101 Switching Protocols

The server agrees to change protocol on this connection, as the client asked in its Upgrade header.

Code: 101
Reason phrase: Switching Protocols
Class: 1xx Informational
Defined in: RFC 9110 section 15.2.2
Cacheable by default: No

What it means

An interim response: the server sends it before the final one, and the client keeps waiting.

When a server should send it. Only in reply to a request with an Upgrade header naming a protocol the server will switch to, such as websocket. The response must carry Upgrade and Connection: Upgrade, and the new protocol starts straight after the blank line.

Common causes. Opening a WebSocket, or upgrading a connection to HTTP/2 over plain text.

What to do. Nothing when it works. If WebSockets fail behind a proxy, make sure the proxy passes the Upgrade and Connection headers.

How clients and crawlers treat it

Browsers and HTTP clients. Browsers see it when a WebSocket opens over HTTP/1.1. HTTP/2 has no Upgrade mechanism, so WebSockets over HTTP/2 use the extended CONNECT method (RFC 8441) and get a 200 instead.

Google Search. An interim response has no effect on indexing; Google acts on the final status that follows it. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Never cached: an interim response is not a final answer to the request.

Example response

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

(the connection now speaks the new protocol)

Applications almost never send a 1xx response by hand: the web server or HTTP library produces it as part of the protocol.

Related codes

All status codes · All 1xx codes

Names and numbers from the IANA HTTP Status Code Registry. Google Search behaviour as documented by Google Search Central.