informational RFC 9110
HTTP 100 Continue
Server received the request headers and the client should proceed with the request body.
What it means
The server received a request that included an `Expect: 100-continue` header and is telling the client to send the request body. This is an optimization for large POST/PUT uploads — the client asks first, and if the server would reject anyway (auth failure, missing header), it can respond with an error before the client wastes bandwidth uploading a large body.
When servers send it
- •Client sent `Expect: 100-continue` in the request headers
- •Server has validated the headers and is ready to accept the body
- •Almost always automatic — libraries handle this transparently
What the client does
Sends the request body. Once the body is fully uploaded, the server responds with the actual status (200, 201, 400, etc.).
When you see it
- •Curl with a POST body larger than 1024 bytes automatically sends `Expect: 100-continue`
- •Java HttpURLConnection and some libraries use it by default for chunked uploads
Similar codes
101 Switching Protocols informational
Server is switching to a different protocol (usually WebSockets or HTTP/2).
200 OK success
The request succeeded and the response body contains the requested data.
103 Early Hints informational
Server is sending hints (usually Link: preload headers) before the final response.
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