All HTTP status codes
4xx Client Error
RFC 9110
417 Expectation Failed
The server cannot meet the requirements of the Expect header.
What it means
HTTP 417 Expectation Failed indicates the expectation given in the request's Expect header (typically 100-continue) cannot be met by the server.
When it's used
When a client sends Expect: 100-continue but the server (or an intermediate proxy) does not understand or support the extension.
Common causes
- Old HTTP/1.0 server receiving HTTP/1.1 Expect header
- Proxy stripping or rejecting the Expect handshake
- Custom Expect values not recognized
How to fix it
- Remove the Expect header from the client request
- Send the request body directly without two-phase handshake
- Upgrade the server to HTTP/1.1 compliance
Example response
HTTP/1.1 417 Expectation Failed Connection: close
SEO impact
Not relevant to SEO.