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Diagnostic Code Reference
Every alert email contains a code from this list. Use this page to look up what each one means and what to do about it.
Liveness (BC-L)
BC-L001 · critical
App returning a non-2xx status code
The app responded with an HTTP status code outside the 200-299 range after a retry. This usually means the server is reachable but is reporting an error (500/502/503) or the URL no longer exists (404).
BC-L002 · critical
App did not respond
Backcheck could not get any response from the app — connection refused, timed out, or network error — even after a retry.
BC-L003 · critical
DNS resolution failed
The hostname couldn't be resolved. Either the domain expired, the DNS record was removed, or the domain was misconfigured.
Content (BC-C)
BC-C001 · warning
Page content changed (your app's HTML)
The HTML returned by your app changed across two consecutive checks. If you didn't deploy, this can indicate an unexpected change — accidental rollout, hack, or wrong environment.
BC-C002 · warning
Content drift on a third-party redirect target
This URL redirects to a third-party host (such as Stripe). Their HTML changed, not yours. Almost always a false positive — A/B tests, rotating banners, etc.
BC-C003 · warning
Required text missing from the page (biotic absent)
A 'biotic' word you configured (something that should always be present, like a CTA or feature label) is missing from the page response.
BC-C004 · warning
Forbidden text found on the page (warning word present)
A 'warning' word you configured (something that should never appear, like 'undefined' or a stack trace fragment) was found in the page response.
Performance (BC-P)
BC-P001 · warning
App is responding slowly
The app responded above the slow threshold across two or more consecutive checks. This isn't downtime, but users will notice the lag.
SSL (BC-S)
BC-S001 · warning
SSL certificate expiring soon
The SSL certificate for this domain will expire within the warning window. After expiration, browsers will show users a security warning.
BC-S002 · critical
SSL certificate expired
The SSL certificate has already expired. Visitors are seeing a browser security warning right now.
Auto-handled (BC-A) — silent, no email
BC-A001 · info · silent
Cold-start auto-recovered
The previous check was slow but the next one was fast. Classified as a normal cold-start and silenced.
BC-A002 · info · silent
Bot challenge page detected
The response was a Cloudflare/Vercel/anti-bot challenge page rather than your real content. Content hashing was skipped to avoid false content-change alerts.
BC-A003 · info · silent
SSL alert deduplicated
An SSL warning was suppressed because we already alerted on this within the last 7 days. We'll re-alert next week if not resolved.
BC-A004 · info · silent
Content-change blip (single check, not yet confirmed)
A content change was seen in one check but didn't repeat. Dampened to avoid alerting on dynamic page elements. Will alert on BC-C001/BC-C002 only if it repeats next check.