Guide · serving-layer diagnosis

Diagnose where an old response came from

Start with the presence or absence of a new fetch, then collect positive evidence from the named worker, intermediary, or controlled origin.

Expected procedure · no live layer attribution was available for the 16 Sep 2026 run.

The quick path

No request? Check pageshow.persisted and back/forward history first. Then check whether a service worker controls the client and log its Cache API decision.
New request? Inspect the named intermediary’s Cache-Status, Age, and Via, then correlate a redacted A/B marker with the origin counter.
No positive signal? Repeat from a fresh client with fixed URL, host, redirects, and client inputs. Treat the source as unknown rather than inferring it from 200 or a missing header.

Each observation narrows only its own layer. A delegated worker fetch still requires downstream inspection.

Small probe checklist

Use a reserved synthetic URL and redacted marker A/B; do not log cookies, authorization, or personal data.
Keep method, URI, Host/SNI, redirect policy, request headers, client state, and edge/POP fixed.
Capture status, redirect chain, Cache-Control, Age, Cache-Status, Via, marker, timestamp, and origin counter.
Run a fresh profile with no service-worker registration and a direct-origin control; neither one erases evidence from other layers.
Repeat serialized and concurrent cases separately; one clean A/B result does not prove absence of leakage at another key, POP, partition, or hop.

Read the result carefully

Positive observation	Scoped reading	Do not infer
`pageshow.persisted === true` and no new request	History/bfcache can explain this display.	That the original bytes came from the HTTP cache.
Worker log says `Cache.match` hit	This worker/Cache API supplied the response in this client.	That the browser HTTP cache or edge is empty.
Named `Cache-Status: edge-test; hit` plus unchanged origin counter	The named cache reports satisfying this request without forwarding.	That hidden hops, other POPs, or other users have the same result.
Origin counter increments with marker B	The controlled origin received this request and emitted B.	That another layer cannot still display A.

Expected, not measured: the runner had no curl, browser, authorized endpoint, or permitted loopback listener. Use the full dated report for the unexecuted command template, warm-up order, standards sources, and edge cases.