First-Party Cookie

First-party cookies are set by the site in the address bar — the operator’s own domain or a branded tracking domain — and are the only cookie class modern browsers reliably keep.

Limits you must design around

Safari’s ITP caps script-set first-party cookies at 7 days (24 hours in some contexts), so a 30-day attribution window promised to affiliates cannot rest on cookies alone. Set cookies via server HTTP headers where possible, persist the click ID server-side, and treat the cookie as a convenience layer, not the system of record.