Thanks for giving it a try and your feedback. I agree, the array splitting is a bit fiddly. It is actually possible to pass JSON directly, you use the :json type on the argument:

    $ jb dependencies:json='["Bash","Grep"]'
    {"dependencies":["Bash","Grep"]}

    $ jb foo=bar bar:json='"this is a well formed string"'
    {"foo":"bar","bar":"this is a well formed string"}

And then you can indeed use command substitution to nest calls:

    $ jb foo:json=$(jb bar=baz)
    {"foo":{"bar":"baz"}}

It works even better to use process substitution, this way the shell gives jb a file path to a file to read, and so you don't need to quote the $() to avoid whitespace breaking things:

    $ jb foo:json@<(jb msg=$'no need\nto quote this!')        
    {"foo":{"msg":"no need\nto quote this!"}}

Another option is to use jb-array to generate arrays. (jb-array is best for tuple-like arrays with varying types):

    $ jb dependencies:json@<(jb-array Bash Grep)
    {"dependencies":["Bash","Grep"]}

And if you use it from bash as a function, you can put values into a bash array and reference it:

    $ source json.bash
    $ dependencies=(Bash Grep)
    $ json @dependencies:[]   
    {"dependencies":["Bash","Grep"]}