505 HTTP Version Not Supported: HTTP Status Code Meaning, Causes, and Fixes

What 505 HTTP Version Not Supported means

505 HTTP Version Not Supported is rare. Modern servers handle HTTP/1.0, HTTP/1.1, HTTP/2, and HTTP/3 ubiquitously. 505 mostly shows up for very old HTTP/0.9 clients hitting modern servers, or for handcrafted requests with malformed version strings.

When servers should return it: Return 505 when a request specifies an HTTP major version the server does not implement.

How to fix 505 HTTP Version Not Supported

Use a modern HTTP client
Verify the request line has a valid HTTP version (HTTP/1.1, HTTP/2.0)

Example response

HTTP/1.1 505
content-type: text/plain
HTTP version not supported

Related status codes

Frequently Asked Questions

When does 505 actually happen?

Almost never in 2026. You can trigger it manually by sending HTTP/0.9 or a bogus version string.

More references

For a one-page reference of all HTTP status codes, see the HTTP cheat sheet. For testing API responses, try the API Tester tool. For inspecting responses on the command line, the curl cheat sheet covers the most common flags.