What must be true about a custom script before it can be executed from within...

For a custom script to be executed from a Fusion SOAR Workflow, it must be made available to workflows by enabling the “Share with workflows” option. This is a permissions and availability requirement: the workflow engine cannot call a custom script unless the script has explicitly been shared for workflow use. The script does not need to already exist locally on the endpoint, because Falcon can invoke scripts through RTR mechanisms depending on configuration. Input and output JSON fields may be useful for structured automation, but they are not the core prerequisite stated in the question. A Response Policy controls RTR permissions and access, but the specific workflow execution requirement is that the custom script must be shared with workflows.