fix: check topic actor status before publishing events

CHANGELOG.md

@@ -4,6 +4,10 @@
 
 NOTE: all releases may include dependency updates, not specifically mentioned
 
+UNRELEASED
+
+- fix: check topic actor status before publishing events [#918](https://github.com/hypermodeinc/modus/pull/918)
+
 ## 2025-06-25 - Runtime v0.18.2
 
 - fix: validate inference history is valid json [#916](https://github.com/hypermodeinc/modus/pull/916)

runtime/actors/agents.go

@@ -267,6 +267,12 @@ func PublishAgentEvent(ctx context.Context, agentId, eventName string, eventData
 	}
 
 	topicActor := _actorSystem.TopicActor()
+	if !topicActor.IsRunning() {
+		// If the topic actor is not running, we cannot publish the event.
+		// This can happen during shutdown if the topic actor is stopped before the agent actors.
+		logger.Warn(ctx).Str("event", eventName).Any("data", eventData).Msg("Topic actor is not running. Cannot publish event.")
+		return nil
+	}
 
 	// if the pid is in context, we're being called as a host function
 	if pid, ok := ctx.Value(pidContextKey{}).(*goakt.PID); ok {