How to enable cross-domain tracking in Google Analytics 4 (GA4)?

This manual explains how to enable cross-domain tracking in Google Analytics 4 (GA4) using query parameters in the URL.
👉What data can be collected from GA4?

Cross-domain tracking allows you to track users as they move across different domains while maintaining a single Analytics session and avoiding duplicated data.

How do you activate cross-domain tracking?

Add the domains parameter to the URL
To enable cross-domain tracking, add the domains parameter to the URL, including the list of domains separated by commas.

Correct domain format
The domains included in the domains parameter must meet the following conditions:

• Separated by commas and without spaces (domain1.com,domain2.com)
• No protocol included (http:// or https://)
• Subdomains can be included (sub.example.com)

System behaviour

• Configuration priority
  The system applies the cross-domain tracking configuration following this priority order:
  
  1. domains query parameter (highest priority)
  2. google-ga4-domains session value
  3. Persistent configuration

• Session persistence
  When the domains parameter is received via URL:
  
  • It is automatically stored in the session.
  • It is reused on subsequent pages for the duration of the session.
  • It can be overwritten by sending a new value via the query parameter.

Verifying that it works:

• From the browser
  
  1. Open the developer tools (F12).
  2. Go to the Network tab.
  3. Filter by gtag or collect.
  4. Check that the request contains the _gl parameter
     (this indicates that the GA4 linker is active).
• From the JavaScript console
  

  Verify that events have been generated with the domain configuration by running:

  console.log(window.dataLayer);

💡 Important notes

• The domains parameter must be present on both involved domains.
• Domains must be correctly written (no spaces, no protocol).
• All domains must use the same GA4 tracking code.
• The value stored in the session is reused automatically.
• You can override the configuration by adding a new value via the query parameter.