Multi-merchant cart/checkout in UCP — is this on the roadmap?
We’re building a multi-brand storefront on Shopify (STAASH) — shoppers add products from several independent Shopify-store brands into one cart, checking out once. Each brand is its own Shopify store/merchant of record; we sit above them as the unified storefront layer.
Testing against UCP (via the ucp-cli, and directly against the Cart/Checkout MCP endpoints), every example we’ve found — in the docs, in this forum, and in third-party UCP guides — assumes a single agent, single merchant relationship: one cart belongs to one Shopify store. We haven’t found documentation or a working pattern for a cart that spans multiple independent Shopify stores in one checkout session.
Specific questions:
- Is a multi-merchant cart (line items from more than one Shopify store, one buyer authorization, one checkout) supported today, or planned? We couldn’t find this in the current capability/checkout spec.
- If it’s not natively supported, is the expected pattern to run separate UCP checkout sessions per merchant and stitch them together at the platform layer (multiple agent-to-merchant negotiations instead of one), rather than a true single cart?
- For payment: does UCP’s checkout capability support splitting a single buyer authorization across multiple merchant payment handlers/accounts (similar to how Shopify’s own Route-style split-payment marketplaces work today), or would each merchant need its own separate payment negotiation entirely?
This seems like a fairly fundamental gap for any multi-brand marketplace model built on Shopify — curious if others building similar platforms have found a working pattern, or if this is something the UCP team is actively scoping. Happy to share more detail on our specific setup if useful.