Is there an existing issue for this?

• I have searched the existing issues

What happened?

The upstream Prometheus Alertmanager supports configuring custom HTTP headers (for example, X-API-Key) via the http_headers field in http_config for webhooks and other HTTP-based receivers (see Alertmanager docs: http_header).

However, the AlertmanagerConfig CRD in prometheus-operator does not currently support this field:

• The Go types in pkg/alertmanager/types.go (specifically httpClientConfig) do not expose any option to set arbitrary HTTP headers (such as http_headers or headers).
• As a result, it is not possible to set custom headers (e.g., for API authentication via X-API-Key) for webhooks or other HTTP receivers using the operator.

This is inconsistent with the upstream Alertmanager and prevents use-cases which require HTTP header-based authentication for notifications.

Steps to reproduce:

1. Try to use the http_headers field under http_config for a webhook_configs receiver in an AlertmanagerConfig.
2. The operator does not recognize this key; the custom header is not set and not present in outgoing requests.

Expected behavior:

It should be possible to configure arbitrary HTTP headers for HTTP-based receivers via http_config.http_headers as supported by the upstream Alertmanager.

Actual behavior:

There is currently no way (via CRD) to declare custom HTTP headers for outgoing webhooks.

Affected versions:

• prometheus-operator:
• This problem is visible in pkg/alertmanager/types.go

Proposed solution:

Add support for the http_headers field (as specified in the Alertmanager documentation) to the httpClientConfig struct and expose it through the AlertmanagerConfig CRD.

Thank you!

Prometheus Operator Version

quay.io/prometheus-operator/prometheus-operator:v0.87.1

labels:
    app: kube-prometheus-stack-operator
    app.kubernetes.io/component: prometheus-operator
    app.kubernetes.io/instance: gst-monitoring
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/name: kube-prometheus-stack-prometheus-operator
    app.kubernetes.io/part-of: kube-prometheus-stack
    app.kubernetes.io/version: 80.4.1
    chart: kube-prometheus-stack-80.4.1

Kubernetes Version

1.32.9

Kubernetes Cluster Type

AKS

How did you deploy Prometheus-Operator?

helm chart:prometheus-community/kube-prometheus-stack

Manifests

prometheus-operator log output

ts=2026-01-29T08:32:02.861226964Z level=error caller=/workspace/pkg/operator/resource_reconciler.go:678 msg="Unhandled Error" logger=UnhandledError err="sync \"monitoring/gst-monitoring-kube-promet-alertmanager\" failed: provision alertmanager configuration: failed to initialize from secret: yaml: unmarshal errors:\n  line 44: field http_headers not found in type alertmanager.httpClientConfig"

Anything else?

No response