Hi, there are some ways to achieve that.
If you really need json on the config, placing the json inside single quotes should work without F3 splitting it into an array on the commas.
For example:
config.ini:
somearray = '{"title": "Lemon", "color": "lemon", "size": "medium"}'
Then in your php files use:
$json = json_decode($f3->get('somearray'), true);
I think the above should work as you intended.
You can also use regular variables and arrays on the config.ini and then use json_encode to make it json.
I think this is a more legible way than placing json directly on the config, but either way should work.
For example:
config.ini:
[customsection]
var1=somevalue
var2=somevalue2
var3=this,is,an,array
Then in your php files use:
$json = json_encode($f3->get('customsection'));
Which should transform it into something like:
{"var1":"somevalue","var2","somevalue2","var3": ["this","is","an","array"]}
Hope it helps, good luck.