The response header, Vary , has been called “ the ugliest part of HTTP that we haven't yet improved. ” The same post describes it as a “horrible, kludgy mechanism” with “pretty abysmal interoperability” across intermediaries. That is usually where sensible engineers back away slowly with their hands raised.

That’s not exactly an endorsement of Vary , but ugly doesn’t mean useless.

One URL can have more than one correct response. A server might, for example, deliver different image formats to different browsers. If a cache ignores Vary , it risks serving the wrong bytes to a request. But if it treats every raw header value as distinct, a handful of similar requests can spread into thousands of barely reusable cache entries. Vary tells a cache which request fields may affect the response, but it does not tell the cache which differences actually matter.

Vary support is now available in Cache Rules on every plan.…