Partially Stringify JSON POST data

I need to POST JSON to an API in a way that one node of the JSON is stringified, if I try to stringify a node in JSON the JSON becomes invalid. Below is an example

Server Side Model that needs to be mapped

class Data {
  public int id;
  public string items;

Client Side JSON

var itemssData = JSON.stringify({"name" : "item1" , "info" : [{"size" : 7 , "color" : "blue"}]})
var data = {
 "id" : 1,
 "items" : itemssData 

Now POST data but data becomes invalid due to following structure


Do I need to encode this data?Is there a performant way to achieve this as items JSON can be very large
NOTE: I cannot stringify the entire model which includes id