When triaging Sentry, the standard acceptance test for a grouping/observability fix is "does tag T now appear" (has:T). The non-obvious part is HOW it appears once the fix deploys.
Setup: a capture_exception call had no stable fingerprint=/companion tag, so a failure family shattered (or grouped tagless). You add fingerprint=[...] plus a companion tag T and ship it.
Surprising behavior: the fix does NOT modify or resolve the existing issue. Sentry computes an issue's grouping hash and metadata at group-creation time and never recomputes them for a live issue. So after deploy:
- A brand-new shortId opens, carrying tag T (this is the fixed path executing).
- The OLD tagless issue simply stops receiving events and goes quiet, usually on the exact date the new one appears.
- That pair — new tagged aggregate + old issue quiet on the same date — IS the acceptance pass. There is no "the issue turned green" moment.
Why it matters for triage bookkeeping: a run whose prior acceptance criterion was written as "issue X stays quiet" or "tag T appears on issue X" will misread this. Issue X going quiet looks like either a fix OR a traffic drop; and T never appears ON X because X's metadata is frozen. The correct check is has:T returning a different shortId, then confirming the old one's lastSeen froze at the cutover date. Resolve the old tagless issue (a genuine recurrence reopens it as the alarm you want); watch the new tagged aggregate.
Concrete instance (FinFam, 2026-09-04): two separate _capture_fetch_failure / source-health fingerprint fixes that a prior triage had marked "never executes" (has:bcu_institution=0 everywhere on 08-20) turned out to be live — surfaced only as new shortIds (FF-FSRV-FAPI-1Z6 with bcu_institution/bcu_http_status; 1ZF with growth_source/growth_source_kind), each with its tagless predecessor quiet since the same day.
Generalizes to any Sentry SDK (Python/JS/etc.): grouping hash + issue metadata are creation-time immutable; changing fingerprint or adding a tag always forks a new issue rather than mutating the old.