The Warning general HTTP header contains information about possible problems with the status of the message. More than one Warning header may appear in a response.

Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only be applied to response messages.

Header type General header
Forbidden header name no


Warning: <warn-code> <warn-agent> <warn-text> [<warn-date>]


A three-digit warning number. The first digit indicates whether the Warning is required to be deleted from a stored response after validation.
  • 1xx warn-codes describe the freshness or validation status of the response and will be deleted by a cache after deletion.
  • 2xx warn-codes describe some aspect of the representation that is not rectified by a validation and won't be deleted by a cache after validation unless a full response is sent.


The name or pseudonym of the server or software adding the Warning header (might be "-" when the agent is unknown).

Advisory text describing the error.
Optional. If more than one Warning header is sent, include a date that matches the Date header.

Warning codes

The HTTP Warn Codes registry at iana.org defines the namespace for warn codes.

Code Text Description
110 Response is Stale A response provided by a cache is stale (the expiration time set for it has passed).
111 Revalidation Failed An attempt to validate the response failed, due to an inability to reach the server.
112 Disconnected Operation The cache is disconnected from the rest of the network.
113 Heuristic Expiration Sent If a cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.
199 Miscellaneous Warning Arbitrary, non-specific warning
214 Transformation Applied Added by a proxy if it applies any transformation to the representation, such as changing the content-coding, media-type or the like.
299 Miscellaneous Warning Same as 199, but indicating a persistent warning


Warning: 110 anderson/1.3.37 "Response is stale"

Date: Wed, 21 Oct 2015 07:28:00 GMT 
Warning: 112 - "cache down" "Wed, 21 Oct 2015 07:28:00 GMT"


Specification Title
RFC 7234, section 5.5: Warning Hypertext Transfer Protocol (HTTP/1.1): Caching

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari Servo
Basic Support(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)
Feature Android Chrome for Android Edge Mobile Firefox for Android IE Mobile Opera Mobile Safari Mobile
Basic Support(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)

See also

Document Tags and Contributors

 Contributors to this page: fscholz
 Last updated by: fscholz,