HTTP 417 Expectation Failed indicates the server cannot meet the requirements specified in the Expect request header. Currently, the only defined expectation is '100-continue.' A server returns 417 to reject the request before the client sends the body, saving bandwidth. This is the 'rejection' counterpart to 100 Continue — the server is saying 'don't bother sending the body.'
Response includes the status code, standard headers (including Content-Type), and a small diagnostic JSON body describing the request and returned status.
Simulator URL (copy in the app after load — not a normal link):
https://httpstatus.com/api/status/417
Example request:
curl -i "https://httpstatus.com/api/status/417"The expectation given in the Expect request header could not be met by the server (e.g., Expect: 100-continue was rejected).
On this code, Inspector focuses on semantics, headers, and correctness warnings that commonly affect clients and caches.
HTTP 417 Expectation Failed has specific technical implications for API design, caching, and client behavior. Understanding the precise semantics helps distinguish it from similar status codes and implement correct error handling. The response should include a descriptive body following a consistent error schema (like RFC 7807 Problem Details) so clients can programmatically handle the error.
// Handle 417 Expectation Failed in Express
app.use((err, req, res, next) => {
if (err.status === 417) {
return res.status(417).json({
type: 'https://api.example.com/errors/expectation-failed',
title: 'Expectation Failed',
status: 417,
detail: err.message
});
}
next(err);
});from fastapi import HTTPException
# Raise 417 Expectation Failed
raise HTTPException(
status_code=417,
detail={
'type': 'expectation_failed',
'message': 'Descriptive error for 417 Expectation Failed'
}
)// Spring Boot 417 Expectation Failed handling
@ExceptionHandler(CustomExpectationFailedException.class)
public ResponseEntity<ErrorResponse> handleExpectationFailed(
CustomExpectationFailedException ex) {
return ResponseEntity.status(417)
.body(new ErrorResponse("Expectation Failed", ex.getMessage()));
}// Return 417 Expectation Failed
func errorHandler(w http.ResponseWriter, message string) {
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(417)
json.NewEncoder(w).Encode(map[string]any{
"status": 417,
"error": "Expectation Failed",
"message": message,
})
}