I am following the official Shopify Agents checkout tutorial, including authentication, product search, cart creation, and then invoking create_checkout on the discovered UCP MCP endpoint from getMcpEndpoint(origin). Cart creation works, but create_checkout returns AuthenticationFailed with Missing required buyer IP header; I tried adding dev.ucp.buyer_ip / dev.ucp.user_agent signals to create_cart, but the same error remains.
Hey @Milan_Jakara - thanks for getting in touch. Would you happen to have the request ID from the response headers for the checkout creation call on hand? Happy to look into our logs and see if I can assist here - hope to hear from you soon!
Hi @Alan_G. I managed to solve this by adding Shopify-Buyer-IP header in POST request to the merchant’s UCP endpoint.
Glad to hear you figured this out @Milan_Jakara !
Let us know if you run into any other issues 
i had same issue, and i resolved it with the header in POST, but i hit now this:
| Internal error calling tool create_checkout: Core client error | ||
|---|---|---|
i put the correct json body with the id of ProductVariant and buyer info, but i get this error above, same error i get with the create_cart tool.
anyone resolved this issue?