What causes "start monitor error: %v" in slimtoolkit/slim?

Inspect evt.Data in the full log output — it carries the underlying sensor error cause.

start monitor error: %v

Error message

start monitor error: %v

What it means

The pod inspector's monitor (sensor) startup protocol expects a StartMonitorDone event from the sensor process inside the pod. If the sensor emits an Error event instead, sensorCommandStart (called from RunPod) surfaces the event payload as "start monitor error". This means the in-pod monitor failed to start, not the slim process itself.

Source

Thrown at pkg/app/master/inspectors/pod/pod_inspector.go:656

			return err
		}

		if evt == nil || evt.Name == "" {
			i.logger.Debug("empty event waiting for the slim container to start (trying again)...")
			continue
		}

		if evt.Name == event.StartMonitorDone {
			i.xc.Out.Info("event.startmonitor.done",
				ovars{
					"status": "received",
				})
			return nil
		}

		if evt.Name == event.Error {
			return fmt.Errorf("start monitor error: %v", evt.Data)
		}

		if evt.Name != event.StartMonitorDone {
			i.xc.Out.Info("event.startmonitor.done",
				ovars{
					"status": "received.unexpected",
					"data":   fmt.Sprintf("%+v", evt),
				})

			//TODO: dump temp container logs
			return event.ErrUnexpectedEvent
		}
	}

	return errors.New("start monitor timeout")
}

func (i *Inspector) sensorCommandStop() error {

View on GitHub (pinned to 81940d17fa)

Solutions

Inspect evt.Data in the full log output — it carries the underlying sensor error cause.
Verify the sensor/slim image matches the cluster's CPU architecture and the runtime.
Check the pod's events/logs (kubectl describe pod, kubectl logs) and service account RBAC for the sensor.
Ensure no admission policies block the sensor container; update slim to the latest release.

Example fix

// before: debugging blind
return fmt.Errorf("start monitor error: %v", evt.Data)
// after: capture full pod diagnostics first
kubectl describe pod <sensor-pod>; kubectl logs <sensor-pod> --previous
// then re-run slim with debug logging:
docker-slim build --debug ...

Defensive patterns

Strategy: try-catch

Validate before calling

// pre-check the pod/sensor can run
kubectl get pod <sensor-pod> -o jsonpath='{.status.containerStatuses[*].ready}'
kubectl auth can-i create pods --as=system:serviceaccount:<ns>:<sa>

Try / catch

if err := runPodInspector(); err != nil {
    var startErr *StartMonitorError
    if errors.As(err, &startErr) || strings.Contains(err.Error(), "start monitor error") {
        log.Printf("sensor failed: %v — check pod logs, image arch, and RBAC", err)
    }
}

Prevention

Match the slim/sensor image architecture to the cluster nodes (amd64 vs arm64).
Grant the service account permissions the sensor needs; verify with kubectl auth can-i.
Check admission policies (PSP/OPA/Kyverno) won't reject the sensor container.

When it happens

Trigger: RunPod -> sensorCommandStart receives evt.Name == event.Error from the sensor: bad sensor binary/arch for the pod's platform, missing RBAC/permissions, sensor crashing on startup, or misconfigured monitor options.

Common situations: Running slim on Kubernetes with a sensor image that doesn't match the node architecture (arm64 vs amd64); security policies (PSP/OPA) blocking the sensor container; pod restarted mid-monitor; insufficient service account permissions.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/d9eb6164c50b0113. Report an issue: GitHub.