HTTP 431 Request Header Fields Too Large means the server refuses to process the request because one or more header fields, or the total header block, exceeds the server's size limit. This often happens with excessively large cookies.
The browser is sending too many or too-large cookies with each request, exceeding the server's header size limit.
The application is setting excessively large custom headers (auth tokens, tracking data).
Delete cookies for the affected domain and try again.
Review the cookies being set and reduce their number or size. Use server-side sessions instead of large session cookies.
On the server, increase the maximum header size.
# Nginx: large_client_header_buffers 4 16k; # Apache: LimitRequestFieldSize 16384 # Node.js: --max-http-header-size=16384