Glossary · HTTP, edge and rendering
Age header
A response header, in seconds, saying how long ago the served copy was built.
What Age header means
A response header, in seconds, saying how long ago the served copy was built. It is the one direct signal that a response came from a cache rather than the origin, and its value is what a scanner reads before deciding whether an old copy is stale: an old copy that matches a fresh one is a note, an old copy that differs is an edit stuck behind the cache.
What Age header can and cannot support
| It can support | It cannot support |
|---|---|
| A response header, in seconds, saying how long ago the served copy was built. | It is the one direct signal that a response came from a cache rather than the origin, and its value is what a scanner reads before deciding whether an old copy is stale: an old copy that matches a fresh one is a note, an old copy that differs is an edit stuck behind the cache. |
How CrawlCheck measures Age header
One finding code in the scanner read this. The share of all scans carrying each is computed from the live corpus when this page is served, not typed into it.
| Finding code | Share of scans | Guide |
|---|---|---|
STALE_CACHE_SERVED | 16.7% | Is Cloudflare blocking AI crawlers on your site? What changes on 15 September 2026 |
Every code, its rule and its current rate: the dataset page. Scan a site to see which apply to it.
Where Age header comes up on this site
- How to write an llms.txt, and how to check it is actually being servedMACHINE_FILE_CACHE_STALE is the code; the fix is a purge, and the thing to check afterwards is the age header rather than your own browser, which has its own cache and will lie to you.
- Cached robots.txt and llms.txt: stale, split, or blockedThe age header says how many seconds ago the stored copy was built.
Related terms in HTTP, edge and rendering
The delivery layer between an origin and a client: proxies, caches, redirects, status codes and the headers that decide what a crawler actually received.
Origin server · Reverse proxy · CDN · Edge node · WAF · Bot score · TLS handshake · DNS resolution · DNS timeout · Connection timeout · Read timeout · Redirect chain · Redirect loop · 301 redirect · 302 redirect · 307 redirect · 308 redirect · 304 response · 404 response · 410 response · 429 response · 500 response · 502 response · 503 response · 504 response · HEAD request · GET request · Content type · MIME sniffing · Content encoding · Compression · Brotli · ETag · Last-Modified · If-None-Match · Cache hit · Cache miss · Cache key · Vary header · Stale response · Stale-while-revalidate · Edge redirect · Origin bypass · Response parity · Header parity · hreflang · canonical URL · HSTS · HSTS preload list · Content-Security-Policy · CSP nonce · edge cache · cache split · cache-buster · Cache-Control · render-blocking resource · payload text share · lazy loading · 404 and 410 status codes
Questions about Age header
What is Age header?
A response header, in seconds, saying how long ago the served copy was built.
What does Age header not show or guarantee?
It is the one direct signal that a response came from a cache rather than the origin, and its value is what a scanner reads before deciding whether an old copy is stale: an old copy that matches a fresh one is a note, an old copy that differs is an edit stuck behind the cache.
How does CrawlCheck measure Age header?
Through the finding code STALE_CACHE_SERVED, each with a stated rule, evidence bytes and a live share of scans on the dataset page.
Which area of the glossary does Age header belong to?
HTTP, edge and rendering: The delivery layer between an origin and a client: proxies, caches, redirects, status codes and the headers that decide what a crawler actually received.
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See it in the full glossary · 668 terms across 19 areas.