The fix for when your funnel breaks across two domains

The fix for when your funnel breaks across two domains

A marketplace ran its catalog on one domain and checkout on another. GA4 showed checkout.example as the top "referral source" and attribution was nonsense. The handoff was severing every session.

The cross-domain playbook:
— Confirm both domains share one GA4 data stream, not two
— Add both to Admin, Data Streams, Configure tag settings, list domains
— Verify the _gl linker parameter appears on the cross-domain links
— Walk a purchase yourself and confirm one unbroken session, one client ID
— Add the partner domain to unwanted referrals as a backstop

Before the fix, 38% of sessions split in two at the handoff, orphaning attribution. After, those sessions stayed whole and the self-referral vanished from the report.

Paid campaigns that looked dead suddenly showed their real checkout revenue.

Lesson: when checkout shows up as your top referrer, you're not measuring marketing, you're measuring your own broken handoff.