Important Update: Google Analytics 4 Session Identifier Format Transition

Data analysts and digital marketing professionals leveraging Google Analytics 4 (GA4) should be aware of a recent modification to the structure of session identifiers. Commencing in early May 2025, GA4 transitioned the format of the ga* cookies, which are instrumental in tracking user session data, from the previous GS1 standard to a new GS2 format. This update was implemented without prior public notification.

The legacy GS1 format utilized a period (.) as a delimiter to segment distinct data points within the session identifier string. This structure maintained a fixed positional arrangement for each data element, such as the session count. Conversely, the newly implemented GS2 format is characterized by the prefix "GS2" and employs a dollar sign ($) to delineate key-value pairs. Notably, the client ID is no longer directly embedded within the cookie value in this revised structure.

For comparative purposes, the structural differences are highlighted below:

Previous Format (GS1): GS1.1.1690013078.2.1.1690013090.0

Current Format (GS2): GS2.1.s1747323152$o28$g0$t1747323152$j60$l0$h69286059

Feature	GS1 Format	GS2 Format
Starting Prefix	GS1	GS2
Main Delimiter	Period (.)	Dollar Sign ($)
Structure	Fixed positional	Key - Value Pairs (Undocumented Keys)

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Rationale for the Format Modification:

While official documentation detailing the impetus for this change has not been extensively disseminated, the adoption of the GS2 format likely stems from strategic considerations aimed at enhancing the platform's adaptability and maintainability. The key-value pair architecture inherent in the GS2 format offers several potential advantages:

• Enhanced Extensibility: This structure facilitates the future integration of additional data fields without necessitating fundamental alterations to the core format, thereby promoting forward compatibility.

• Improved Intrinsic Clarity: The explicit key-value pairings contribute to a more self-documenting structure, potentially streamlining internal debugging and data interpretation processes.

• Robust Version Management: The explicit "GS2" prefix enables more effective parallel management of different cookie versions, facilitating smoother transitions during future platform updates and ensuring a degree of backward compatibility.

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Implications for Existing Implementations:

The transition to the GS2 format carries significant implications for any custom implementations that rely on direct parsing of the ga* cookies to extract session-level or client-level data. This includes:

• Third-party analytics platforms and browser extensions that access GA4 cookie data.

• Server-side analytics tracking systems that depend on cookie information.

• Custom reporting tools or dashboards that process raw cookie values.

• Customer Relationship Management (CRM) and advertising platforms that integrate with GA4 data via direct cookie access.

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Recommended Mitigation Strategy:

To ensure the continued functionality and accuracy of data retrieval, it is strongly recommended to adopt Google's officially supported method for accessing session and client identifiers: the gtag('get', ...) API. This programmatic interface provides a stable and future-proof mechanism for obtaining these critical data points, abstracting away the underlying cookie structure.

The following JavaScript code snippet demonstrates the retrieval of the client ID using the gtag('get', ...) API:

gtag('get', 'YOUR_GA4_MEASUREMENT_ID', 'client_id', function(clientId) {
  console.log('Client ID:', clientId);
});

(Replace YOUR_GA4_MEASUREMENT_ID with your specific GA4 property identifier.)

For users of Google Tag Manager (GTM), leveraging community-developed solutions such as Simo Ahava's GTAG GET API Template offers a streamlined approach to accessing these identifiers without direct cookie manipulation.

While adapting existing parsing logic to accommodate the new GS2 format might appear to be an immediate solution, it is generally discouraged due to the inherent volatility of undocumented internal structures. Reliance on official APIs ensures greater resilience against future platform updates.

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Concluding Remarks:

The recent transition in the GA4 session identifier format from GS1 to GS2 necessitates a review of any custom implementations that directly interact with the ga* cookies. To maintain data integrity and ensure long-term stability, migrating to the officially supported gtag('get', ...) API is the recommended course of action. This proactive adjustment will mitigate the risk of data acquisition failures resulting from future modifications to the underlying data storage mechanisms within Google Analytics 4.

I encourage data professionals to assess their current GA4 integrations and implement the necessary updates to ensure seamless data flow and accurate reporting.