Warning
The HTTP Warning header was a general-purpose header carrying additional information about potential problems with a message. This header has been removed from the HTTP standard.
Usage
The Warning header was defined in the original caching specification to communicate supplementary status information beyond what the response status code conveyed. Caches and intermediaries attached warning codes to flag conditions such as stale responses, failed revalidation, or applied transformations.
The current caching specification removed Warning from the HTTP Caching specification. The header is no longer part of the standard, and modern clients, servers, and proxies are not expected to generate or interpret the value. Legacy systems still in operation are the only remaining source of this header.
Deprecated
The Warning header has been removed from the HTTP standard. Modern implementations do not generate or process this header.
The general syntax was:
Warning: <warn-code> <warn-agent> <warn-text>
[<warn-date>]
Values
warn-code
A three-digit number identifying the warning type. The first digit distinguished two categories:
- 1xx codes described the freshness or validation status of a cached response. These entries were deleted after successful revalidation.
- 2xx codes described a persistent condition of the representation not remedied by revalidation.
Previously defined warning codes:
| Code | Text |
|---|---|
| 110 | Response is Stale |
| 111 | Revalidation Failed |
| 112 | Disconnected Operation |
| 113 | Heuristic Expiration |
| 199 | Miscellaneous Warning |
| 214 | Transformation Applied |
| 299 | Miscellaneous Persistent Warning |
warn-agent
The name or pseudonym of the server or proxy adding
the warning. When the agent identity is unknown, a
hyphen (-) served as a placeholder.
warn-text
A human-readable description of the problem.
warn-date
An optional timestamp indicating when the warning was generated.
Example
A cache serving a stale response attaches warning code 110 to alert downstream consumers. The warn-agent identifies the cache, and the text provides a human-readable explanation.
Warning: 110 cdn.example.re "Response is Stale"
A proxy applied a content transformation (such as image Compression) and flagged the modification with code 214.
Warning: 214 proxy.example.re
"Transformation Applied"
Takeaway
The Warning header was part of the HTTP Caching specification until the current revision removed the header from the standard. Legacy systems still emit the header, but modern HTTP stacks neither generate nor act on Warning values.