304 Not Modified

Not modified: the client's cached copy is still valid, so no body is sent.

304 at a glance

Code: 304
Name: Not Modified
Class: 3xx Redirection
Status: IANA-registered standard code
Defined in: RFC 9110, Section 15.4.5
Cacheable by default: It updates a stored response rather than being stored itself: caches refresh the cached 200 with its headers.

What 304 means

304 Not Modified answers a conditional GET or HEAD. The client sent If-None-Match (ETag) or If-Modified-Since, the resource has not changed, and the server tells the client to reuse its cached copy. The response has no body, which saves bandwidth, but it still costs a round trip; long max-age avoids even that.

What typically causes a 304

Browsers revalidating cached CSS, JS and images after max-age expires, or on reload.
CDNs revalidating with the origin.
API clients using ETags to poll efficiently.

Fix it on the server

Send the same Cache-Control, ETag, Expires, Vary and Content-Location headers you would send on the 200.
If ETags differ between servers behind a load balancer (Apache used to include inode numbers), clients never get 304s; use content hashes or FileETag MTime Size.
Weak ETags (W/"...") are fine for 304; nginx turns strong ETags into weak ones when it gzips.

Fix it as a client or visitor

A 304 with stale content means the validator matched wrongly; check that the server changes ETag or Last-Modified when content changes.
Hard reload (Ctrl+Shift+R) bypasses revalidation if you need a fresh copy.

Headers that go with 304

Header	Role
`ETag`	Current validator, compared against If-None-Match.
`Last-Modified`	Compared against If-Modified-Since.
`Cache-Control`	Refreshes freshness lifetime of the cached copy.

Example response

HTTP/1.1 304 Not Modified
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
Cache-Control: max-age=600
Date: Tue, 10 Nov 2026 12:00:00 GMT

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