First-Party Cookies

What are first-party cookies?

First-party cookies are cookies set by the website domain a user is actively visiting. When you log into a site and it remembers your session on your next visit, that is a first-party cookie at work. When an analytics platform tracks your pageviews within a single site, it typically relies on a first-party cookie to identify you as the same visitor across pages and sessions.

The "first-party" distinction is about origin: the cookie's domain matches the domain in the browser's address bar. This is in contrast to third-party cookies, which are set by external domains (ad networks, social widgets, tracking pixels) embedded on the page. First-party cookies are widely considered less invasive because they only function within the context of the site the user chose to visit.

Why it matters

First-party cookies are the foundation of website functionality and analytics. Without them, every pageview would look like a brand-new visitor, login sessions would break on navigation, and shopping carts would empty between pages.

For marketing teams, first-party cookies are increasingly important for a specific reason: they are the last reliable identifier standing. As browsers phase out third-party cookies and privacy regulations tighten, first-party cookies remain the primary mechanism for:

• Session continuity — Identifying a returning visitor across multiple sessions over days or weeks.
• Accurate analytics — Distinguishing unique visitors from repeat pageviews.
• Attribution — Connecting an ad click to a conversion that happens hours or days later.
• Personalization — Remembering user preferences, language settings, and content choices.

However, not all first-party cookies are equal. Cookies set via JavaScript in the browser (client-side) are subject to browser restrictions that significantly limit their lifespan.

How first-party cookies work

1. Cookie creation — When a user visits a website, the server (or a client-side script) sets a cookie on the site's domain. The cookie contains an identifier and optional metadata.
2. Storage — The browser stores the cookie locally, associated with the issuing domain.
3. Subsequent requests — On every following request to that domain, the browser automatically includes the cookie in the request headers, allowing the server to recognize the visitor.
4. Expiration — The cookie persists until its expiration date or until the user clears browser data. The maximum lifespan depends on how the cookie was set and which browser is being used.

Client-side vs. server-side first-party cookies

The distinction between client-set and server-set first-party cookies is critical. Safari's ITP caps JavaScript-set cookies at 7 days, meaning any analytics or identity cookie set via a tag manager or client-side script will expire within a week. Server-set cookies issued via an HTTP response header from a first-party subdomain are not subject to this restriction.

How Ingest Labs handles first-party cookies

Ingest Labs sets its identity cookie (MPID) server-side via an HTTP response header on the customer's own subdomain (e.g., collect.yourdomain.com). Because it is a genuine server-set first-party cookie, the MPID maintains an approximately two-year lifespan across all browsers — including Safari with ITP and Firefox with Enhanced Tracking Protection. This gives marketing teams a durable, privacy-compliant identifier that persists far longer than any client-side cookie alternative.