HTTP 505 HTTP Version Not Supported — Meaning, Causes & How to Fix
The server does not support the HTTP protocol version used in the request.
- Code
- 505
- Name
- HTTP Version Not Supported
- Class
- 5xx Server Error
- Specification
- RFC 9110 §15.6.6
- Cacheable by default
- Only with explicit Cache-Control / Expires
What HTTP 505 means
505 HTTP Version Not Supported means the server refuses the major HTTP version in the request line. It is very rare in practice, because clients and servers negotiate versions (via ALPN for HTTP/2 and HTTP/3).
Common causes
- A malformed request line, e.g.
GET / HTTP/3.0sent as plain text. - Custom clients writing the version incorrectly.
How to fix it
For site owners
- Usually nothing to change; check for broken clients or proxies rewriting the request line.
For visitors & developers
- Use a standard HTTP library, which sends a valid version automatically.
SEO impact
No practical SEO impact; search engine crawlers send standard HTTP/1.1 or HTTP/2 requests.
Example 505 response
HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain
Connection: closeRelated status codes
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Open HTTP Status Codes tool →All status codesFrequently asked questions
What does HTTP 505 mean?
HTTP 505 HTTP Version Not Supported is a 5xx Server Error status code. The server does not support the HTTP protocol version used in the request.
Does 505 mean my server needs HTTP/3?
No. It means the request used a version the server cannot speak; modern clients negotiate the version automatically.
Which specification defines 505?
It is defined in RFC 9110 §15.6.6.
Is a 505 response cacheable?
Not by default. A 505 response is only stored by caches when it carries explicit freshness information such as Cache-Control: max-age or Expires.