Yes (5.) is possible, but now your API type becomes "List of List of String" and you need documentation and examples to explain that every list will have two en

Yes (5.) is possible, but now your API type becomes "List of List of String" and you need documentation and examples to explain that every list will have two entries and the first one will be the key. On top of that, if you have a different types for keys and values you have an even bigger problem, because lists in graphql need to have one type for all elements. And while you can use a uniontype (like String | Integer) to have strings as keys and integers as values, your user now ends up with an ugly typ in their code and has to untangle it by hand. So the user will have to 1.) know about that it is supposed to be a map, 2.) untangle the list types in their code, 3.) convert it into a map while not mixing up key and value Therefore I usually go with explicit key/value objects if it's okay for performance or go with a custom JSON type if performance trumps the rest.