# Vary header > Lists the request headers a response depends on, controlling cache key selection. Source: https://apiguide.dev/headers/vary Direction: response Registered: yes Defined in: RFC 9110, Section 12.5.5 Updated: 2026-07-10 ## What is the Vary header? The `Vary` response header, defined in RFC 9110, names the request headers that a cache must consider before serving a stored response. It tells caches that the same URL can produce different representations depending on those headers. ## API Usage & Best Practices * **Correct content negotiation**: Listing `Accept`, `Accept-Encoding`, or `Accept-Language` ensures a client asking for JSON is never served a cached HTML response, and a gzip-capable client is not handed a brotli body it cannot read. * **Protect private data**: Including `Authorization` prevents a shared cache from returning one user's personalized response to another. * **Use the wildcard sparingly**: `Vary: *` means the response varies on factors outside the request headers, which effectively makes it uncacheable by shared caches. ## Examples ```http HTTP/1.1 200 OK Content-Type: application/json Vary: Accept, Accept-Encoding ```