Cross-domain tracking issues
A single user crossing from one domain to another (site to a separate checkout or booking host) should stay one user and one session. Without cross-domain tracking, the second domain starts a fresh session and often a self-referral, double-counting users and breaking attribution. This page explains how the GA4 linker passes the client ID and the common reasons it does not arrive.
What this means
GA4 configures cross-domain measurement in the data stream settings. When set, GA4 decorates links and forms to a configured domain with a `_gl` linker parameter carrying the client ID, so the destination continues the same session and user.
When the destination domain is not in the configured list, or the link is built in a way the SDK cannot decorate, the parameter never arrives and the second site mints a new client ID.
Where it fails
Common failures: the destination domain is missing from cross-domain settings; redirects strip the `_gl` parameter; links are opened via JavaScript the tag cannot intercept; or the second property uses a different measurement ID entirely. Each produces split users and a self-referral from the originating domain.
- Destination domain not listed in cross-domain settings
- Redirect chains drop the linker parameter
- Different measurement IDs on each domain
- Originating domain appears as a referrer on the destination
How it appears in analytics and logs
If sessions jump and the second domain shows the first as a referrer, the client ID is not crossing — the linker is missing, the destination is not configured, or the parameter is being stripped.
Diagnostic use case
Diagnose split sessions and self-referrals when traffic moves between domains, and confirm the linker parameter is decorating outbound links.
What WebmasterID can help detect
WebmasterID can show whether the second domain is appearing as a referrer to the first — a direct symptom of a broken cross-domain handoff — without cross-site cookies.
Common mistakes
- Forgetting to add the destination domain to cross-domain settings.
- Letting redirects strip the linker parameter.
- Not adding the partner domain to the referral exclusion list.
Privacy and accuracy notes
Cross-domain tracking in GA4 passes a client ID in the URL, not personal data; it stitches the same browser's visit. WebmasterID stitches first-party without exposing identifiers across sites.
Related pages
- Self-referrals and lost attribution
A self-referral is when your own site shows up as a referring source in your reports. It usually means a session was broken and a new one started attributed to your domain, often when a visitor crosses subdomains or returns from a payment provider. Self-referrals fragment sessions and steal credit from the real source. This page explains the causes and the fix.
- Subdomain tracking issues
Subdomains under the same registrable domain (blog.example.com, shop.example.com) typically share a first-party cookie set on the parent domain, so a user stays continuous. Problems arise when the cookie domain is scoped too narrowly, when subdomains use separate properties, or when one subdomain appears as a referrer to another. This page distinguishes subdomain handling from true cross-domain tracking.
- Referrer exclusion list mistakes
GA4's unwanted-referrals (referrer exclusion) configuration tells analytics not to treat certain domains — payment gateways, SSO providers, your own domains — as new traffic sources. Get it wrong and you either fragment sessions (under-listing a gateway) or erase real referrers (over-listing). This page explains the mechanism and the two-sided error.
- Attribution Analytics
See where multi-domain journeys break attribution.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.