DevArbor
5xx Server Error

505 HTTP Version Not Supported

What it means

The server doesn't support the HTTP protocol version used in the request line. Rarely encountered in practice — most servers support HTTP/1.0, HTTP/1.1, and HTTP/2. Might be seen if a client sends HTTP/3 to a server that only supports older versions.

Site Visitor

What can I do?

  • This is a server-side error — not your fault.
  • Wait a few minutes and try refreshing the page.
  • If the error persists, contact the site owner or check their status page.
Developer

How to debug & fix

  1. Update server to support current HTTP versions
  2. Configure CDN/proxy to negotiate protocol versions appropriately
  3. Rarely need to return this manually
  4. Configure your CDN or reverse proxy to handle HTTP version negotiation — application code rarely needs to handle this directly.

Code Example

Node.js / Express
// Extremely rare — Node.js http/https modules handle this automatically

Related Status Codes

501 Not Implemented

How HTTP Status Codes Work

Every HTTP response carries a three-digit status code that tells the client — browser, API consumer, or search-engine crawler — exactly what happened. The first digit defines the class: 1xx informational (request in progress), 2xx success, 3xx redirection, 4xx client error (bad request, missing auth, not found), and 5xx server failure.

Status codes are standardised in RFC 9110 (HTTP Semantics, 2022). Extensions like WebDAV (RFC 4918) and rate-limit headers (RFC 6585) added codes beyond the core set. When a client receives an unrecognised code, the rule is to treat it as the generic x00 of its class.

Why the Right Code Matters

Semantically correct codes help search engines index accurately (301 passes link equity; 410 removes pages faster than 404), allow API clients to implement correct retry logic (429 + Retry-After, 503 + Retry-After), and let monitoring systems distinguish bugs (500) from load issues (503) from auth failures (401/403).

Looking up a different status code? The full reference covers all HTTP codes with causes, fix guides, and copyable code examples for Node.js and Python.

Browse the full HTTP Status Code reference →

Frequently Asked Questions

What does HTTP 505 HTTP Version Not Supported mean?
The server doesn't support the HTTP protocol version used in the request line. Rarely encountered in practice — most servers support HTTP/1.0, HTTP/1.1, and HTTP/2. Might be seen if a client sends HTTP/3 to a server that only supports older versions.
Is HTTP 505 the visitor's fault?
No. HTTP 505 HTTP Version Not Supported is a server-side error. The problem lies with the server, not the visitor. The site owner is responsible for fixing it.
How do I fix HTTP 505 HTTP Version Not Supported?
As a visitor: wait a few minutes and refresh — server errors are usually temporary. As a developer: check your server error logs immediately, restart the application server if needed, and set up uptime monitoring.