The cart behavior in Hydrogen 2 + Remix often relates to how Shopify handles cart sessions and optimistic UI updates. Below is a summary of potential reasons and troubleshooting techniques:

Why does totalQuantity temporarily display 1 before dropping to 0?

• Optimistic UI kicks in immediately after CartForm.ACTIONS.LinesAdd, so your cart state temporarily shows the added line and updated quantity.

• After the server processes the cart update, your app fetches the real cart state from Shopify.

• If your server-side action or Shopify's API returns a cart with totalQuantity: 0, the optimistic state gets replaced by that empty cart, causing prices to show $0 and quantity to flip back.

Typical Causes for This

• The cart session is not properly persisted between client and server (Hydrogen relies on cart cookies or session).

• Your server-side cart.addLines() call might be using an empty or expired cart ID, causing Shopify to create a new empty cart silently.

• The cart context on the server side might be missing or not properly wired, so your cart object in the action is invalid.

Things You Can Try and Look Into

• Verify Cart Context Setup

( Ensure that the action receives a valid cart from your server-side context )

export async function loader({ request, context }: LoaderFunctionArgs) {
  const cart = await getCartFromRequest(request, context);
  return json({ cart });
}

• Confirm Cart Session is Present

  In browser DevTools:

  -- Check cookies for cart or similar session identifier

  -- Ensure the same cart ID is used between optimistic state and server response

• Log Cart ID and Server Cart Object ( Make changes to your action to record what occurs: )

export async function action({ request, context }: ActionFunctionArgs) {
  const { cart } = context;
  console.log('Cart context on server:', cart);

  const formData = await request.formData();
  const { action, inputs } = CartForm.getFormInput(formData);

  if (action === CartForm.ACTIONS.LinesAdd) {
    const result = await cart.addLines(inputs.lines);
    console.log('Cart result from server:', result.cart);
    return result;
  }
}

Search for:

1. is cart.id valid?

2. Is totalQuantity correct on the server response?

3. Are prices correct?

• Force Re-Fetch After Add

useEffect(() => {
  refetchCart();
}, [optimisticCart?.totalQuantity]);

(Replace refetchCart with your method for forcing a fresh cart query from server after changes)

Additionally : Try a Clean Storefront

Sometimes old cookies or dev server cache cause weird cart behavior. Try:

• Clear cookies

• Restart dev server

• Test in private/incognito window

I hope this is useful! If you need assistance debugging logs or reviewing your server-side context configuration, please let me know.