Cannot serialize built-in classes.

[phosxd]

You cannot serialize any built-in Godot class to JSON because they do not return a script with get_script so it fails with an error in your implementation.

Am I misusing this or is this not a feature?

[phosxd]

Also for custom classes none of the properties transfer over into the JSON representation. I have a basic class with one property "id", passing an instance with "id" set to "test" returns a JSON object with one key/value pair with the key being ".type" & the value being "Object".