HTTP Status Codes

417 Expectation Failed

The server cannot satisfy the request's Expect header — almost always an Expect: 100-continue that an intermediary refuses.

What it means

417 Expectation Failed is returned when the value in the Expect request header cannot be met. In practice the only common expectation is 100-continue; a 417 usually means a proxy in the path does not support it. Clients typically retry without the Expect header.

Why it matters

  • It tells clients to drop an unsupported expectation and retry, avoiding a stall.

Common mistakes

  • Proxies that break the 100-continue handshake and surface 417 unexpectedly.

How WebInspect checks this

  • WebInspect reports the HTTP version and framing behaviour that govern the Expect handshake.