Error can be reproduced by running the below monitor.

heartbeat.run_once: true
heartbeat.monitors:
- type: browser
  enabled: true
  id: my-monitor
  name: My Monitor
  screenshots: "off"
  source:
    inline:
      script:
        asdas
        step("load homepage", async () => {
            await page.goto('https://www.elastic.co');
        });
  schedule: "@every 1m"

output.console: ~

• The synthetics runner writes the errors back to stderr without any JSON marshalling as it cannot get the registered journeys. In these cases, HB cannot write the summary event as there are no acrtive journeys.

By disabling this check, we get the summary event-

But we need to swallow the error and do it correctly.