Partitioned cookies (CHIPS) in depth
Partitioned cookies, standardised as CHIPS (Cookies Having Independent Partitioned State), let a cookie opt into per-top-level-site storage with the Partitioned attribute. A cookie set by an embedded third party is then stored under a partition key tied to the top-level site, so the same third party cannot read it across different sites. This preserves legitimate cross-site embeds while removing the cross-site tracking ability. This page is educational.
How CHIPS partitioning works
Normally a third-party cookie is stored in one jar keyed only to the cookie's own domain, so the same embed reads it on every site — the basis of cross-site tracking. With CHIPS, a cookie that carries the Partitioned attribute (and Secure) is stored under a key that includes the top-level site. The embed on news.example can set a partitioned cookie, but when it loads on shop.example it sees a separate, empty partition. The use cases are legitimate same-embed state like load balancing or session continuity within one site.
What it does and does not change
CHIPS is part of the broader move to phase out unpartitioned third-party cookies: embeds that need per-site state can keep working, while cross-site identity joining via cookies is removed. It does not anonymise anything — a partitioned cookie still identifies a returning user within the same top-level site, which is exactly what a first-party analytics cookie does too. And it does not replace cross-site advertising measurement; the Privacy Sandbox APIs target that separately. Treat CHIPS as scoping, not anonymisation.
The attribute is defined in the CHIPS specification and documented on MDN.
- Partitioned attribute keys the cookie to the top-level site
- Preserves legitimate same-embed state across navigations
- Removes cross-site linkage but not within-site identification
How it appears in analytics and logs
If a third-party embed sets a Set-Cookie with Partitioned, its cookie is isolated per top-level site; it cannot link the same user across unrelated sites.
Diagnostic use case
Understand how the Partitioned attribute lets embedded services keep state per top-level site without enabling cross-site tracking of the same user.
What WebmasterID can help detect
WebmasterID's first-party, cookieless approach does not rely on third-party cookies; CHIPS shows how browsers isolate the cross-site cookies it avoids.
Common mistakes
- Thinking partitioning makes a cookie anonymous.
- Expecting CHIPS to replace cross-site ad measurement.
- Forgetting Partitioned cookies must also be Secure.
Privacy and accuracy notes
This page is educational, not legal advice. Partitioning reduces cross-site linkage but a cookie still identifies a user within one top-level site.
Related pages
- Storage partitioning and CHIPS
Storage partitioning keys client-side storage (cookies, localStorage, caches) by the top-level site, so an embedded third party cannot use the same storage to recognise a user across different sites. CHIPS (Cookies Having Independent Partitioned State) lets a cookie opt into per-site partitioned storage. This page explains both and their effect on analytics.
- Third-party cookie phase-out: current status
The phase-out of third-party cookies has unfolded unevenly across browsers. Safari (ITP) and Firefox (Total Cookie Protection) block or partition third-party cookies by default. Chrome originally planned to remove them via the Privacy Sandbox, but in 2024-2025 changed course toward a user-choice prompt rather than automatic deprecation. This page describes the data-model consequences even-handedly, not a timeline or winner. This page is educational.
- Chrome Privacy Sandbox and analytics
The Privacy Sandbox is a set of Chrome web-platform APIs intended to support advertising and measurement use cases without cross-site tracking of individuals. It includes interest-based targeting, conversion measurement, and anti-abuse APIs that return aggregated or noised results rather than per-user identifiers. This page maps the pieces and what they mean for analytics.
- Privacy-first analytics
First-party measurement does not depend on third-party cookies.
Sources and verification notes
- MDN — Set-Cookie Partitioned attribute (CHIPS)Primary documentation for the Partitioned attribute.
- CHIPS explainer (Privacy CG)Specification explainer for partitioned cookies.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.