Why does a 421 appear only in some browsers?
Browsers differ in how aggressively they reuse HTTP/2 connections across hostnames. One that coalesces two domains onto a single connection may hit a server that serves only one.
HTTP 421 MISDIRECTED REQUEST
The request reached a server that cannot answer for this host, often from connection reuse. 421 belongs to the 4xx client error class and is defined in RFC 9110.
Loading the workbench.
About HTTP 421
What Does HTTP 421 Misdirected Request Mean?
The request reached a server that cannot answer for this host, often from connection reuse. The code is registered with IANA and defined in RFC 9110.
Client error: the request has a problem, so it cannot be fulfilled as sent. Every code from 400 to 499 shares that general meaning, so a client that does not recognise 421 must treat it like 400.
When Do You See a 421 Response?
421 Misdirected Request means the request reached a server that is not configured to answer for the host it named. It usually comes from connection reuse in HTTP/2 and HTTP/3: a browser coalesces requests for two hostnames onto one connection because they share an IP address and a certificate, but the server behind that connection serves only one of them. TLS SNI mismatches on multi-tenant servers and CDNs cause it too.
How Do You Fix a 421?
Clients may retry the request on a new connection, which browsers do automatically after a 421. Server operators should ensure that every hostname covered by a certificate on a given IP is actually served there, or split certificates so browsers do not coalesce connections they cannot fulfil. A 421 must not be generated by proxies, only by origin servers.
What Does a 421 Response Look Like?
HTTP/2 421
content-type: text/html
content-length: 0421 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 421?
- 400 Bad Request: The request is malformed: bad syntax, invalid framing, or an unreadable body.
- 502 Bad Gateway: A proxy or gateway got an invalid response from the upstream server.
- 404 Not Found: Nothing at this URL, or the server will not say whether there is.
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
Should my app retry on 421?
Yes, on a fresh connection. The request was not processed, so retrying is safe, and browsers do it automatically. Custom clients should open a new connection to that host first.
Is HTTP 421 an error?
Yes. 421 is a client error response: the problem is in the request, so repeating it unchanged will usually fail again.
Is a 421 response cacheable by default?
No. Caches store a 421 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.