What causes a 417 Expectation Failed?
Almost always a server or proxy that does not handle Expect: 100-continue. Removing that header from the request usually makes the error go away.
HTTP 417 EXPECTATION FAILED
The server cannot meet the Expect header. 417 belongs to the 4xx client error class and is defined in RFC 9110.
Loading the workbench.
About HTTP 417
What Does HTTP 417 Expectation Failed Mean?
The server cannot meet the Expect header. 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 417 must treat it like 400.
When Do You See a 417 Response?
417 Expectation Failed means the server cannot meet a requirement in the request's Expect header. In practice the only standard expectation is 100-continue, so 417 comes from servers or proxies that do not support that handshake. Some older proxies and a few application servers reply 417 instead of quietly ignoring it, which causes large uploads from clients such as curl or .NET HttpClient to fail outright.
How Do You Fix a 417?
Clients can repeat the request without the Expect header; in curl, pass -H 'Expect:', and in .NET set Expect100Continue to false. RFC 9110 says a client receiving 417 for 100-continue should retry without the expectation. Server operators should either support 100-continue or ignore the header rather than rejecting the request. Handling it correctly costs nothing and spares clients a confusing failure on large uploads.
What Does a 417 Response Look Like?
HTTP/1.1 417 Expectation Failed
Content-Length: 0
Connection: close417 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 417?
- 100 Continue: The headers were fine; send the body. Used with Expect: 100-continue so a client does not upload a large body that would be rejected.
- 413 Content Too Large: The body is bigger than the server accepts. Formerly "Payload Too Large".
- 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
Is it safe to drop Expect: 100-continue?
Yes. It is only an optimization that avoids sending a body the server would reject. Without it the client just sends the body straight away.
Is HTTP 417 an error?
Yes. 417 is a client error response: the problem is in the request, so repeating it unchanged will usually fail again.
Is a 417 response cacheable by default?
No. Caches store a 417 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.