What it means
The server cannot meet the requirement in the request's Expect header, usually Expect: 100-continue.
Common causes
- A proxy or old server that does not support 100-continue
- Unknown Expect values
How to fix it
As a client or visitor
Remove the Expect header (curl -H 'Expect:', or disable it in your HTTP client) and resend.
As the site or API owner
Support 100-continue or ignore Expect gracefully.
Example response
HTTP/1.1
HTTP/1.1 417 Expectation Failed
Date: Tue, 07 Oct 2025 09:30:00 GMT
Server: nginx
Content-Length: 0Specification
FAQ
› Why does .NET or curl trigger 417?
They add Expect: 100-continue to uploads; some proxies reject it. Disable the header in the client.
› Is 417 common?
No; it mostly appears with legacy proxies.