What this tool does
Paste your cache directives, the request, and the upstream response. The tool answers two separate questions — will this response be stored, and will a later matching request be served from it — with the ordered rule walk behind each, plus the computed cache key.
They are separate on purpose. A response can be perfectly cacheable and never served because the key differs; a response can be served when it should not be because the key ignores something that mattered.
The store rules, in order
Caching is off entirely unless proxy_cache names a zone — by far the most common reason nothing is ever cached. Only the methods in proxy_cache_methods are cached, defaulting to GET and HEAD. A response carrying Set-Cookie is not stored, on the assumption it is user-specific. Upstream Cache-Control: no-cache, no-store or private prevents storage. A status with no proxy_cache_valid and no freshness header has no lifetime to be stored under. And proxy_no_cache with a non-empty, non-zero value prevents the write.
The pair people swap
proxy_no_cache prevents the write. proxy_cache_bypass skips the lookup and still writes the result. They read like synonyms and are opposites in the half that matters — using bypass where you meant no_cache fills the cache with exactly what you were trying to keep out.
The asymmetry that leaks
NGINX excludes cookied responses from storage. It does not exclude cookied requests from being served a shared entry.
That is usually harmless, because the personalised responses were never stored — until someone adds proxy_ignore_headers Set-Cookie to make caching "work" on a backend that sets a session cookie on every response. Now per-user responses sit under a key every user shares. The tool warns on exactly this combination, because it is a two-line change that looks like a performance fix.
Honest limits
One exchange, not the cache lifecycle: no proxy_cache_use_stale, no proxy_cache_lock, no revalidation, no min_uses accounting, no Vary handling. Freshness is treated as present or absent rather than computed, so an expired entry is out of scope. Key rendering substitutes the common variables and leaves anything else as written.