Cross-Domain Tracking Not Working

If you’re seeing users show up as new visitors after crossing between two of your domains, here are the most common causes and how to fix them. Cross-domain tracking in GA4 relies on the _gl URL parameter carrying the client ID and session state between domains. Every fault here is a failure to either generate, propagate, or read that parameter.

Note

Before debugging, confirm you actually have two separate registrable domains. shop.example.com and account.example.com are subdomains of the same registrable domain and share cookies automatically — cross-domain setup is not required, and configuring it can cause harm.

1. Google Tag not configured with the domain list

Verify. Open the Google Tag in GTM (the one with your G-XXXXXXX). Click Configuration settings → Configure your domains. If the domain list is empty or missing the partner domain, _gl will never be appended to outgoing links.

Fix. Add every domain involved in the cross-domain flow — both your primary domain and every partner. Use bare hostnames (example.com, partner.com), not URLs. Save and republish.

2. Measurement ID mismatch between the two sites

Verify. Both sites must send to the same GA4 property (same G-XXXXXXX). If Site A fires G-ABC123 and Site B fires G-XYZ789, cross-domain stitching can never work — they are two separate properties.

Fix. Consolidate. Either both sites point at the same Measurement ID, or you accept that they are two different audiences. Web stream IDs within a property are fine to differ — what must match is the property.

3. _gl parameter stripped by a server-side redirect

Verify. Open the cross-domain link in a clean browser with DevTools → Network → Preserve log. Click the link. You should see ?_gl=... on the outbound URL. Follow each redirect in the chain (301/302 rows) and check whether _gl survives. If you see https://partner.com/?_gl=1*abcd... followed by https://partner.com/ (no query string), a redirect stripped it.

Fix. Update the redirect logic to preserve query parameters. Nginx: rewrite ^/old$ /new$is_args$args permanent;. Apache: add [QSA] to the RewriteRule. CDN/edge redirects (Cloudflare, Fastly) usually have a “preserve query string” setting — enable it.

4. Legacy linker config on gtag (not the Google Tag UI)

Verify. If the site still uses raw gtag('config', 'G-XXXXXXX', {...}) outside of GTM, the linker object must be configured explicitly: linker: { domains: ['partner.com'], accept_incoming: true }. Missing accept_incoming means the destination site generates its own cookies instead of reading _gl.

Fix. Add accept_incoming: true. Or better, move the configuration into the Google Tag UI in GTM and delete the raw gtag config to avoid the two disagreeing.

5. Missing accept_incoming on the destination

Verify. Outgoing link from Site A correctly has _gl=1*.... Destination Site B ignores it and generates a fresh client ID. In DevTools on Site B, check _ga cookie value after arrival — if it doesn’t match the decoded client ID in _gl, accept_incoming is off.

Fix. In the Google Tag on Site B, enable accepting cross-domain linker parameters. In the Google Tag UI it’s on by default when you add the domain — in raw gtag it requires accept_incoming: true.

6. SameSite cookie policy breaks on cross-domain redirect chains

Verify. A cross-domain redirect chain (A → B → A) with Set-Cookie: SameSite=Strict on the _ga cookie fails on the return leg. Modern browsers (Chrome 80+, Safari 16+) drop Strict cookies on cross-site navigations. This manifests as the client ID reverting to a new value after the round trip.

Fix. Ensure GA4’s own cookies use SameSite=Lax (the default since gtag.js 2020+). If your own server sets a session cookie that participates in tracking, it must be SameSite=Lax for top-level navigations, or the flow breaks.

7. URL parameter blocked by a campaign-tagging rule

Verify. GA4 → Admin → Data streams → pick stream → Configure tag settings → List unwanted referrals / exclude URL query parameters. If _gl was added to the excluded parameter list (someone thought it was PII), the incoming URL has the data stripped before GA4 reads it.

Fix. Remove _gl from any exclusion list. _gl is not PII — it’s a hashed client-ID transport.

Other things that look like cross-domain failures but aren’t

• Iframes — an iframe embed is not cross-domain navigation. Cross-domain config doesn’t apply. See Iframe Tracking.
• Hash-based redirects — #_gl=... fragments are not sent to the server, and some JavaScript navigators strip them. Use query-string _gl, not fragment.
• Referral exclusion missing — even with _gl working end-to-end, if the partner domain isn’t in your Configure your domains list, returning from the partner creates a new session (referral). Configure both directions.

Related articles

Cross-Domain Tracking Setup The full happy-path setup, which is the starting point before troubleshooting.

Cross-Domain Setup Recipe Step-by-step recipe including subdomains and multi-country setups.

Subdomain Tracking When the 'two sites' are actually subdomains — different rules apply.

Why GA4 Shows 'Unassigned' Traffic The session arrives but channel grouping falls over — a related symptom.