ETag
A version tag for the response, so a client can ask "has this changed?" without downloading it again.
Explained
Examples
Example
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"Watch out
Weak tags start with W/ and only promise the content is equivalent. A tag that changes on every server gives no caching benefit behind a load balancer.
Defined in RFC 9110 §8.8.3.