diff --git a/docs/observability.md b/docs/observability.md
index 1fddc27..1caf30b 100644
--- a/docs/observability.md
+++ b/docs/observability.md
@@ -434,7 +434,7 @@ as loaded and healthy and could not fire for any input ([#63](https://github.com
 `prometheus/tests/*.test.yaml` holds `promtool test rules` unit tests, which
 feed a rule synthetic series and assert it fires — paired with a case asserting
 it stays quiet, because a test that only ever expects silence would have passed
-against the broken rule too. Coverage is fifty-seven rules of 78 so far — the five
+against the broken rule too. Coverage is fifty-eight rules of 78 so far — the five
 in `blackbox.rules.yaml`, both in `dns.rules.yaml`, `ContainerHighMemory`,
 `ContainerNearMemoryLimit`, `ContainerRestartLoop`, `ContainerCpuThrottled` and
 `PrometheusSizeRetentionActive`, `Watchdog`, the three iLO rules from
@@ -449,7 +449,7 @@ SMART rules from [#351](https://github.com/Gerrrt/HomeLab/issues/351),
 `PatchStateStopped` from [#360](https://github.com/Gerrrt/HomeLab/issues/360), and
 `SystemUpdateAvailable` from [#378](https://github.com/Gerrrt/HomeLab/issues/378),
 and the two guest rules from [#257](https://github.com/Gerrrt/HomeLab/issues/257).
-The other 21 are still validated for syntax only, which is exactly the
+The other 20 are still validated for syntax only, which is exactly the
 standing #63 had. Both numbers are checked by `scripts/check_docs.py` — the
 sentence they replaced claimed six and named two, and had been wrong for
 weeks.
diff --git a/stacks/observability/prometheus/rules/stack.rules.yaml b/stacks/observability/prometheus/rules/stack.rules.yaml
index 3b9994c..6e5c70a 100644
--- a/stacks/observability/prometheus/rules/stack.rules.yaml
+++ b/stacks/observability/prometheus/rules/stack.rules.yaml
@@ -72,6 +72,11 @@ groups:
             PROMETHEUS_RETENTION, so history is shorter than 30 days.
 
       - alert: AlertmanagerNotificationsFailing
+        # The `reason` label is the first thing to read, not the integration.
+        # On 2026-09-08 this fired with reason="rateLimited": every receiver
+        # was reachable and answering, and ntfy.sh was refusing with 429
+        # because the day's free message budget was spent (#407). "The
+        # webhook is unreachable" is what a reader assumes, and it was wrong.
         expr: increase(alertmanager_notifications_failed_total[15m]) > 0
         for: 5m
         labels:
@@ -79,10 +84,15 @@ groups:
           severity: critical
           category: correctness
         annotations:
-          summary: "Alertmanager cannot deliver notifications via {{ $labels.integration }}"
+          summary: "Alertmanager cannot deliver via {{ $labels.integration }}: {{ $labels.reason }}"
           description: >-
             Alerts are firing but not reaching anyone — this makes every other
-            rule in this repository silently useless.
+            rule in this repository silently useless. Read the reason before
+            the URL: rateLimited means the destination answered and refused
+            (ntfy.sh's daily quota, #407), clientError or serverError means it
+            answered with an error, and contextDeadlineExceeded means it did
+            not answer at all. This alert travels the same path it reports on,
+            so if it reached you, at least one receiver still works.
 
       - alert: LokiIngestionStalled
         expr: |
diff --git a/stacks/observability/prometheus/tests/stack.test.yaml b/stacks/observability/prometheus/tests/stack.test.yaml
index 064ac27..e20a7aa 100644
--- a/stacks/observability/prometheus/tests/stack.test.yaml
+++ b/stacks/observability/prometheus/tests/stack.test.yaml
@@ -13,10 +13,11 @@
 # alert for a real input; the quiet case proves it does not produce one for the
 # input production actually supplies.
 #
-# What these do NOT prove: four rules in stack.rules.yaml have no unit test.
-# PrometheusConfigReloadFailed, PrometheusRuleEvaluationFailures,
-# AlertmanagerNotificationsFailing and LokiIngestionStalled are validated for
-# syntax only, which is exactly the standing #63 had.
+# What these do NOT prove: three rules in stack.rules.yaml have no unit test.
+# PrometheusConfigReloadFailed, PrometheusRuleEvaluationFailures and
+# LokiIngestionStalled are validated for syntax only, which is exactly the
+# standing #63 had. AlertmanagerNotificationsFailing gained its test the day
+# after it fired for real with a reason nobody had expected (#407).
 #
 # RemoteWriteJobStale is tested below and needs to be, because the obvious way
 # to write it cannot fire for any input. `time() - timestamp(up) > 300` reads
@@ -55,6 +56,55 @@ tests:
                 Retention is now bounded by PROMETHEUS_RETENTION_SIZE rather
                 than PROMETHEUS_RETENTION, so history is shorter than 30 days.
 
+  # --- AlertmanagerNotificationsFailing: fires, and names the reason ---------
+  # The fixture is the 2026-09-08 incident: the webhook integration reachable
+  # and refusing with 429 for the day's quota. The point of the case is the
+  # rendered summary — `reason` must come through, because it is the label
+  # that told the two failure modes apart on the night and nothing else did.
+  - interval: 1m
+    input_series:
+      - series: 'alertmanager_notifications_failed_total{instance="alertmanager:9093",job="alertmanager",integration="webhook",reason="rateLimited"}'
+        values: "0+0x10 1+1x20"
+    alert_rule_test:
+      - eval_time: 20m
+        alertname: AlertmanagerNotificationsFailing
+        exp_alerts:
+          - exp_labels:
+              alertname: AlertmanagerNotificationsFailing
+              instance: alertmanager:9093
+              job: alertmanager
+              integration: webhook
+              reason: rateLimited
+              component: stack
+              severity: critical
+              category: correctness
+            exp_annotations:
+              summary: "Alertmanager cannot deliver via webhook: rateLimited"
+              description: >-
+                Alerts are firing but not reaching anyone — this makes every other
+                rule in this repository silently useless. Read the reason before
+                the URL: rateLimited means the destination answered and refused
+                (ntfy.sh's daily quota, #407), clientError or serverError means it
+                answered with an error, and contextDeadlineExceeded means it did
+                not answer at all. This alert travels the same path it reports on,
+                so if it reached you, at least one receiver still works.
+
+  # --- AlertmanagerNotificationsFailing: quiet on a flat counter ------------
+  # Alertmanager exports this counter for every integration it knows, at zero,
+  # forever. A rule written as `> 0` on the counter itself rather than on its
+  # increase would be fine here and wrong the first time any receiver ever
+  # failed once, months ago; this case is what pins the increase() form.
+  - interval: 1m
+    input_series:
+      - series: 'alertmanager_notifications_failed_total{instance="alertmanager:9093",job="alertmanager",integration="webhook",reason="rateLimited"}'
+        values: "3x40"
+      - series: 'alertmanager_notifications_failed_total{instance="alertmanager:9093",job="alertmanager",integration="email",reason="other"}'
+        values: "0x40"
+    alert_rule_test:
+      - eval_time: 30m
+        alertname: AlertmanagerNotificationsFailing
+        exp_alerts: []
+
   # --- PrometheusSizeRetentionActive: quiet while the ceiling is not bound --
   # This is the production state and the one the rule must not alert on: the
   # counter exists and reads 0 forever. Without this case the rule would pass