Is HTTP 101 a client or server error?

101 is an informational code (1xx). It's an intermediate response — the final status will follow.

How is HTTP 101 different from 100 Continue?

Server received the request headers and the client should proceed with the request body. Both codes look similar but signal different situations — see the "Similar Codes" section below for a full comparison.

informational RFC 9110

HTTP 101 Switching Protocols

Server is switching to a different protocol (usually WebSockets or HTTP/2).

What it means

The client requested a protocol upgrade via the `Upgrade` header and the server is agreeing to switch. The connection stays open but subsequent frames are in the new protocol — most commonly WebSocket (`ws://` handshake completes here), but also HTTP/2 upgrades from HTTP/1.1.

When servers send it

•Client sent `Upgrade: websocket` — server responds 101 to complete the WS handshake
•HTTP/1.1 client upgrading to HTTP/2 (rare; modern clients start on HTTP/2 directly via ALPN)

What the client does

Stops speaking HTTP/1.1 on this connection and starts sending WebSocket frames (or the new protocol). The TCP connection is reused.

When you see it

•WebSocket handshake — expected and normal
•HTTP/2 cleartext upgrade (`h2c`) — expected on some server configs

Similar codes

100 Continue informational

Server received the request headers and the client should proceed with the request body.

103 Early Hints informational

Server is sending hints (usually Link: preload headers) before the final response.

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