417 Expectation Failed

HTTP/1.1 417 Expectation Failed
Connection: close
Content-Type: text/plain

Expectation not met.

const http = require("http");

const server = http.createServer((req, res) => {
  if (req.headers.expect && req.headers.expect !== "100-continue") {
    res.writeHead(417);
    res.end("Expectation Failed");
  }
});

# To avoid 417 errors in requests when dealing with bad proxies, you might need to ensure Expect is not sent.
import requests

headers = {"Expect": ""}
response = requests.post("https://api.example.com/upload", data="large_data", headers=headers)

func handler(w http.ResponseWriter, r *http.Request) {
    if r.Header.Get("Expect") != "" && r.Header.Get("Expect") != "100-continue" {
        http.Error(w, "Expectation Failed", http.StatusExpectationFailed)
        return
    }
    w.WriteHeader(http.StatusOK)
}

// In .NET, HttpClient sends Expect: 100-continue by default for large POSTs.
// To disable it and avoid 417 errors with non-compliant servers:
var client = new HttpClient();
client.DefaultRequestHeaders.ExpectContinue = false;

# cURL sends Expect: 100-continue automatically for payloads > 1024 bytes.
# To disable it and avoid a 417:
curl -X POST -H "Expect:" -d @largefile.bin https://api.example.com/upload