The index.html boot guard now fires a fire-and-forget beacon (POST /telemetry/unsupported)
when it turns a client away on the unsupported-engine screen, so the owner can see — on the
Users dashboard beside "app opens" — how many real clients hit it and on which engines.
- Client: navigator.sendBeacon (fetch fallback) with a localStorage dedup keyed by app version
+ reason + Chromium, so a user reopening the app is one report, not ten.
- Gateway: a new unauthenticated POST /telemetry/unsupported handler (the client never booted,
so it carries no session), per-IP public-limited and body-capped, mirroring the export-download
route. It folds the beacon into the OTel counter unsupported_engine_total {reason =
no_bigint/no_proxy/boot_error/other, chromium}, with reason allow-listed and the Chromium major
reduced to a bounded range (normalizeUnsupported) so a spoofed beacon cannot inflate the metric
cardinality; the full user agent is logged, not labelled.
- Caddy: /telemetry/* added to the @gateway matcher (else it falls to the landing catch-all).
- Grafana: two panels on the Scrabble — Users dashboard (by reason, by Chromium major).
- Docs: ARCHITECTURE.md §11.
Tests: recordUnsupportedEngine (metric split), normalizeUnsupported (cardinality bounding), and
the handler route end to end (204 / 405 / counter). go build+vet+test and gofmt clean; ui
check/build/e2e green; the client beacon + dedup verified against a forced hard-gate (BigInt
removed) — one POST, deduped on reopen, correct reason/chromium/version labels.
Ilia Denisov