What does the Vary header do for caching?

Vary is an HTTP response header that lists the request headers the origin used to choose the response, such as Accept-Encoding or Accept-Language. A cache may reuse a stored response only for later requests whose listed headers match the original ones, so each combination of values becomes its own cache entry.

Vary: Accept-Encoding

Vary: *

Vary: * means the response depends on something outside the request headers, so no stored response can be matched to a later request without asking the origin. In practice it disables reuse.

• Each value adds variants. Vary: Accept-Language can mean dozens of copies of one page, each of which starts cold after a purge.
• Variants are part of the effective cache key, and a broad Vary lowers the cache hit ratio.
• CDNs limit it in their own ways. Vercel, for example, does not cache a response whose Vary names Cookie, and returns it with x-vercel-cache: MISS.

When you warm a cache, send the header values your visitors send, or the warmed variant is not the one they get.