100 Continue

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

When a server should send it. Only when the request carried Expect: 100-continue and the server is willing to receive the body. If it will reject the request anyway, for example because the body is too large or the login is missing, it should send that final status straight away instead.

Common causes. A client sent Expect: 100-continue before uploading a large body, to check the server would accept it first.

What to do. Nothing. If uploads stall at this point, a proxy in between may not support Expect: 100-continue; disable it in the client.

Browsers and HTTP clients. Browsers never ask for it. curl sends Expect: 100-continue for larger uploads and waits up to a second for the 100 before sending the body anyway. HTTP/2 and HTTP/3 support it as an interim response too.

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.

HTTP/1.1 100 Continue

(the final response follows on the same connection)