Keep One Session Across Two Sites (Cross-Domain Tracking)

Why this matters

When a user journey spans two domains — your marketing site and a separate checkout or SaaS app — GA4 breaks the session and creates a phantom new user by default, making your funnel look far leakier than it actually is.

Try it yourself

1. In GA4 Admin > Data Streams > Configure tag settings > Configure your domains, add both domains to the cross-domain list and save; then use DebugView on the second domain and verify that the _gl linker parameter arrives in the URL on navigation.
2. Without cross-domain configured, open your checkout domain and inspect the client_id cookie; then enable cross-domain and repeat the journey — confirm the client_id values match across both domains.
3. Check your GA4 Acquisition report for 'self / referral' entries with your own domain as source — their presence before the fix and absence after confirms the configuration worked.

References

• https://support.google.com/analytics/answer/10071811