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.
Status
431 Request Header Fields Too LargeExample
GET /account/settings HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html
Cookie: session=abc123; pref=dark-mode; cart=item1%2Citem2%2Citem3; tracking=GA1.2.9876543210; ads=eyJ0eXBlIjoiYWRzIiwidmFsdWUiOiJ0cnVlIn0%3D; notices=dismissedHTTP/1.1 431 Request Header Fields Too Large
Content-Type: text/html; charset=utf-8
<!doctype html>
<html>
<body>
<h1>431 Request Header Fields Too Large</h1>
<p>Your browser sent too many cookies. Try clearing your cookies and loading the page again.</p>
</body>
</html>How to Troubleshoot
Clear browser cookies
Delete cookies for the affected domain and try again.
Reduce cookie size
Review the cookies being set and reduce their number or size. Use server-side sessions instead of large session cookies.
Increase server header limits
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
Common Causes
Cookies too large
highThe browser is sending too many or too-large cookies with each request, exceeding the server's header size limit.
Custom headers too large
mediumThe application is setting excessively large custom headers (auth tokens, tracking data).
Related Status Codes
References
| Specification | Section |
|---|---|
| Additional HTTP Status Codes | RFC 6585 §5 |
Frequently Asked Questions
This reference was compiled from official RFCs, protocol specifications, and hands-on troubleshooting experience. AI tools were used primarily for formatting and organizing the content on the page.