431 Request Header Fields Too Large

431

Request Header Fields Too Large

Example HTTP Response

HTTP Response

HTTP/1.1 431 Request Header Fields Too Large

Common Causes

Cookie header too large
Too many custom headers
Excessively long header values

Technical Details

What does this mean?

TMI in the headers! You're sending so much metadata that the server's eyes glazed over. Trim those headers down!

Technical Definition

The server is unwilling to process the request because its header fields are too large.

RFC Says

"The 431 (Request Header Fields Too Large) status code indicates that the server is unwilling to process the request because its header fields are too large. The request MAY be resubmitted after reducing the size of the request header fields."

Plain English:

431 means 'Your HTTP headers are too big.' This can happen when you have too many cookies, very large authorization tokens, or numerous custom headers. The server has a limit on total header size (often 8KB) and you've exceeded it.

Common Misinterpretation

If only one specific header is too large, consider mentioning which one in the error message. This often happens with JWT tokens or cookies. Don't confuse this with 413 (payload too large) - 431 is about headers, 413 is about the request body.

View RFC Documentation

Code Snippets

Ready-to-use code for returning this HTTP status in your application:

Node.js

// Express.js
app.get('/example', (req, res) => {
  res.status(431).json({
    error: 'Request Header Fields Too Large',
    message: 'Your error message here'
  });
});

// Native HTTP
const http = require('http');

http.createServer((req, res) => {
  res.writeHead(431, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({
    error: 'Request Header Fields Too Large',
    message: 'Your error message here'
  }));
}).listen(3000);

431 Request Header Fields Too Large FAQ

What causes a 431 Request Header Fields Too Large error?

Cookie header too large. Too many custom headers. Excessively long header values.

// Express.js app.get('/example', (req, res) => { res.status(431).json({ error: 'Request Header Fields Too Large', message: 'Your error message here' }); }); // Native HTTP const http = require('http'); http.createServer((req, res) => { res.writeHead(431, { 'Content-Type': 'application/json' }); res.end(JSON.stringify({ error: 'Request Header Fields Too Large', message: 'Your error message here' })); }).listen(3000);