Cross-domain tracking

Cross-domain tracking keeps one visitor recognized as one continuous session when they move between domains you own (typically from your marketing site to a checkout, booking, or payment domain). Because cookies don't cross domain boundaries, the identifier has to be passed explicitly; GA4 does it by appending a linker parameter (_gl) to cross-domain links.

#Subdomains aren't cross-domain, separate domains are

shop.example.com and www.example.com share first-party cookies, so subdomains need no special handling. The trap is the genuinely separate domain (example-checkout.com, a hosted booking or payment page), where the session breaks at exactly the moment of highest intent. The signal impact is nasty because nothing errors: the buyer arrives at checkout as a "new" visitor, the conversion books to Direct or Referral (often as a self-referral from your own site), and ad platforms go blind on precisely your best-converting traffic while every dashboard stays green. Broken cross-domain tracking is a standing entry in any platform-discrepancy diagnosis for stacks with external checkouts.

Buron's first-party pixel sidesteps the subdomain version of this entirely. It sets its cookie at the root domain (.example.com), so one visitor stays recognized across shop., checkout., and www. with no linker and no manual setup. A genuinely separate registrable domain (a standalone checkout domain, not a subdomain) still can't share a cookie by browser rule, so that case needs the client ID passed across. Either way, this failure mode is one of the checks Buron runs continuously: a spike in self-referrals or direct-attributed conversions from a checkout domain becomes a finding, not a mystery.

The diagnosis-first walkthrough (symptoms, confirming it's cross-domain loss, the fix per stack) is Cross-domain tracking: the checkout-domain trap. It also feeds the broader triage in Why your GA4 and Google Ads conversions don't match, and Conversion tracking & signal quality maps the wider territory.