The Shopify Cart API is not responding correctly inside the Theme Store demos. Users cannot add more than one product to their cart. When attempted, the Shopify API responds with an empty cart. This is causing several downstream effects. For example, if a user adds a Recommended Item to her cart from a PDP, it removes the original item from her cart. If a user adds gift wrapping or an add-on product to her cart, it empties the cart. For clarity, this is only occurring inside the Theme Store demo frame, not on any live demo stores (which is probably why no one has caught it yet or reported it). Further, this is impacting ALL themes, including Horizon and Dawn.
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Hi @Melanie_Moore! Thanks for the detailed report - I tried to replicate this across a few Theme Store demos - I was able to reproduce the behavior on Eurus (adding a different item replaced the existing one, and increasing quantity returned an empty cart), but I couldn’t replicate it on Dawn or Crave.
Before I raise this with the Theme Store team, could you confirm which specific theme demos you’re seeing this on? And if possible, the exact steps you’re taking - are you using the quick add, going through the PDP, or something else? Just want to make sure I can give them a solid reproducible case. Thanks!
Hi @Donal-Shopify I can replicate this error on Dawn, Horizon, Dwell, Stretch, Maravella, and Roam. Please try testing on Safari; try to add more than one item to the cart from anywhere (Quickshop or PDP). I have screen recordings of all of these errors, but I’m not able to upload videos to the forums. If you would like a copy of the screen recordings, please let me know where I can send them.
Thanks for the context @Melanie_Moore! I DM’d you just now so you can forward the screen recording. I’ll get this in front of the Theme Store team shortly so they can investigate why this is happening.
Quick update: I’ve raised this with the Theme Store team internally and will update this thread once I hear back from them 
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See the same in Safari with all of our theme demos (Aurora, Auriga, Stella, Astra, Vega).
Hey @Melanie_Moore and @Ann_Ashen, thanks for the additional context and for confirming this across your themes.
I raised this with the Theme Store team and they’ve confirmed this is a known limitation. The root cause is likely SameSite cookie restrictions - when themes are rendered in the Theme Store demo iframe, browsers (particularly Safari) block the cross-site cookies needed for cart state to persist properly. This is why adding a second item either replaces the first or empties the cart entirely.
The behavior affects all themes equally in the demo view, so it’s not specific to any particular theme.
I understand the frustration here, especially around how this might affect buyer perception during the evaluation process. I’ve logged this as partner feedback internally but I don’t have a timeline to share as to when Demo Store cart behaviour will have parity with that of an actual store.
I’ll update this thread if anything changes on the platform side. Thanks again for reporting!
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