417 - Expectation Failed

Start Here

Use the closest compare guide, playbook, or adjacent error page to narrow the decision faster before you start changing production systems.

This page is part of the Error Reference library. Learn more about the project or report a correction.

What Does Expectation Failed Mean?

The request is blocked before body processing because expectation requirements declared by the client cannot be satisfied by this server path.

Common Causes

• -Client sends Expect: 100-continue, but intermediary proxy does not implement expectation handling correctly.
• -Legacy SDK always adds Expect header even for tiny payloads, and origin policy rejects that negotiation path.
• -WAF rule flags Expect header as anomalous and blocks request before application reads request body.

How to Fix Expectation Failed

1. 1Remove unsupported Expect values or use only Expect: 100-continue where explicitly supported.
2. 2Verify intermediaries preserve expectation headers and properly relay interim 100 Continue responses.
3. 3Retest large-body requests with expectation flow enabled and disabled to isolate compatibility gaps.

Step-by-Step Diagnosis for Expectation Failed

1. 1Capture raw request/response headers to inspect exact Expect value and server reaction.
2. 2Confirm whether failure occurs at origin or intermediary (proxy/CDN/WAF) layer.
3. 3Validate 100-continue support across the full path, including timeout behavior before body send.
4. 4Retest with minimal payload and explicit expectation settings to isolate mismatch source.

curl -i -X POST https://api.example.com/v1/upload -H "Expect: 100-continue" -H "Content-Type: application/octet-stream" --data-binary @large.bin
# Response:
# HTTP/1.1 417 Expectation Failed
# {"error":"Expectation Failed","code":"417"}

const response = await fetch('https://api.example.com/v1/upload', {
  method: 'POST',
  headers: { 'Accept': 'application/json', 'Content-Type': 'application/octet-stream', 'Expect': '100-continue' },
  body: new Uint8Array([1, 2, 3])
});
if (response.status === 417) {
  console.error('Handle 417 Expectation Failed');
}

import requests

response = requests.post(
    'https://api.example.com/v1/upload',
    headers={'Accept': 'application/json', 'Content-Type': 'application/octet-stream', 'Expect': '100-continue'},
    data=b'example-binary'
)
if response.status_code == 417:
    print('Handle 417 Expectation Failed')

Seen in Production

Debugging Tools

• -curl -v with Expect header and body tracing
• -Gateway/intermediary raw header logs
• -Origin parser debug logs for 100-continue path
• -Upload handshake integration tests

How to Verify the Fix

• -Repeat affected requests and confirm expectation handling succeeds without 417.
• -Validate large-body upload flows complete with correct 100 Continue behavior.
• -Ensure expectation-related retries do not cause duplicate body transmission.

How to Prevent Recurrence

• -Standardize Expect usage policy in SDKs and disable custom expectation tokens by default.
• -Add integration tests for 100 Continue across every ingress path.
• -Version gateway header policies and enforce no-drop rules for expectation handshake headers.

Pro Tip

Related Errors

Official References

• -RFC 9110 Section 15.5.18 - 417 Expectation Failed definition
• -IANA HTTP Status Code Registry - 417 Expectation Failed
• -MDN Web Docs — 417 Expectation Failed
• -HTTP Official Documentation