HTTP 103 (HTTP_EARLY_HINTS): meaning and handling

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: HTTP 103 lets a server send selected response headers before its final response.

Could be also:
Constant	Type	OS
ERROR_TOO_MANY_SEM_REQUESTS	Win32 error	Windows
ECONNABORTED	errno	Linux
ENOPOLICY	errno	Mac
EALREADY	errno	Windows
CONFIG_INITIALIZATION_FAILED	BugCheck Code	Windows

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HTTP_STATUS_PROCESSING	HTTP_STATUS_OK

HTTP_EARLY_HINTS

HTTP 103 is an informational response. It is normally used with Link headers so a user agent can begin preloading resources while the origin server is still preparing the final response.

A client must treat the eventual final response as authoritative. Do not use 103 as an application result or as proof that the requested resource was successfully generated; it only carries early, non-final metadata.

RFC 9110: HTTP Semantics

Operational meaning of HTTP 103 for HTTP_EARLY_HINTS

HTTP 103 (this result) is a informational response. AllStat records its meaning as “103 Early Hints”. 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 103, 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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