510 Not Extended

What HTTP 510 Not Extended Means

Further extensions to the request are required for the server to fulfill it.

The server needs an additional protocol extension (per RFC 2774) that the client did not declare. Rarely used in practice.

Common Causes

• Required HTTP extension not announced by the client
• Legacy extension-negotiation mechanism
• Non-standard server requirement

How to Fix It (For Visitors)

• Contact the website administrator

How to Fix It (For Developers/Admins)

• Declare the required extension in the request
• Reconsider using the (obsolete) extension framework
• Document the expected client behavior

Returning a 510 Not Extended (Code Examples)

If you build APIs or web apps, here is how to send an HTTP 510 response and how to test for it:

Node.js (Express)

app.get('/resource', (req, res) => {
  res.status(510).json({ error: 'Not Extended' });
});

Python (Flask)

@app.route('/resource')
def resource():
    return jsonify(error='Not Extended'), 510

PHP

<?php
http_response_code(510);
header('Content-Type: application/json');
echo json_encode(['error' => 'Not Extended']);

Check the status with curl

curl -I https://example.com/resource
# Look for: HTTP/1.1 510 Not Extended

Frequently Asked Questions

What does HTTP 510 Not Extended mean?

The server needs an additional protocol extension (per RFC 2774) that the client did not declare. Rarely used in practice. In short: Further extensions to the request are required for the server to fulfill it.

Is a 510 Not Extended error my fault or the website's?

510 is a 5xx server-error code, so the problem is on the server side, not your browser or device. As a visitor you can usually only retry; if you run the site, investigate the server.

How do I fix a 510 Not Extended error?

Declare the required extension in the request. Reconsider using the (obsolete) extension framework. Document the expected client behavior.

Official Specification

The 510 Not Extended status code is defined in RFC 2774.

View the IANA HTTP Status Code Registry →