already sitting in this browser's cart?
• Does the add request return a status code and a reason in its response body?

Stock is counted per variant. A medium shirt can be sold out while the large sits on the shelf, and the add fails on the variant the shopper chose. A per-order limit caps the quantity no matter how much stock is left, which is common with bulk goods such as paper. A per-customer limit counts past orders on the same account or address, so a shopper who bought last month can be refused today.

A shipping restriction blocks an item from going to a region. Some stores check it at the add, and others only at checkout, so the same item can pass one step and fail the other. A pairing rule blocks a subscription and a one-time purchase in one cart, or a pre-order and an in-stock item. A well built store says which rule fired. A poorly built one says nothing, and the shopper has no way to tell the rules apart.

Open a private window, sign out of anything you are signed into, and add the same item at the same quantity with an empty cart. If the add works there, the cause sits in the cart saved in your normal browser, such as a subscription, a pre-order or an item that ships somewhere else. Empty that cart on its own page and try again. If the add is refused in the private window too, the cause is the product rule, so check the variant's stock and the quantity limit in your product settings.

Then read the add request in the Network tab. The response body usually carries the reason as a code or a sentence, such as out of stock or limit reached. Copy the text exactly. A developer can search for those words in the store's code, and the same words make a precise prompt for a coding agent. A plain message on screen can hide a reason the server had.

An admin login has order history of its own, or none, and a cart you emptied last week carries none, so a per-customer limit rarely fires for you. A new visitor arrives with no history, so a run reaches the stock and per-order rules a first-time shopper meets. A per-customer limit only fires for a shopper with past orders, which means a visitor signed in with a test account you give it, and that takes paid credits. Every run records the message it saw and a screenshot of the product page at that moment.

A restock that sells out early, a new per-order limit or a shipping rule added for one region can refuse the same item again, with no change to the page. Rerun the same shopper after each stock or rule change and compare the message they saw. With paid credits, visitors can also open a protected preview, so each deploy gets the same run before a launch.