In early May 2025, Google silently changed the format of a key GA4 session cookie (_ga_<property-id>) without prior notice. This cookie, central to user and session tracking, shifted from a simple dot-separated string to a labeled, dollar-sign-delimited format (e.g., GS2.1.s1747323152$o28$g0$t1747323152$j60$l0$h69286059). While the update improves flexibility for Google, it broke countless analytics setups that assumed the old format causing data gaps, broken tracking, and inaccurate attribution.

What Changed

• Old Format: GA1.1.860784081.1732738496 (fixed position, dot-separated)
• New Format (GS2): GS2.1.s<session_id>$o<session_num>$g<engaged_flag>$... (key=value style, $-separated)

The new structure is more extensible and readable, using prefixed segments like s, o, t for session ID, session number, and timestamps, respectively. But Google offered no heads-up leaving many organizations scrambling.

Who’s Affected by the GA4 Cookie Format Change?

While the transition from the GS1 to GS2 cookie format in Google Analytics 4 (GA4) is seamless for most users, some developers and analysts need to take action – especially those working with cookies at a technical level. Here’s who should pay attention:

You’re Affected If:

• You Directly Parse GA4 Cookies in Your Code
• You Use Measurement Protocol with Cookie-Derived Data
• You Have Custom Integrations (e.g., CRMs, Analytics Layers, Server-Side Tracking)

You’re not Affected If:

• You’re Using Standard GA4 Tags (gtag.js or Google Tag Manager)

Immediate Actions to Take

1. Audit Cookie Parsing: Search for any system parsing _ga_<id> cookies directly. Fix logic to recognize GS2 format.
2. Patch Tracking Code: Use GA4 APIs (e.g. gtag('get', ...)) instead of reading cookies manually.
3. Test & Monitor: Verify sessions, users, and conversions are tracking properly. Look for spikes in “(not set)” traffic post-May.
4. Flag Lost Data: Annotate reports from early May to clarify possible data gaps or inaccuracies.

Long-Term Fixes

• Stop Parsing GA Cookies Directly: Treat GA cookies as opaque. Use official APIs.
• Adopt Server-Side Tagging: Centralize control, reduce exposure to client-side changes.
• Use First-Party Identifiers: Implement GA4’s User-ID and tie GA data to internal user/session IDs.
• Make Data Pipelines Format-Aware: Add version detection (GS1 vs GS2) to ETL logic.
• Stay Informed: Monitor Google changelogs, forums, and analytics blogs to catch future changes early.

Conclusion

The GS2 format represents more than a simple technical change—it’s a significant improvement in how GA4 handles session tracking. Its structured, extensible design lays the groundwork for more reliable and adaptable analytics moving forward.

If you're using custom or manual implementations, now is the time to update your tracking logic. For those leveraging Google’s built-in Conversion tools, the transition happens automatically and requires no action.