Also, I found something interesting. I initially wrote the test like this (see code snippet attached), but it failed with the following error message:

java.lang.AssertionError: 
Expected :{"primitive": 0}
Actual   :{"primitive":0}

As you can see, there's an additional space between the semicolon and the value when I serialize the json using

json { "primitive" to 0 }.toString()

, compared to

JSON.stringify(serializer, obj)

. Is this intentional ? It seems to me that both should have the exact same output

Is this intentional?

Not really. Filed https://github.com/Kotlin/kotlinx.serialization/issues/323