diff --git a/argocd/manifests/grafana/alerting.yaml b/argocd/manifests/grafana/alerting.yaml
index dfcc5a3..dcb6762 100644
--- a/argocd/manifests/grafana/alerting.yaml
+++ b/argocd/manifests/grafana/alerting.yaml
@@ -315,6 +315,64 @@ groups:
                     type: and
               refId: C
 
+  - orgId: 1
+    name: argocd-health
+    folder: Infrastructure Alerts
+    interval: 60s
+    rules:
+      - uid: argocd-app-out-of-sync
+        title: ArgoCDAppOutOfSync
+        condition: C
+        for: 30m
+        noDataState: OK
+        execErrState: Alerting
+        annotations:
+          summary: >-
+            ArgoCD app {{ index $labels "name" }} is {{ index $labels "sync_status" }}
+          runbook_url: https://docs.eblu.me/how-to/alerts/runbook-argocd-out-of-sync
+        labels:
+          severity: warning
+          service: argocd
+        data:
+          - refId: A
+            datasourceUid: prometheus
+            relativeTimeRange:
+              from: 300
+              to: 0
+            model:
+              expr: >-
+                argocd_app_info{sync_status!="Synced"}
+              interval: ""
+              refId: A
+          - refId: B
+            datasourceUid: "__expr__"
+            relativeTimeRange:
+              from: 0
+              to: 0
+            model:
+              type: reduce
+              expression: A
+              reducer: last
+              settings:
+                mode: dropNN
+              refId: B
+          - refId: C
+            datasourceUid: "__expr__"
+            relativeTimeRange:
+              from: 0
+              to: 0
+            model:
+              type: threshold
+              expression: B
+              conditions:
+                - evaluator:
+                    type: gt
+                    params:
+                      - 0
+                  operator:
+                    type: and
+              refId: C
+
 templates:
   - orgId: 1
     name: ntfy-infra
diff --git a/argocd/manifests/prometheus/prometheus.yml b/argocd/manifests/prometheus/prometheus.yml
index 2d2dbcf..f96ce12 100644
--- a/argocd/manifests/prometheus/prometheus.yml
+++ b/argocd/manifests/prometheus/prometheus.yml
@@ -80,6 +80,14 @@ scrape_configs:
       - target_label: cluster
         replacement: indri
 
+  # ArgoCD application metrics
+  - job_name: "argocd"
+    static_configs:
+      - targets: ["argocd-metrics.argocd.svc.cluster.local:8082"]
+    metric_relabel_configs:
+      - target_label: cluster
+        replacement: indri
+
   # Frigate NVR metrics (via Caddy on indri — Frigate runs on ringtail)
   - job_name: "frigate"
     scheme: https
diff --git a/docs/how-to/alerts/runbook-argocd-out-of-sync.md b/docs/how-to/alerts/runbook-argocd-out-of-sync.md
new file mode 100644
index 0000000..753b336
--- /dev/null
+++ b/docs/how-to/alerts/runbook-argocd-out-of-sync.md
@@ -0,0 +1,65 @@
+---
+title: "Runbook: ArgoCD App Out of Sync"
+modified: 2026-03-22
+tags:
+  - how-to
+  - alerting
+  - runbook
+---
+
+# Runbook: ArgoCD App Out of Sync
+
+**Alert name:** `ArgoCDAppOutOfSync`
+
+An ArgoCD application has been out of sync for 30+ minutes. This means the live state in Kubernetes differs from what's declared in Git.
+
+## Diagnostic Steps
+
+1. **Check which app is out of sync** — the `name` label in the alert tells you:
+   ```fish
+   argocd app get <app-name>
+   ```
+
+2. **View the diff**:
+   ```fish
+   argocd app diff <app-name>
+   ```
+
+3. **Check if it's a branch revision issue** — during C1/C2 work, apps may be pointed at a feature branch. After merge, they need to be reset to main:
+   ```fish
+   argocd app get <app-name> -o json | python3 -c "import json,sys; print(json.load(sys.stdin)['spec']['source']['targetRevision'])"
+   ```
+
+4. **Check ArgoCD UI** — https://argocd.ops.eblu.me — look for sync errors or degraded status.
+
+## Common Causes
+
+- **Forgot to sync after push** — ArgoCD uses manual sync; changes require explicit `argocd app sync`
+- **Branch revision not reset after PR merge** — app still points at a deleted branch
+- **Kustomize/manifest error** — invalid YAML or unsatisfiable resource requirements
+- **Pruning needed** — old ConfigMaps from `configMapGenerator` need pruning
+
+## Resolution
+
+```fish
+# Simple sync
+argocd app sync <app-name>
+
+# If pruning is needed
+argocd app sync <app-name> --prune
+
+# If stuck on a deleted branch
+argocd app set <app-name> --revision main
+argocd app sync <app-name>
+```
+
+## Silencing
+
+During active C1/C2 development, apps may intentionally be out of sync:
+1. Grafana → Alerting → Silences → Create Silence
+2. Match `alertname = ArgoCDAppOutOfSync` and `name = <app-name>`
+
+## Related
+
+- [[argocd]] — ArgoCD reference
+- [[deploy-infra-alerting]] — Alerting pipeline overview
diff --git a/docs/reference/operations/observability.md b/docs/reference/operations/observability.md
index 9d4a7a0..35136d5 100644
--- a/docs/reference/operations/observability.md
+++ b/docs/reference/operations/observability.md
@@ -25,3 +25,4 @@ Metrics, logs, traces, and dashboards for BlumeOps infrastructure.
 - [[runbook-pod-not-ready]] - Pod not ready runbook
 - [[runbook-textfile-stale]] - Metrics textfile freshness runbook
 - [[runbook-frigate-camera-down]] - Frigate camera health runbook
+- [[runbook-argocd-out-of-sync]] - ArgoCD sync status runbook