HTTP 417 Expectation Failed

What is HTTP 417 Expectation Failed?

The 417 (Expectation Failed) status code indicates that the expectation given in the request's Expect header field could not be met by at least one of the inbound servers. This is most commonly seen when using Expect: 100-continue and the server decides not to accept the request based on the headers.

Common Use Cases

• Server rejecting Expect: 100-continue
• Proxy server unable to meet client expectations

// PHP - rejecting a request at the Expect: 100-continue stage
if ($_SERVER['HTTP_EXPECT'] === '100-continue' && !isAuthenticated()) {
    http_response_code(417);
    echo json_encode(['error' => 'Authentication required']);
    exit;
}