Can a browser trigger a 505?
Very rarely. Browsers negotiate HTTP/2 and HTTP/3 safely and fall back to HTTP/1.1, so a 505 usually points to a custom client or an unusual server.
HTTP 505 HTTP VERSION NOT SUPPORTED
The server refuses this major HTTP version. 505 belongs to the 5xx server error class and is defined in RFC 9110.
Loading the workbench.
About HTTP 505
What Does HTTP 505 HTTP Version Not Supported Mean?
The server refuses this major HTTP version. The code is registered with IANA and defined in RFC 9110.
Server error: the server failed to fulfill a valid request. Every code from 500 to 599 shares that general meaning, so a client that does not recognise 505 must treat it like 500.
When Do You See a 505 Response?
505 HTTP Version Not Supported means the server refuses the major HTTP version used in the request. It is rare, because versions are negotiated: HTTP/2 and HTTP/3 are chosen during the TLS handshake or advertised with Alt-Svc rather than attempted blindly. You might see it from minimal or embedded servers that only speak HTTP/1.0, from hand-written requests with a typo in the version, or from security appliances rejecting unusual traffic.
How Do You Fix a 505?
Clients should send a version the server supports, normally HTTP/1.1, and check for typos such as HTTP/1.2 in hand-crafted requests or raw sockets. Servers should describe in the body which versions they support. Operators who meant to support HTTP/2 or HTTP/3 should verify ALPN configuration rather than expecting clients to guess. Testing with curl and its --http1.1 or --http2 options shows what each endpoint accepts.
What Does a 505 Response Look Like?
HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain
Connection: close505 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.
To inspect a real one without sending it anywhere:
- Copy the response headers from your browser Network tab, or from
curl -i. - Paste them into the Header decoder to see every header explained.
- Check caching in the Cache lane.
Which Status Codes Are Related to 505?
- 426 Upgrade Required: The client must switch protocols, named in the Upgrade header.
- 501 Not Implemented: The server does not support the method at all.
- 400 Bad Request: The request is malformed: bad syntax, invalid framing, or an unreadable body.
Browse every code on the HTTP status codes page.
What Other Lanes Are in the Reference Section?
- HTTP status codes: Every IANA-registered status code, with a page per code.
- MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
- HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.
Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.
Frequently Asked Questions
What should a 505 response contain?
RFC 9110 suggests a body describing why the version is not supported and which other protocols the server does support, so the client can try again with one of them.
Is HTTP 505 an error?
Yes. 505 is a server error response: the server failed to handle a valid request, so retrying later may work.
Is a 505 response cacheable by default?
No. Caches store a 505 only when Cache-Control or Expires explicitly allows it.
How Can You Verify That Nothing Leaves This Browser?
You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.
- Open your browser developer tools with
F12, orCmd+Option+Ion a Mac, and select the Network tab. - Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
- Click the clear icon in the Network tab so that the log is empty.
- Paste real headers into any lane and read the result.
- Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.
Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.
2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.