Understand cache headers without guessing

"Is this cached?" is not obvious

A resource keeps returning stale data, or never caches, or revalidates on every request. The answer is in the response headers — Cache-Control, ETag, Expires, Last-Modified — but reading them by eye is easy to misread.

Decode the headers

Paste the raw headers into the Cache Header Analyzer. It explains each directive: max-age (fresh window), no-cache (cache but revalidate), no-store (never cache), immutable (no revalidation), plus whether ETag/Last-Modified enable conditional requests.

Worked example

Cache-Control: max-age=3600, immutable + ETag means the browser can reuse the asset for an hour without even a conditional request. Remove immutable and it will revalidate via ETag instead.

Boundaries

• Interprets headers you paste; it does not fetch URLs or measure real cache hits.
• In HTTP/1.1, max-age overrides Expires when both are present.

FAQ

no-cache vs no-store? no-cache still caches but revalidates; no-store never caches at all.

Why revalidate? ETag/Last-Modified let the server answer 304, saving bandwidth while staying correct.