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Put context in the Prometheus alert rule first, not in the webhook. Use labels for short, stable fields that Alertmanager may route or group on, and annotations for human-readable explanations such as summaries, descriptions, and runbook references. Use an Alertmanager webhook to deliver that already-authored context to an external system, or to transform it when the destination has no native integration.
A webhook is therefore usually a delivery and transformation layer, not a competing authoring location. Writing the same context separately in a webhook makes maintenance harder and can produce inconsistent notifications.
What is different about the two choices?
Native labels and annotations are fields on a Prometheus alerting rule. Prometheus evaluates the rule expression, creates an alert when the condition is true, and evaluates any templates in those fields. The resulting alert is sent to Alertmanager with its labels and annotations.
An Alertmanager webhook is a receiver. Alertmanager accepts alerts from Prometheus, groups and routes them, deduplicates notifications, renders a notification payload, and sends that payload to an HTTP endpoint. The standard webhook envelope contains group-level data and the original fields for each alert. A custom payload can reshape that data, but it does not remove the need to author useful labels and annotations in the rule.
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| Decision axis | Native labels and annotations | Alertmanager webhook |
|---|---|---|
| Main role | Author context with the alert rule. | Deliver grouped alert data to an HTTP receiver and optionally transform it. |
| Evaluation stage | Prometheus rule evaluation. | Alertmanager notification rendering and delivery. |
| Routing and grouping | Labels are available for route matchers and can be included in grouping. | Receives the grouping and routing result after Alertmanager processes the alerts. |
| Best content | Severity, team, service, environment, summary, description, and links to operational documentation. | Fields required by an incident, ticketing, chat, or automation endpoint. |
| Operational burden | Keep names stable and values purposeful; keep prose out of labels. | Maintain endpoint compatibility, authentication, JSON validity, and downstream handling. |
Choose the field before choosing the receiver
Use labels for compact, structured metadata
Labels identify the alert in a machine-readable way. Typical values include severity, team, service, cluster, and environment. Alertmanager can match routes on these values, and a route can group alerts by selected labels. Because labels participate in alert identity and processing, changing a label can change routing, grouping, deduplication, and the apparent identity of an alert.
Keep label values stable enough to support those operations. A label should answer a routing or classification question, not carry a paragraph. Do not put a changing explanation, stack trace, or incident narrative in a label.
Use annotations for explanations people read
Annotations are intended for additional information. A summary can be a short sentence suitable for a notification title; a description can explain what was measured and where; a runbook or dashboard reference can tell the responder what to open next. Annotations are not the usual basis for Alertmanager grouping or route matching, so changing explanatory wording does not have the same identity effect as changing a label.
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How context travels through the alerting pipeline
- Prometheus evaluates the rule. The expression is evaluated on each rule interval. For a firing alert, Prometheus evaluates templates in the rule’s label and annotation values. Rule templates can use alert labels and the evaluated value.
- Prometheus sends the alert object. The alert contains its labels, annotations, start and end state information, and other alert metadata.
- Alertmanager groups and routes. Alertmanager uses route matchers and the configured grouping labels to decide which notification group a firing alert belongs to. It also deduplicates notifications and handles resolved alerts according to receiver settings.
- Alertmanager renders the notification. A built-in integration can render a message from Alertmanager notification templates. A generic webhook renders the standard webhook envelope unless you configure a custom payload.
- The receiver processes the HTTP request. The external service receives grouped fields such as
groupLabels,commonLabels, andcommonAnnotations, plus a per-alert object containing that alert’s labels and annotations.
This sequence explains why a webhook is rarely the right place to invent basic context. By the time the webhook runs, Alertmanager has already made grouping and routing decisions using the alert’s labels.
Authoring a useful Prometheus alert rule
A practical rule layout
The following pattern separates routing data from responder-facing text. The expression is illustrative; adapt metric names and thresholds to your own workload.
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groups:
- name: api
rules:
- alert: ApiErrorRateHigh
expr: rate(http_requests_total{job='api',status=~'5..'}[5m]) / rate(http_requests_total{job='api'}[5m]) > 0.05
for: 10m
labels:
severity: page
team: payments
service: api
environment: production
annotations:
summary: 'API error rate is high on {{ $labels.instance }}'
description: 'The measured 5xx ratio for {{ $labels.instance }} is {{ $value }}.'The labels are short and useful to a route or grouping policy. The annotations explain the condition and can be rendered in a human-facing notification. Add a runbook or dashboard annotation when you have a real destination for it; keep that link in the annotation rather than embedding it in a label.
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Prometheus alert-field templates use the rule-evaluation context. Common variables include $labels for labels from the expression and $value for the evaluated result. A template that is valid in a Prometheus rule is not automatically valid in an Alertmanager notification template.
Keep these expressions lightweight. Prometheus evaluates alert-field templates during every rule iteration for each firing alert. For complicated investigation or presentation, link to a console or dashboard and let the responder obtain the richer view there instead of constructing a large string on every evaluation.
Configuring a webhook without losing native context
Use the standard webhook envelope first
The generic webhook is intended for notification mechanisms that Alertmanager does not support natively. It is also useful when a separate incident or automation system needs the complete grouped alert data. Start with the standard payload; it already carries the information most receivers need.
A minimal receiver can keep the endpoint in a protected file and request resolved notifications as well as firing notifications:
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route:
group_by: ['alertname', 'cluster', 'service']
receiver: 'ops-webhook'
receivers:
- name: 'ops-webhook'
webhook_configs:
- url_file: '/etc/alertmanager/webhook-url'
send_resolved: trueIf you configure the endpoint directly instead of using url_file, use the HTTPS address and the authentication settings required by the receiver. The exact available options depend on the Alertmanager version deployed; check that version’s configuration reference before rollout.
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Understand grouped and common fields
A webhook request represents a notification group, not necessarily one alert. Group-level fields describe the group and the labels shared by all alerts. The payload normally includes:
groupLabels: labels that define the group according to the route.commonLabels: labels shared by every alert in the notification.commonAnnotations: annotations shared by every alert in the notification.alerts: an array of individual alerts, each with its own status, labels, annotations, timestamps, and fingerprint.- Notification metadata such as the receiver name, status, group key, and Alertmanager’s external URL.
Read common fields for a concise group-level title, but iterate over the alerts array when every alert must be preserved. A group can contain alerts with different instances or other non-common labels, so a receiver that reads only commonLabels can silently discard important detail.
When a built-in integration is enough
If Alertmanager has a native integration for the destination, prefer it when its message format meets your needs. Native integrations use Alertmanager notification templates, which expose group and alert data after routing and grouping. You can render annotations without writing a second copy of the text.
{{ range .Alerts -}}
{{ .Labels.alertname }}: {{ .Annotations.summary }}
{{ end -}}In this context, .Alerts is the group of alerts being notified, while .Labels and .Annotations belong to the current alert in the loop. Group-wide maps such as .CommonLabels and .CommonAnnotations are useful only when the value is genuinely common to the whole group.
When to customize the webhook payload
Use custom payloads for a contract, not for basic authoring
Some receivers require a different JSON shape, a fixed field name, or a reduced set of values. Alertmanager supports a custom payload template for that advanced case. The template can select common fields, iterate over alerts, and map native annotations into the receiver’s contract.
For example, a receiver might need one object per alert with a field called title sourced from the alert’s summary annotation. That mapping belongs in the webhook payload because it is a transport concern. The summary itself should still be authored once in the Prometheus rule.
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Validate the output yourself
Alertmanager does not validate that a custom payload is valid JSON or that it satisfies the receiver’s schema. Your configuration must therefore account for:
- JSON quoting and escaping when annotation text contains quotes, line breaks, or backslashes.
- Empty or missing labels and annotations.
- Groups containing multiple alerts rather than one fixed alert.
- Firing and resolved statuses, including timestamps and end-state values.
- Receiver limits on body size, field length, or number of alert objects.
- Authentication, TLS, retries, timeout behavior, and idempotent handling on the receiving side.
Test the rendered body against a JSON parser and the receiver’s schema before enabling it for production traffic. Keep the custom template small enough to review; complex transformation logic is usually easier to maintain in a dedicated adapter service.
Keep the two template contexts separate
| Question | Prometheus rule template | Alertmanager notification or webhook template |
|---|---|---|
| When does it run? | During rule evaluation for each firing alert. | When Alertmanager renders a notification group. |
| Typical label variable | $labels.instance |
.Labels.instance for the current alert or .CommonLabels.instance for a shared value. |
| Typical value variable | $value |
Read the value only if it was included as an alert label or annotation; the rule-evaluation variable is not automatically available. |
| Annotation access | Values are authored in the rule and can themselves contain templates. | .Annotations.summary, .CommonAnnotations.summary, or an iteration over .Alerts. |
| Main risk | Expensive or overly dynamic text is evaluated repeatedly. | Assuming one alert, using the wrong field scope, or emitting invalid custom JSON. |
Write each expression for the stage that evaluates it. Copying a Prometheus expression such as {{ $labels.instance }} into an Alertmanager template will not produce the intended value, and copying an Alertmanager expression such as {{ .CommonAnnotations.summary }} into a Prometheus rule is equally wrong.
A decision framework for common requirements
You need routing by severity or team
Put the value in a label. Alertmanager route matchers can then select the appropriate receiver, and grouping can include the label when separate notifications are required.
You need a responder-readable explanation
Put it in an annotation. Use a concise summary for the notification title and a description for the measured condition, affected scope, and useful next action.
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Author it once in the rule. Native integrations and the generic webhook can both read the annotation, avoiding divergent copies.
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Use the generic webhook with its standard envelope. The receiver can consume labels and annotations directly, or an adapter can translate the envelope into the destination’s API.
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Use a custom payload only for the required transformation. Keep the source meaning in labels and annotations, validate the generated JSON, and test groups containing multiple alerts.
Implementation procedure
- Inventory the context. List the fields the receiver and responders need. Mark each as structured routing data or explanatory text.
- Define stable labels. Choose names such as severity, team, service, cluster, and environment only when they have a clear routing or grouping purpose.
- Write annotations. Add summary and description text that remains useful in a notification, plus operational references where available.
- Check rule templates. Confirm that every Prometheus expression uses the rule context, including
$labelsand$value, and keep the rendering inexpensive. - Configure grouping deliberately. Set
group_byso alerts that should be notified together share the intended labels. Remember that grouping changes the shape of webhook requests. - Start with the standard receiver. Send the unmodified webhook envelope to a test endpoint and inspect both common fields and each alert object.
- Add transformation only if required. Introduce a custom payload for a documented receiver contract, not to compensate for missing rule annotations.
- Exercise state changes. Test firing, repeated notifications, multiple alerts in one group, and resolved notifications when
send_resolvedis enabled. - Monitor delivery. Check HTTP status handling, authentication failures, timeouts, retries, and downstream deduplication.
Troubleshooting by symptom
The webhook has no summary or description
Confirm that the text is under annotations, not labels, in the Prometheus rule. Then inspect the per-alert annotations object in the webhook payload. If it is present there but absent from the final message, the notification or custom payload template is selecting the wrong field.
A route stops matching after a rule edit
Compare the alert’s labels before and after the edit. A renamed label, changed value, or newly templated value can prevent a route matcher from matching. Restore the stable value or update the route intentionally; do not move the routing field into an annotation.
One notification unexpectedly contains many alerts
That is usually grouping behavior, not a webhook failure. Inspect groupLabels and the route’s group_by setting, then iterate over the alerts array in the receiver. If separate notifications are required, choose grouping labels that distinguish the alerts.
The custom receiver rejects the request as malformed JSON
Render the payload with representative annotation text, including quotes and line breaks, and run it through a JSON parser. Ensure commas are handled correctly when iterating over multiple alerts, and escape inserted values. Alertmanager will not certify the generated body for you.
The template prints an empty value
Check the evaluation stage and scope. Prometheus rule templates use dollar-prefixed variables; Alertmanager templates use dot-prefixed data such as .Alerts, .Labels, and .CommonAnnotations. Also verify whether the requested label is common to the group before reading it from a common-label map.
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Notifications repeat after a label change
Because labels contribute to alert identity and Alertmanager processing, changing a label can make an alert appear new. Treat label names and values as an interface: change them only with a corresponding review of routes, grouping, dashboards, receivers, and downstream deduplication.
Operational checklist
- Labels contain concise, stable metadata that a machine may route or group.
- Annotations contain summaries, descriptions, and operational references intended for people.
- Rule templates use Prometheus variables and remain lightweight.
- Notification templates use Alertmanager variables and account for grouped alerts.
- The receiver reads per-alert fields when common fields are insufficient.
- A custom payload is used only when the destination requires a different contract.
- Custom output is valid JSON, handles empty values, and passes the receiver’s schema checks.
- Grouping, resolved notifications, authentication, timeouts, and retries are tested with the deployed Alertmanager version.
Bottom line
Author context once in the Prometheus rule: labels for structured routing and grouping metadata, annotations for explanatory text. Let Alertmanager carry those fields through its standard webhook envelope, and customize the payload only when an external receiver requires a different schema. This separation keeps alert identity predictable, messages consistent, and webhook integrations maintainable.
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