staleiferror

Stale-if-error is an HTTP Cache-Control extension directive used by caching systems to improve the availability of content when the origin server is unavailable or returns an error. It allows a cache to serve a previously stored, now stale, response to a new request under certain error conditions, rather than returning a hard failure to the client.

The directive is specified as stale-if-error=<seconds>, and is used in conjunction with other caching directives such as max-age. When a cached response has expired and a request would normally require revalidation, a cache that supports stale-if-error may deliver the stale response if the origin cannot be reached or returns a 5xx error, provided the time since expiration does not exceed the specified number of seconds. If the origin later provides a fresh, valid response, the cache updates accordingly.

Example: Cache-Control: public, max-age=60, stale-if-error=3000

Support and behavior can vary by implementation. Many modern caches, CDNs, and proxy servers implement stale-if-error to improve resilience, especially for content that is acceptable to serve slightly out of date during brief outages. However, it may expose clients to outdated data, so its use should align with the tolerance for staleness and the criticality of up-to-date information for the content in question.

Relation to other directives: stale-if-error is related to, but distinct from, stale-while-revalidate (which serves stale content while revalidating in the background). Together, these directives give caches options to balance freshness and availability.