UCP profile answers on apex and www

What we check

If your store is at shop.example we ask www.shop.example for /.well-known/ucp, and if it is at www.shop.example we ask shop.example. We compare that answer with the profile on your own host: its status, whether it redirected, and its ucp.version.

Why it matters

An agent may be sent to either host. If one of the two has no profile, an agent sent there finds none. UCP Checker notes that the spec says a profile must not redirect, and lists an apex and www redirect as something to fix.

How we check

We read the other host's robots.txt first and ask for its profile only if robots.txt allows it. If that robots.txt redirects to your own host, we use your own robots.txt, which the scan already read. That is two reads at most (robots.txt, then the profile), plus any redirects they follow. PASS: both hosts answer 200 with the same version and no redirect. WARN: the other host redirects (score 70) or serves a different version (score 50). FAIL: it answers 404 or 410, its HTML page, or a document with no ucp.version. SKIPPED: there is no profile, your store runs on its own subdomain or has no public domain, your profile has no ucp.version, or we could not read the other host.

How to fix it

1. Serve the same /.well-known/ucp on your bare domain and on www, as application/json with a 200 and no redirect.
2. If one host forwards everything to the other, exempt /.well-known/ucp from that redirect.

What good looks like

GET https://shop.example/.well-known/ucp      200 application/json
GET https://www.shop.example/.well-known/ucp  200 application/json
(both with "version": "2026-08-25")

Sources

• UCP 2026-08-25 specification: overview
• UCP Checker: how we check

Read more

• UCP explained

  The Universal Commerce Protocol: the profile a store publishes at /.well-known/ucp, the capabilities it declares there, and the checkout flow from a new basket to a placed order.