How to expose custom fields from alert JSON in the Slack API connector?

Elastic Stack Kibana
1

Hello Team

I'm implementing detection rules in Elastic Security (Kibana v8.x) and want to notify alerts in Slack using the Slack API connector. However, I can only use the predefined variables listed in the + Add variable selector (e.g., alert.severity, alert.risk_score, alerts.new.count, etc.) Elastic.

My goal is to also include custom fields that I add to the alert document (e.g., user metadata or labels defined in the index), but when I try to reference them with Mustache ({{my_custom_field}} or {{context.alerts.0.my_custom_field}}), Slack ignores them or expands them to empty...

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What I've tried

  1. Slack API connector with standard Mustache ({{alert.*}}, {{alerts.*}}, {{context.results_link}}): works only with predefined variables.
  2. Trying to reference context.rule.*: It doesn't work because those scopes only contain rule metadata, not the alert itself Stack Overflow.
  3. In Kibana 8.8+, I've seen that context.alerts exists as an array, but when using it in the Slack connector, it doesn't recognize it directly Discuss the Elastic Stack.

Is there a native way, without using intermediate Webhooks, for the Slack API connector to expose arbitrary fields that come in the alert JSON?