505 HTTP Version Not Supported

What HTTP 505 HTTP Version Not Supported Means

The server does not support the HTTP protocol version used in the request.

Server doesn't support the HTTP version in the request.

Common Causes

• Request line uses a major HTTP version the server does not implement
• Malformed request line (e.g. a broken client or a port scanner)
• Server or proxy deliberately configured to refuse an HTTP version

How to Fix It (For Visitors)

• Update your browser
• Contact website administrator

How to Fix It (For Developers/Admins)

• Check the raw request line the client sends (e.g. with curl -v)
• Make sure the client speaks HTTP/1.1 or negotiates HTTP/2/3 via ALPN
• Review proxy settings that restrict protocol versions
• Update outdated server or client software

Returning a 505 HTTP Version Not Supported (Code Examples)

If you build APIs or web apps, here is how to send an HTTP 505 response and how to test for it:

Node.js (Express)

app.get('/resource', (req, res) => {
  res.status(505).json({ error: 'HTTP Version Not Supported' });
});

Python (Flask)

@app.route('/resource')
def resource():
    return jsonify(error='HTTP Version Not Supported'), 505

PHP

<?php
http_response_code(505);
header('Content-Type: application/json');
echo json_encode(['error' => 'HTTP Version Not Supported']);

Check the status with curl

curl -I https://example.com/resource
# Look for: HTTP/1.1 505 HTTP Version Not Supported

Frequently Asked Questions

What does HTTP 505 HTTP Version Not Supported mean?

Server doesn't support the HTTP version in the request. In short: The server does not support the HTTP protocol version used in the request.

Is a 505 HTTP Version Not Supported error my fault or the website's?

505 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 505 HTTP Version Not Supported error?

Check the raw request line the client sends (e.g. with curl -v). Make sure the client speaks HTTP/1.1 or negotiates HTTP/2/3 via ALPN. Review proxy settings that restrict protocol versions.

Official Specification

The 505 HTTP Version Not Supported status code is defined in RFC 9110 Section 15.6.6.

View the IANA HTTP Status Code Registry →