1xx Informational

Informational responses

Interim responses. The server acknowledges the request and continues working before sending a final status.

A 1xx response is never the end of an exchange. The server sends it, then keeps going and sends a final status on the same connection. Clients that treat the interim response as the result will misread every request that produces one.

HTTP/1.0 predates interim responses, so a server must not send a 1xx to an HTTP/1.0 client. Intermediaries that buffer whole responses tend to swallow them, which is why 100 Continue and 103 Early Hints often disappear behind a proxy or CDN.

Codes in this class

• 100 Continue: The server has read the request headers, found nothing wrong with them, and is ready for the client to send the request body.
• 101 Switching Protocols: The server is switching the connection to the protocol the client asked for in the Upgrade header.
• 102 Processing: WebDAV interim response telling the client that a long-running request is still being worked on so it does not time out.
• 103 Early Hints: An interim response carrying Link headers so the browser can start preloading assets before the final response is ready.