431 Request Header Fields Too Large
Server is unwilling to process request because header fields are too large.
Usually happens because:
- ☑ Accumulation of huge browser cookie
- ☑ Extremely long Referer URLs.
- ☑ Excessive custom HTTP headers sent
🔍 Quick Checklist:
Meaning
The HTTP 431 Request Header Fields Too Large client error status code indicates that the server is unwilling to process the request because either an individual header field, or all the header fields in collective, are too large.
Root Causes
- Accumulation of huge browser cookie files.
- Extremely long Referer URLs.
- Excessive custom HTTP headers sent by client libraries.
| Cause | Frequency |
|---|---|
| Accumulation of huge browser cookie | ⭐⭐⭐⭐⭐ |
| Extremely long Referer URLs. | ⭐⭐⭐⭐ |
| Excessive custom HTTP headers sent | ⭐⭐⭐ |
Common Mistakes
- Storing massive session objects inside client cookies instead of database sessions.
- Confusing 431 (headers too large) with 414 (URL too long).
How to Fix
Framework-Specific Examples
Express router checking header sizes.
app.use((req, res, next) => {
const headersStr = JSON.stringify(req.headers);
if (headersStr.length > 8000) {
return res.status(431).send('Header fields too large');
}
next();
});Server Configuration Examples
Increasing Nginx client header buffer parameters.
large_client_header_buffers 4 16k;Prevention
- Store large user session data parameters in backend caching datastores (like Redis), keeping cookies light.
Frequently Asked Questions (FAQ)
Q: What is the most common cause of a 431 error?
The accumulation of cookies. Over time, web apps might append multiple authorization, tracker, and session cookies, eventually bloating headers past server size limits.
Q: How can users fix 431 errors in their browser?
Users can clear browser history, cookies, and site cache for the affected domain to wipe the bloated headers.
Q: What is the default header size limit on servers?
Most modern web servers (Node.js, Apache, Nginx) default to limits between 8KB and 16KB for header buffers.
Q: Is 431 cacheable?
No. The 431 Request Header Fields Too Large status code is never cached.