Getting 'Error 431: Request Header Fields Too Large' – How Do I Fix This? or What Causes Error 431 an

Fun fact: error 431 can pop up if you’ve got too many cookies stored for a site. Like, WAY too many.

Clear ‘em manually (not just cache):
Chrome: `chrome://settings/siteData` → search the site → nuke all cookies.

Still no luck? Try accessing the site in a fresh browser profile. Sometimes Chrome’s profile gets weirdly bloated.