HTTP
417 - Expectation Failed
HTTP 417 Expectation Failed means the server cannot meet requirements in the Expect request header.
Last reviewed: February 12, 2026|Editorial standard: source-backed technical guidance
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 unsupported Expect header values.
- -Intermediary does not handle 100-continue handshake correctly.
- -Request path strips or rewrites expectation-related headers.
How to Fix Expectation Failed
- 1Remove unsupported `Expect` values or use only `Expect: 100-continue` where explicitly supported.
- 2Verify intermediaries preserve expectation headers and properly relay interim `100 Continue` responses.
- 3Retest large-body requests with expectation flow enabled and disabled to isolate compatibility gaps.
Step-by-Step Diagnosis for Expectation Failed
- 1Capture raw request/response headers to inspect exact `Expect` value and server reaction.
- 2Confirm whether failure occurs at origin or intermediary (proxy/CDN/WAF) layer.
- 3Validate 100-continue support across the full path, including timeout behavior before body send.
- 4Retest with minimal payload and explicit expectation settings to isolate mismatch source.
Expect Header and Interim Response Validation
- -Inspect whether client sends unsupported expectation token (example: custom `Expect` value rejected by origin).
- -Verify server/intermediary handles `100 Continue` handshake correctly (example: proxy drops interim response and origin emits 417).
Intermediary Compatibility and Request Flow Trace
- -Audit header normalization rules in gateways (example: WAF strips `Expect` and leaves client waiting for `100 Continue`).
- -Compare direct-origin vs edge-path behavior (example: direct succeeds, CDN path returns 417 due to legacy proxy behavior).
Implementation Examples
Reproduce HTTP 417 Expectation Failedcurl
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"}Handle 417 in JavaScriptjavascript
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');
}Handle 417 in Pythonpython
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')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
- -simulate slow-upload clients in CI to catch expectation timeout bugs that only appear under partial-body send conditions.
Decision Support
Official References
Provider Context
This guidance is specific to HTTP services. Always validate implementation details against official provider documentation before deploying to production.