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.