The FPID cookie in server-side tagging for Google Tag Manager is an HttpOnly, server-managed ID cookie that’s designed to replace the JavaScript-managed _ga cookie used by Google Analytics 4 and Universal Analytics.
For more details about the cookie itself, check out my previous article on FPID.
In that article, I mentioned one caveat for adopting FPID being the fact that cross-domain tracking will not work.
I mean, how could it? FPID is an HttpOnly cookie, which means it’s not available to JavaScript in the browser.
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