100 Continue

POST /upload HTTP/1.1
Host: example.com
Content-Length: 9999999
Expect: 100-continue

HTTP/1.1 100 Continue

[...client sends body here...]

const http = require('http');

const server = http.createServer();
server.on('checkContinue', (req, res) => {
  if (req.headers['authorization']) {
    res.writeContinue();
    req.pipe(processUpload());
  } else {
    res.statusCode = 401;
    res.end('Unauthorized');
  }
});

import requests

# Python requests does not support Expect: 100-continue natively.
# You must use a session or a custom transport adapter, 
# or rely on lower-level libraries like http.client for granular control.
import http.client
conn = http.client.HTTPSConnection("example.com")
conn.putrequest("POST", "/upload")
conn.putheader("Expect", "100-continue")
conn.endheaders()

package main

import (
	"net/http"
	"strings"
)

func main() {
	req, _ := http.NewRequest("POST", "http://example.com/upload", strings.NewReader("body data"))
	// Go's net/http client automatically adds Expect: 100-continue 
	// for requests with bodies larger than a certain threshold.
	req.Header.Set("Expect", "100-continue")
	client := &http.Client{}
	client.Do(req)
}

# curl automatically sends Expect: 100-continue for payloads > 1024 bytes.
# To explicitly disable it:
curl -X POST -H "Expect:" -d @largefile.zip https://example.com/upload

# To explicitly force it (rarely needed):
curl -X POST -H "Expect: 100-continue" -d @file.txt https://example.com/upload