Following the instructions here, I made a blunder and provided my butane file instead of an ignition file.
kvpctl was happy to apply this file without error, but of course my VM failed to boot.

If it's not too much trouble, would it be possible for kvpctl to simply evaluate the file to be sure it's formatted in JSON and not YAML (or otherwise) before applying it to the image?

It doesn't need to be a full "ignition-validate", just a simple JSON check.

Thanks