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Issue reference

Updating the checkout through browser tools fails

Issue code
WEBMCP_UPDATE_CHECKOUT_FAILED
Stage
Any
Protocol
WEBMCP
Severity
High

What it means

It concerns WebMCP, the browser agent checkout. The Web Model Context Protocol. Your pages tell a browser agent which actions they support — Shopify's get_checkout and complete_checkout, for example — so it calls them instead of guessing at buttons.

What it does to an agent

Most agents fail at this point. The few that find a way round it spend more time and model calls doing so, and may give up before checkout.

How to fix it

Accept the checkout object get_checkout returns as update_checkout's input. An agent edits what it read, so a tool that rejects its own output cannot be used.

Check the fix

Run the check that reported WEBMCP_UPDATE_CHECKOUT_FAILED again after the change. The scan needs no account; an issue found during an AI shopping test needs that test run again.

Terminal
npx @tragent/cli scan https://your-store.com