diff --git a/services/monitoring/prometheus.nix b/services/monitoring/prometheus.nix
index 271b514..9feab09 100644
--- a/services/monitoring/prometheus.nix
+++ b/services/monitoring/prometheus.nix
@@ -147,6 +147,14 @@
                 annotations:
                   summary: "The build-flakes service on {{ $labels.instance }} has failed"
                   description: "The build-flakes service on {{ $labels.instance }} has failed"
+              - alert: harmonia_down
+                expr: node_systemd_unit_state {instance="nix-cache01.home.2rjus.net:9100", name = "harmonia.service", state = "active"} == 0
+                for: 5m
+                labels:
+                  severity: critical
+                annotations:
+                  summary: "Harmonia not running on {{ $labels.instance }}"
+                  description: "Harmonia has been down on {{ $labels.instance }} more than 5 minutes."
               - alert: low_disk_space_nix
                 expr: node_filesystem_free_bytes{instance="nix-cache01.home.2rjus.net:9100", mountpoint="/nix"} / node_filesystem_size_bytes{instance="nix-cache01.home.2rjus.net:9100", mountpoint="/nix"} * 100 < 10
                 for: 5m