First-Party vs Third-Party Cookies

A first-party cookie is set by the domain the visitor is actually on; a third-party cookie is set by a different domain embedded in that page. Browsers treat the two very differently. Safari and Firefox have blocked third-party cookies by default for years, and Chrome has spent years restricting them, while first-party cookies continue to work normally everywhere.

For affiliate tracking the consequence is direct. A program whose tracking cookie is set by a third-party tracking domain loses attribution on a large share of traffic, silently. A program that redirects the click through the merchant's own domain, or a subdomain of it, sets a first-party cookie that the browser keeps — which is why the redirect architecture matters more than the cookie duration.

This is the most common invisible cause of a partner earning less than their traffic should produce. Nothing looks broken: clicks are recorded, conversions simply never appear, and the partner concludes the audience did not buy.

You cannot fix it from outside, but you can notice it. A conversion rate that collapses specifically on mobile and in-app traffic, while holding up on desktop, is the classic fingerprint.

Moving the click redirect onto the brand's own domain converts the tracking cookie from third-party to first-party and recovers attribution that was being lost. It is usually a DNS and configuration change rather than a rewrite.

First-party cookies are not immune either — Safari caps the lifetime of some script-set cookies to days rather than months, which can silently truncate a long cookie window. Setting the cookie server-side on an HTTP response avoids that cap, and a server-to-server postback avoids the question entirely.