The Vary response header is used for indicating to browsers and other user agents which request header values to check when deciding whether to load content from a cache or to instead fetch a new response over the network.

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Vary is one of the most powerful HTTP response headers. Used correctly, it can do wonderful things. Unfortunately, this header is frequently used incorrectly, which can lead to abysmal hit ratios. Worse still, if it's not used when it should be, the wrong content could be delivered.

The Vary HTTP header is sent in billions of HTTP responses every day. But its use has never fulfilled its original vision, and many developers misunderstand what it does or don’t even realize that their web server is sending it.