The Vary header silently doubling a store's cache

The Vary header silently doubling a store's cache

A homeware shop in Manchester had a healthy 88% hit ratio that should have been 97%. Storage costs were oddly high too.

We ran a Vary header audit:

— Listed every distinct Vary value the origin emitted per route
— Found Vary: User-Agent on all HTML — that splits cache across thousands of UA strings
— Found Vary: Cookie on pages that didn't use cookies at all
— Replaced UA-varying with edge device detection, normalized to three buckets: mobile, tablet, desktop
— Removed Cookie from Vary on anonymous routes

The cache stopped storing one copy per browser fingerprint. Hit ratio reached 96% and storage dropped by a third. Vary is the most expensive header on the web — every value you add multiplies your cache. Audit it route by route; most sites Vary on things they don't need.

The number that mattered: 8 points of hit ratio recovered by deleting two words.