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Last-Modified Header

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

Header TypeResponse Header
SpecificationStandard RFC compliant
Associated CodesHTTP 200,HTTP 304,HTTP 412

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

Associated HTTP Status Codes

HTTP 200 — OK

The request has succeeded. The response payload contains the requested resource representation.

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HTTP 304 — Not Modified

Instructs the client that the cached version of the resource is still valid.

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HTTP 412 — Precondition Failed

One or more preconditions in the request headers evaluated to false.

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