HTTP headers / response header
Vary: Accept-Encoding, Origin
Tells caches which request headers change the response — each combination gets its own cache entry.
Missing Vary: Origin on CORS responses makes CDNs serve one origin's ACAO header to another origin: intermittent, infuriating CORS failures.
The fastest way to see what Vary is actually doing is to send the request and read both sides raw — ReqPad shows auto-generated and custom headers for every request, on all six protocols, with history. Related references: status codes · WWW-Authenticate · Content-Encoding · Transfer-Encoding · ETag · Last-Modified · Expires
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.