When is 100 Continue used?

It is used when a client sends a large request body and includes the Expect: 100-continue header to check if the server will accept the request before transmitting the full payload.

Do all servers support 100 Continue?

Most modern HTTP/1.1 servers support it, but some older servers or proxies may ignore the Expect header.

HTTP 100 Continue — What It Means & How to Fix It

Informational1xx informational

Overview

The HTTP 100 Continue status code means the server has received the initial part of the request and the client should continue sending the rest of the request body.

Key Details

The 100 Continue status is an interim response indicating the initial request has been received.
It is commonly used when a client sends a large request body and wants confirmation before transmitting all data.
The client must send the Expect: 100-continue header to trigger this response.
If the server rejects the request, it responds with a final status code instead of 100.

Common Causes

Client sends Expect: 100-continue header before uploading a large payload.
Server acknowledges it is ready to accept the full request body.
Proxy servers forwarding the Expect header from the original client.

Steps

1Verify your client is correctly handling the 100 Continue interim response before sending the body.
2Ensure the server is configured to support the Expect header.
3If uploads stall, check timeout settings on both client and server.
4Review proxy configurations that might strip the Expect header.