HTTP 100 Continue
An interim response telling the client to go ahead and send the request body — normal when the client sent Expect: 100-continue.
100 ContinueHTTP/1.1 100 ContinueExpect: 100-continue Meaning
Clients uploading large bodies can first send only the headers with Expect: 100-continue. The server answers 100 Continue if it will accept the body, or a final error (401, 413, 417) if it won’t, saving a wasted upload.
You mostly see it in curl -v output or logs. Problems come from servers/proxies that don’t support the handshake: the client waits about a second for a 100 that never comes, or the proxy answers 417.
Common causes
- Client (curl, .NET HttpClient, PHP cURL) automatically sends Expect: 100-continue for large POST bodies
- Old proxy or server that doesn’t implement the handshake, causing a delay
- Logs/monitoring treating 100 as a final status
⚡ Quick fix
- Treat 100 as interim — the final status follows
- Remove the Expect header if a proxy mishandles it
- Look past the 100 to the real final response
Detailed fix by platform
curl
curl -H 'Expect:' -T big.zip https://example.com/upload # empty value disables Expect: 100-continue
PHP
curl_setopt($ch, CURLOPT_HTTPHEADER, ['Expect:']); // stop libcurl adding Expect: 100-continue
.NET
client.DefaultRequestHeaders.ExpectContinue = false;
How to diagnose
- Request headers — Is Expect: 100-continue being sent?
- Final status — What comes after the 100?
- Delay — Does the upload stall ~1s before sending?
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Last updated 13 Oct 2026