field Registered
Verified 2026-09-18 · registry updated
2026-08-28If-Modified-Since
Asks a server to send a representation only if it changed after a given date.
Use it with Last-Modified for cache validation. If-None-Match takes precedence when both validators are present.
httpheadercacheconditional
Reference (http)
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
Use it with Last-Modified for cache validation. If-None-Match takes precedence when both validators are present.
Common mistakes
- Treating a 304 response as containing a response body.
IANA registry: http-fields/field-names
Registry reference: RFC 9110, Section 13.1.3: HTTP Semantics