The CUE Command Line automatically infers a files format based on its extension. In cases where a different file extension is used like templates, the correct format can be hinted using qualifiers as follows.
The cue tool will infer a file’s type from its extension by default. The user my override this behavior by using qualifiers. A qualifier takes the form
<tag>{'+'<tag>}':'
$ cue vet test.cue test.yml.hbs
unknown file extension .hbs
$ cue vet test.cue yaml: test.yml.hbs