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kotlinlang

it does not offer CSV as a built-in serialization target but the plugin is agnostic to formats meaning you are able to support CSV yourself

I have not seen documentation on how to do this. How can this be done?

There is a link right on Readme of the project: `Writing custom serializers`
<https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/custom_serializers.md>

also you can check existing formats, all of them just library code, no magic
And there is a couple of examples of custom formats:
<https://github.com/Kotlin/kotlinx.serialization/blob/master/examples/example-jvm/src/>

That seems really simple, but I had a problem while trying to use it :
<https://github.com/Kotlin/kotlinx.serialization/issues/526>