Google just changed the rules and didn’t tell anyone.

In early May, without any announcement or warning, Google silently altered the format of one of GA4’s session-related cookies. If your systems depend on GA cookie parsing, even indirectly, this matters. A lot.

What Changed?

Specifically, the format of the ga<container-id> cookie in Google Analytics 4 (GA4) has shifted.

Old format:‍

GA1.2.123456789.987654321

(Structured, parseable, and familiar)

New format:

GS2.1.s1823456789$o2$g1$t1823456890$j1$l1$h1

(Looks like someone spilled coffee on the keyboard)

This change moves GA4 session cookies from the GS1 format to GS2, a more complex, opaque structure.

Why It Matters

If your analytics or marketing stack directly references GA cookies for Client ID, Session ID, or attribution logic, this update can silently break things across the board. That includes:

• Data pipelines that extract client/session info from cookies
• CRM integrations pulling GA cookie data
• Attribution models based on client/session IDs
• Server-side tagging setups using cookie parsing
• Measurement Protocol events
• CDPs like Segment, RudderStack, etc.

In other words: if you’re manually reading GA cookies, it’s time to stop or update your logic now.

Implications for Launch Labs

At Launch Labs, we do capture GA session IDs behind the scenes as part of our tracking pipeline, especially to associate website sessions with offline events, like follow-up calls, in-store visits, or offline conversions.

As of Tuesday, May 20, all of our systems have been updated to:

• Correctly interpret the new GS2 cookie format
• Fall back to previously cached versions using the older GS1 format (where still available)

This means:

• No disruption to attribution
• Offline event tracking remains accurate
• No developer action is needed by our partners or clients

We’re actively monitoring cookie behaviors to ensure continued reliability and we’ll make proactive adjustments as needed.

What’s Safe (For Now)

Thankfully, the original _ga cookie used to store Client ID only, remains unchanged and continues to use the familiar format:

GA1.1.123456789.987654321

Why this matters:

• This cookie powers many legacy setups.
• It’s essential for user ID stitching in CRMs, custom dashboards, and backend systems.
• Breaking it would disrupt millions of websites using older or hybrid GA implementations.

Because of its stability and widespread usage, it’s very unlikely Google will change the _ga format anytime soon. But that doesn’t mean you should rely on it forever.

Takeaways & Action Steps

• Audit your cookie dependencies. If you’re reading GA4 cookies directly, verify that your code supports the new GS2 structure.
• Review server-side tagging or custom integrations — especially if they rely on GA cookie parsing.
• Avoid relying on raw GA cookie formats moving forward. Use official GA APIs or gtag/gtm variables whenever possible.
• Monitor any attribution, session, or user stitching issues starting in early May 2025 — they might trace back to this change.

Learn More

For a deeper technical breakdown of the GS1 → GS2 format change, check out this excellent write-up by Thyngster.