What does HTTP status 100 (Continue) mean?

HTTP_STATUS_CONTINUE

HTTP 100 is used with the Expect: 100-continue mechanism. It allows a client to send request headers first and wait before sending a potentially large request body.

This response is informational, not a final success result. A server can still reject the request with a final status after examining the headers, so clients must continue to read the response stream.

RFC 9110: HTTP Semantics

Operational meaning of HTTP 100 for HTTP_STATUS_CONTINUE

HTTP 100 (this result) is a informational response. AllStat records its meaning as “100 Continue”. Interpret it with the request method, selected representation, response headers, and the specification or product that defines the code; the number alone does not prove that application work completed.

Client and server handling

• Keep reading until a final response arrives; this result is not the terminal application result.
• do not commit user-visible success or failure from this informational response.
• record intermediaries because some gateways suppress or transform 1xx responses.

For telemetry, record HTTP 100, method, authority, route template rather than secrets, response headers relevant to caching or retry, latency, upstream identity, and whether the response originated at the application or an intermediary.

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