This week in caching: the Vary header, friend and foe

This week in caching: the Vary header, friend and foe
Three reads on the most quietly destructive response header.
— Vary: User-Agent shreds your cache — thousands of UA strings means near-zero shared-cache reuse; almost never what you want.
— Vary: Accept-Encoding is the one you do want — separates gzip/br/identity correctly; most CDNs handle it internally so you may not even need to send it.
— Client Hints + Vary: Sec-CH-* — the modern way to vary on viewport/DPR without the UA-string explosion. Opt-in via Accept-CH.
Bookmark: Andrew Betts' "Best practices for Vary" — the rule "vary on as little as possible" is the whole discipline in one line.