Partitioned cookies (CHIPS)

How it works

The browser stores a Partitioned cookie with two keys: the origin that set it and the site of the top-level page. The top-level site includes the scheme. So the widget on https://shop.example cannot read the cookie that it set on https://news.example.

The cookie cannot follow the user from site to site. MDN lists CHIPS as a way to keep state in a frame when browsers limit third-party cookies. MDN says CHIPS works in the latest browsers since December 2025.

Rules

• A Partitioned cookie must have Secure.
• Use SameSite=None so the browser sends the cookie in a cross-site frame.
• The __Host- prefix keeps the cookie on one host. It needs Path=/ and no Domain. MDN recommends it but does not require it.
• Subdomains of the same top-level site share the partition. A widget on support.shop.example and checkout.shop.example reads the same cookie.

Firefox also partitions third-party cookies by default. CHIPS is the opt-in way, and MDN recommends it for the most compatible partitioned cookies.