Why the favicon matters in search
Google shows your favicon next to the site name on every result. A clear icon makes your result easier to spot and remember. A missing or broken one shows as a grey globe.
Google uses one favicon per hostname, taken from the homepage. That's why this tool always checks your homepage, even if you paste the address of another page.
What Google needs
A <link rel="icon"> tag in the homepage's <head> that points to the icon. The icon must be square (1:1), at least 8×8 pixels, and Google recommends larger than 48×48. SVG works at any size. Both the homepage and the icon file must be reachable for Google's crawlers, so robots.txt must not block them.
Keep the icon's address stable. When you change it, Google picks it up the next time it crawls your homepage, which can take days or weeks.
What this tool checks
It reads every icon tag on your homepage plus the default /favicon.ico, downloads each file, reads its real format and size from the file itself (not from the sizes attribute), checks robots.txt for Googlebot and Googlebot-Image, and picks the icon that Google is most likely to use.