EC
ERR_HTTP_CONTENT_LENGTH_MISMATCH
Node.jsERRORNotableHTTPHIGH confidence

Response body size does not match Content-Length header

Production Risk

Produces malformed HTTP responses that can truncate or corrupt data seen by clients.

What this means

Thrown when an HTTP response is finalised with res.end() but the number of bytes actually written does not match the value declared in the Content-Length header. This mismatch would produce an invalid HTTP response that confuses clients and intermediaries.

Why it happens
  1. 1Content-Length was calculated from the wrong string (e.g. character count vs byte count for multi-byte UTF-8)
  2. 2The response body was modified after Content-Length was set
  3. 3Dynamic content that grew or shrank between the header calculation and the write
How to reproduce

Triggered at res.end() when the byte count of all written data differs from the Content-Length header.

trigger — this will error
trigger — this will error
const http = require('http');
http.createServer((req, res) => {
  const body = 'héllo'; // 6 bytes in UTF-8, not 5
  res.writeHead(200, { 'Content-Length': '5' }); // wrong
  res.end(body);
}).listen(3000);

expected output

Error [ERR_HTTP_CONTENT_LENGTH_MISMATCH]: Response body's content-length mismatch

Fix 1

Use Buffer.byteLength() to compute Content-Length

WHEN Always when setting Content-Length for string bodies

Use Buffer.byteLength() to compute Content-Length
const body = 'héllo';
const len = Buffer.byteLength(body, 'utf8'); // 6
res.writeHead(200, { 'Content-Length': String(len) });
res.end(body);

Why this works

Buffer.byteLength() counts encoded bytes, not Unicode code units, giving the correct byte length.

Fix 2

Omit Content-Length and let Node use chunked encoding

WHEN When the body size is not known in advance

Omit Content-Length and let Node use chunked encoding
res.writeHead(200, { 'Content-Type': 'text/plain' });
res.end('héllo'); // chunked, no mismatch possible

Why this works

Without Content-Length the HTTP stack uses Transfer-Encoding: chunked, which never mismatches.

Code examples
Triggerjs
const http = require('http');
http.createServer((req, res) => {
  const body = 'héllo'; // 6 bytes in UTF-8, not 5
  res.writeHead(200, { 'Content-Length': '5' }); // wrong
  res.end(body);
}).listen(3000);  // this triggers ERR_HTTP_CONTENT_LENGTH_MISMATCH
Handle in try/catchjs
try {
  // operation that may throw ERR_HTTP_CONTENT_LENGTH_MISMATCH
  riskyOperation()
} catch (err) {
  if (err.code === 'ERR_HTTP_CONTENT_LENGTH_MISMATCH') {
    console.error('ERR_HTTP_CONTENT_LENGTH_MISMATCH:', err.message)
  } else {
    throw err
  }
}
Defensive pattern to avoid itjs
// Validate inputs before calling the operation
function safe_err_http_content_length_mismatch(...args) {
  // validate args here
  return performOperation(...args)
}
What not to do

✕ Use string.length as the Content-Length for UTF-8 encoded text

String length counts characters, not bytes; multi-byte characters will produce a mismatch.

Sources
Official documentation ↗

Node.js Error Codes Documentation

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