The server has agreed to change protocol.
What does HTTP 101 mean?
The 101 Switching Protocols code is part of the 1XX class. This is an informational response — the server received the request and the client should continue or wait.
When is it used?
A server returns 101 Switching Protocols in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.