Webhook bridge that converts Prometheus alertmanager alert payloads into rich Slack Block Kit messages with runbook links, labels, and action buttons

Prometheus alertmanager sends Slack notifications using legacy attachment formatting that Slack has deprecated in favor of Block Kit. The feature request to support Block Kit directly in alertmanager has 189 reactions and 24 comments since March 2020. No alertmanager maintainer has ever responded to the issue. The Slack app auth PR merged in November 2025 added bearer token support but left the visual format unchanged. Teams still receive plain-text wall-of-labels alerts with no action buttons, no grouped sections, and no runbook links. The product is a lightweight webhook proxy that alertmanager POSTs to instead of Slack directly. The proxy parses the alertmanager payload, renders it as a Block Kit body (severity badge, labels table, generator URL, runbook button, silence button), and forwards to the Slack API. Configurable via YAML per alertname or severity.