Continue
ActiveHTTP 100 Continue indicates the server has received the request headers and the client should proceed to send the request body. Defined in RFC 9110 §15.2.1. Used with the Expect: 100-continue header to avoid sending large bodies to servers that would reject them.
Description
The 100 Continue status code indicates that the initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.
When a client sends a request with an `Expect: 100-continue` header, it is asking the server to validate the headers before the client sends the (potentially large) request body. If the server responds with 100 Continue, the client proceeds. If it responds with an error (like 413 or 401), the client avoids sending the body unnecessarily.
Examples
POST /upload HTTP/1.1
Host: example.com
Content-Length: 50000000
Expect: 100-continue
(client waits for 100 Continue before sending body)HTTP/1.1 100 Continue
(client now sends the request body)Edge Cases
- •Not all servers support Expect: 100-continue. Clients should implement a timeout and send the body anyway.
- •Proxies must forward the 100 response or generate one themselves.
- •HTTP/2 and HTTP/3 handle this differently — the mechanism is implicit in stream flow control.
When You'll See This
- →Uploading large files via POST/PUT
- →APIs that validate authentication before accepting body
- →Proxies that need to check forwarding rules before relaying body
Implementation References
| Language | Constant |
|---|---|
| Go | http.StatusContinue |
| Rust | http::StatusCode::CONTINUE |
| Python | http.HTTPStatus.CONTINUE |
| Node.js | http.STATUS_CODES[100] |
| .NET | HttpStatusCode.Continue |
| Java | HttpURLConnection.HTTP_CONTINUE |
History
Introduced in HTTP/1.1 (RFC 2068, 1997) to solve the problem of clients sending large request bodies to servers that would immediately reject them. Refined in RFC 9110 (2022).
Related Status Codes
Related Headers
FAQ
What does HTTP 100 Continue mean?
HTTP 100 Continue tells the client that the server has received the request headers and is ready to accept the request body. The client should proceed to send the body.
When is HTTP 100 used?
It is used when a client sends an Expect: 100-continue header, typically before uploading large files, to verify the server will accept the request before transmitting the entire body.