HTTP 505 HTTP Version Not Supported

What It Means

The HTTP 505 HTTP Version Not Supported status code indicates that the server does not support, or refuses to support, the major version of HTTP that was used in the request message.

HTTP Version History

Version	Year	Key Features
HTTP/0.9	1991	GET only, no headers
HTTP/1.0	1996	Headers, status codes, content types
HTTP/1.1	1997	Keep-alive, chunked transfer, Host header
HTTP/2	2015	Binary, multiplexing, header compression
HTTP/3	2022	QUIC protocol, improved performance

Common Causes

Very old servers: Server only supports HTTP/1.0
Proxy issues: Intermediate proxy doesn't support the version
Malformed requests: Invalid version string in request line
Future versions: Client using a version newer than server supports

Example Request/Response

# Client sends request with unsupported version
GET / HTTP/9.9
Host: example.com

# Server response
HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain

This server only supports HTTP/1.0 and HTTP/1.1

How to Fix

Check client: Ensure your HTTP client is using a supported version
Update server: Upgrade to a web server version that supports modern HTTP
Check proxies: Verify intermediate proxies support the HTTP version
Fallback: Configure clients to fall back to HTTP/1.1 if needed

Modern Considerations

This error is rare in practice. Most modern servers support HTTP/1.1 and HTTP/2, with HTTP/3 becoming more common. Browsers automatically negotiate the best supported version using ALPN (Application-Layer Protocol Negotiation) during the TLS handshake.

Frequently Asked Questions

What does HTTP 505 HTTP Version Not Supported mean?

HTTP 505 means the server does not support, or refuses to support, the major version of HTTP that was used in the request message. For example, a server that only supports HTTP/1.1 would return 505 if it received a request using an unsupported version like HTTP/9.9.

How common is the 505 status code?

505 is extremely rare in practice. Modern web servers support HTTP/1.1 and HTTP/2, and browsers automatically negotiate the best supported version using ALPN (Application-Layer Protocol Negotiation) during the TLS handshake. You would typically only see this with misconfigured clients or very old servers.

What are the differences between HTTP/1.1, HTTP/2, and HTTP/3?

HTTP/1.1 (1997) introduced persistent connections and chunked transfer encoding. HTTP/2 (2015) added binary framing, multiplexing, header compression, and server push. HTTP/3 (2022) replaced TCP with QUIC for improved performance, especially on unreliable networks.

How do I fix a 505 error?

If you are the client, ensure your HTTP library is using a supported version (HTTP/1.1 is universally supported). If you are the server operator, update your web server software to support modern HTTP versions. Also check intermediate proxies that may not support the HTTP version being used.

Can browsers trigger a 505 error?

It is very unlikely for a browser to trigger a 505 error because browsers negotiate the HTTP version automatically. This error is more commonly caused by custom HTTP clients sending malformed version strings, or very old proxy servers that do not understand newer protocol versions.