304 Not Modified
The cached copy the client already has is still valid, so the server sends no body. It answers a conditional GET or HEAD.
3xx · Redirection
Typical causes
A browser or CDN revalidating with If-None-Match (an ETag) or If-Modified-Since. Stale content usually means the cache never revalidates, not that 304 is wrong.
Defined in RFC 9110, §15.4.5