100 Continue

What is HTTP 100?

This interim response tells the client that the initial part of the request has been received and not yet rejected by the server. The client should continue sending the remainder of the request, or ignore this response if the request is already complete. It is commonly used when a client sends an Expect: 100-continue header before transmitting a large request body. The server uses this to confirm it is willing to accept the data before the client commits to sending it.

Defined in RFC 9110

Common Causes

• Client sent an Expect: 100-continue header with a POST or PUT request
• Large file uploads where the client wants confirmation before transmitting the body
• API calls that check server readiness before sending substantial payloads

How to Fix

• No action needed; this is normal behavior indicating the server is ready to receive data
• If you see unexpected delays, check whether your HTTP client library handles 100 Continue correctly
• Some proxies or older servers may not support this; consider removing the Expect header if issues arise