Resource cache stores only third-party tag/personalization scripts (0 first-party) — it should be restricted by host

Start with packages/browser/src/ResourceCache.ts and its isCacheableRequest entry point; review how resourceCache configuration is defined and how cache hosts are currently handled. Define completion around host-or-subdomain matching, empty-list compatibility, and tests covering the chosen default behavior and replay eligibility.

What happens

ResourceCache will store any host's script or stylesheet that passes the HTTP policy check, so
third-party tag-manager, A/B-testing and personalization scripts get cached alongside the site's own
assets. There is no host restriction of any kind.

Measured 2026-09-16 against a live storefront, with the cache warm: a mobile home-page render stores
54 entries / 5.1 MB, and every one of them is third-party — not a single first-party asset. Across
home + a product page it is 69 entries, same story:

host	entries
assets.adobedtm.com (Adobe Launch + its rule bundles)	47
display.ugc.bazaarvoice.com / apps. / analytics-static.	6
cdn.cookielaw.org (OneTrust)	2
pub.loudcrowd.com	2
one each: personalization vendor, Clicktale, Constructor, Taboola, PubMatic, ID5, GTM, impactradius, coherentpath, cnnx, fohr, Google shopping agent	12
first-party (*.<storefront>)	0

CACHEABLE_RESOURCE_TYPES is ['stylesheet','script'], so the stored population is bundles — not the
XHR/fetch responses a vendor delivers its per-session decisions over.

#109 is why there are no first-party entries, and the two compound: that issue excludes everything
safe to cache, this one keeps everything that isn't.

Mechanism

isCacheableRequest (packages/browser/src/ResourceCache.ts) gates on method, navigation, resource
type and auth/cookie headers only:

isCacheableRequest(req) {
    if (req.method() !== 'GET') return false;
    if (req.isNavigationRequest()) return false;
    if (!CACHEABLE_RESOURCE_TYPES.has(req.resourceType())) return false;
    const headers = req.headers();
    if (headers['authorization'] || headers['cookie']) return false;
    return true;
}

Anything with a positive max-age is then eligible, and tag vendors serve their bundles with long
max-ages (observed: 1200s to 30 days).

The content loss this issue was filed for has been explained, and fixed

Filed on the hypothesis that replaying session-scoped scripts changes the snapshot. The measurement
was real — on a live storefront's mobile home page, with the cache enabled, the served HTML lost
~5,290 characters of text, 22 links and 16 images on every render, interleaved off/on/off/on with the
arms internally identical. The attribution was wrong.

c515d76 (browser v1.24.0) found the actual mechanism: a cached response whose headers repeat
reaches us as puppeteer's \n-join of the values, and CDP refuses to fulfil a response carrying one
(Fetch.fulfillRequest → Invalid header: <name>, rejecting the whole payload). The rejection was
swallowed (.catch(noop)), so the request was never answered at all — the resource silently never
loaded, for the rest of the render. Identical signature: content missing, timing unchanged, 200,
non-empty, nothing logged.

Confirmed by reverting the fix (2026-09-16, same page, same interleave, released v1.24.0 build with
both halves of c515d76 reverted to their 1.16.0 behaviour):

build	text	links	imgs	bytes	cacheReplaysRefused
v1.24.0 as released	20,641 → 20,641 (0)	421 → 421 (0)	250 → 250 (0)	+1	0
fix reverted	20,641 → 18,800	421 → 418	250 → 247	−48,864	8 (2/render)

Arms internally identical (spread 0), so every delta is real by this issue's own bar. Three of the 54
cached entries carry a repeated header and two are refused per render:

d.impactradius-event.com/…/A375953-….js            x-goog-hash                   (2 values)
agents.cloud.google.com/shopping_agent/static.js    content-security-policy      (3 values)
cdn.cookielaw.org/consent/…/otSDKStub.js            access-control-expose-headers (2 values)
                                                     cross-origin-resource-policy (2 values)

The −48,864 bytes independently reproduces the "~49 KB smaller" c515d76 measured on a product page.

What is still open

The host restriction. It no longer rests on a demonstrated content loss; it rests on this: the
cache's entire value is the site's own bundles (megabytes per render), third-party tags contribute
nothing to the snapshot, and on this deployment they are 100% of what it stores. Three of them
were actively costing content until this week. Caching them is all risk and no return.

Proposal unchanged: resourceCache.hosts: string[], matched as host-or-subdomain suffixes, empty list
preserving today's cache-any-host behaviour. The library can't infer the right set — a storefront's
asset host is frequently a different registrable domain from the navigation host — so a same-site
default derived from the navigation URL handles the simple case but can't be the only mechanism.

Worth deciding whether the restrictive behaviour should be the default. A deployment that already
blocks tag hosts through block.urlPatterns never sees any of this, which is a good way for it to stay
unnoticed.

Untested, and no longer load-bearing for this issue: whether a stale configuration bundle changes the
snapshot. Only script/stylesheet are cached, so a per-session decision fetched over XHR cannot be
replayed; what can go stale is a rule bundle that bakes decisions in — which is precisely what those
47 assets.adobedtm.com/.../RC*.js entries are. Testing it needs an aged cache (warm on a pod,
replay a day later) or a diff of a replayed bundle against a fresh fetch. A cache warmed minutes before
the replay cannot see it, and will read as a clean null result.

How to test for it

Interleave the arms (off/on/off/on) rather than running all-off then all-on: these pages vary on their
own between renders, and an A,A,B,B ordering attributes site-side session variance to the cache. Then
treat a cross-arm difference as real only when it exceeds the within-arm spread. Comparing text length,
anchor count, image count and JSON-LD count of the served HTML is enough; a raw byte diff is too noisy.
Read cacheReplaysRefused (added in v1.24.0) alongside — a non-zero count means resources are being
dropped rather than replayed.