A visitor browses your site, clicks through to checkout on a separate domain, and completes a purchase. If cross-domain tracking isn't configured, GA4 can quietly record that as two unrelated sessions from two different people.
GA4 identifies a visitor using a client ID stored in a cookie, scoped to the domain that set it. The moment a visitor moves to a different domain or subdomain — a separate checkout provider, a booking system, a payment processor — that cookie doesn't automatically travel with them, because cookies are domain-scoped by design. Without specific configuration telling GA4 these domains belong to the same journey, the visitor gets a new client ID on the second domain, and GA4 has no way to know it's the same person.
This isn't just a reporting inconvenience — it breaks attribution specifically at the moment that matters most: conversion. A visitor who arrived from a Google Ads click, browsed your site, then converted on a separate checkout domain can show up as an anonymous, sourceless session on the checkout side, disconnected entirely from the ad click that actually brought them in. Ad platforms and analytics reports end up showing incomplete or misattributed conversion data for real, paying customers.
_ga cookie value before and after the domain change — the client ID should stay the sameWe test whether the client ID actually carries across domains as a best-effort check on every audit.
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