It is currently the case that if debugging is enabled, synthetics will log to the --out-fd value passed by heartbeat . This FD is reserved purely for JSON formatted messages, and creates ugly heartbeat logs like the following:

2021-12-15T17:55:22.123Z    WARN    synthexec/synthexec.go:226  error parsing line: invalid character 'a' looking for beginning of value for line: at 1822 ms register step: my-step-name

Heartbeat will properly pick up output to stderr/stdout and forward to ES as synthetics.type: console/log documents that the UI knows how to display.

We probably just need to change this line to point elsewhere