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Prometheus Alertmanager

INFRA · APIS
Velocity2.5

Alert routing, grouping, silencing and notification dispatcher for Prometheus

prometheus.io/docs/alerting/latest/alertmanager ↗

Alertmanager 0.34.2 turns off CORS by default, breaking cross-origin API calls.

alertingprometheusobservabilitybreaking-changessecurity
◆Current state
Alertmanager handles deduplication, grouping, silencing and routing of Prometheus alerts. 0.34.2 (October 29) stops sending CORS headers by default, so cross-origin calls to api/v2 now fail unless --web.cors.origin is set, and rebuilds with Go 1.26.10 to distrust the compromised eMudhra intermediates. 0.34.1 fixed inhibitions that could un-mute alerts, and 0.34.0 split auth and rate-limit failures out of the clientError metric reason.
◆Where it's heading
The 0.3x line keeps shipping breaking changes outside major versions: metric reason splits in 0.34.0, per-group markers in 0.33.0 and now a CORS default in a patch. Operators need to read every release note, not just minors.
◆Prediction
Expect 0.35.0 to carry further breaking changes in the vein of 0.33 and 0.34, though the entries do not say what it will contain.

◆Recent moves

  1. 11d ago

    0.34.2: CORS off by default, eMudhra CAs distrusted

    A security patch with a breaking default: browser tools calling api/v2 cross-origin now need the new --web.cors.origin flag. The Go rebuild picks up the eMudhra CA distrust.

    View source ↗
  2. 1mo ago

    0.34.1: inhibition fixes stop alerts being wrongly un-muted

    Fixes several inhibition bugs that could un-mute alerts that should have stayed silent. A correctness fix in a core path.

    View source ↗
  3. 2mo ago

    Notification failure reasons split; templatable route labels added

    Splits authError and rateLimited out of clientError on alertmanager_notifications_failed_total, which silently changes what existing dashboards match — the release notes flag it as requiring updates. Adds templatable labels on alert routes, server-side active, expired and pending filters on the silences API, SNS external_id support, and toDate template functions.

    View source ↗
  4. 4mo ago

    Silence snapshot legacy matchers field restored

    Three bug fixes, the notable one restoring the legacy matchers field in silence snapshots so older Alertmanager versions can read newer snapshots again. A compatibility repair on the 0.33 line.

    View source ↗
  5. 5mo ago

    Per-group alert markers replace the global one; event recorder lands

    ⚡ SPARK

    Replaces the global alert marker with per-aggregation-group AlertMarkers, removing the alertmanager_marked_alerts metric and relocating marker types out of the public types package. The structured event recorder also arrives behind a feature flag with file, webhook and kafka outputs — the foundation the next release keeps building on.

    View source ↗
  6. 5mo ago

    Dispatcher goroutine leak on alertgroup swap fixed

    A single fix for dispatcher goroutine leaks when a destroyed alertgroup is swapped. One of the contention symptoms the per-group marker rework in 0.33.0 addresses structurally.

    View source ↗