“Vary tells a cache which request fields may affect the response, but it does not tell the cache which differences actually matter.”

Cloudflare Blog · Cloudflare ships Vary header support in Cache Rules

“The origin declares what may vary, and you decide how much variation is actually meaningful for the cache.”

Cloudflare Blog · Cloudflare ships Vary header support in Cache Rules

“Without Vary, whichever response enters the cache first can be served to both clients. If HTML wins, the API client receives markup and its JSON parser fails.”

Cloudflare Blog · Cloudflare ships Vary header support in Cache Rules