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Response Header

Last-Modified

: Wed, 08 Jul 2026 10:15:00 GMT

The date and time a resource was last changed, used for cache validation.

DirectionResponse Header
StatusStandardized
Seen with200, 304, 412
Defined inRFC 9110, Section 8.8.2
Updated2026-10-14

What is the Last-Modified header?

The Last-Modified response header, defined in RFC 9110, carries an HTTP-date indicating when the origin server believes the resource was last modified. Clients store it and use it to ask whether their cached copy is still current.

API Usage & Best Practices

  • Conditional requests: Clients send the stored value back in If-Modified-Since; if the resource has not changed, the server replies with 304 Not Modified and no body, saving bandwidth.
  • Guarding writes: Sending the value in If-Unmodified-Since lets the server reject a request with 412 Precondition Failed if the resource changed in the meantime.
  • Weaker than ETag: Its one-second resolution can miss rapid updates, so ETag is preferred when available and takes priority when both are present.

Examples

HTTP/1.1 200 OK
Last-Modified: Wed, 08 Jul 2026 10:15:00 GMT

Status codes seen with it

3

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