# Web Experiment Bucketing ID Shared Across Subdomains — Amplitude Changelog

Web experiments now use a bucketing ID stored in a cookie, allowing sharing across subdomains, improving consistency in bucketing.

Source: https://amplitude.com/en-us/releases/web-experiment-bucketing-id-shared-across-subdomains

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[Changelog](/releases)/Web Experiment Bucketing ID Shared Across Subdomains

ExperimentJul 13, 2026

# Web Experiment Bucketing ID Shared Across Subdomains

Web experiments now use a bucketing ID stored in a cookie, allowing sharing across subdomains, improving consistency in bucketing.

## What it is

Under the hood, web experiment maintains its own `device_id` (stored as `web_exp_id` and `web_exp_id_v2`) to use for bucketing, instead of using Amplitude Analytics' device id. This was implemented to ensure a bucketing ID would be immediately available on web experiment initialization and to simplify implementation for customers using third-party analytics trackers.

Previously, this bucketing ID was only stored in `localStorage`, scoped to each subdomain, meaning each subdomain had its own bucketing ID.

Now, for web experiments created after 13th July 2026, the bucketing ID will be stored in a `cookie`, which can be shared across subdomains.

Release details

- Release ID

  web-experiment-bucketing-id-shared-across-subdomains

- Shipped

  Jul 13, 2026

- Area

  Experiment

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