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101 Switching Protocols and the WebSocket handshake

101 Switching Protocols confirms the server accepted an Upgrade request, typically switching the connection from HTTP/1.1 to WebSocket.

Check a URL’s status code

101 Switching Protocols means the server agreed to the client’s request to change protocols on the current connection. Right after the 101, both sides stop speaking HTTP/1.1 and start speaking the new protocol, which in practice is almost always WebSocket.

How the upgrade works

RFC 9110, section 15.2.2 ties 101 to the Upgrade header: the server switches only when the client asks, and it must send an Upgrade header naming the protocol that takes over. The WebSocket handshake from RFC 6455 is the best-known case:

GET /chat HTTP/1.1
Host: app.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

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

101 belongs to HTTP/1.1. HTTP/2 dropped support for it: WebSockets over HTTP/2 use an extended CONNECT request instead (RFC 8441).

Why the upgrade fails

  • A reverse proxy or load balancer drops the hop-by-hop Upgrade and Connection headers, so the app sees a normal request and replies 200 or 400 instead of 101.
  • The proxy closes idle connections after its read timeout, cutting live sockets.
  • A CDN or hosting plan doesn’t allow WebSockets on that hostname.

For Nginx in front of a Node.js or Python app, forward the headers explicitly:

location /ws/ {
    proxy_pass http://127.0.0.1:3000;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_read_timeout 3600s;
}

SEO impact

A 101 never applies to the page itself. Crawlers request HTML with ordinary GET requests and get a normal final status. The risk is content: text that only arrives through a WebSocket after the page loads isn’t in the server HTML that many crawlers, and our checkers, read. Put headings, body copy and links in the initial response (see JavaScript SEO) and use the socket for live updates. Confirm the page URL itself returns 200 with the HTTP status checker.

Frequently asked questions

Is 101 Switching Protocols an error?

No. It means a protocol upgrade succeeded, usually a WebSocket handshake. In browser DevTools, a working WebSocket connection shows status 101.

Why does my WebSocket get 200 or 400 instead of 101?

The Upgrade and Connection headers were lost on the way, most often at a reverse proxy or load balancer that isn’t configured for WebSockets. It can also mean the URL isn’t a WebSocket endpoint at all.

Does HTTP/2 use 101 Switching Protocols?

No. HTTP/2 removed support for 101. Over HTTP/2, WebSockets are opened with an extended CONNECT request defined in RFC 8441.