SNIPPET JSON parsing failure despite 200 OK

How do I correctly to parse the JSON response in a SNIPPET action when GetRESTProxy returns a 200 OK? The docs state “The response body is stored as a string,” yet my ASSIGN action fails to extract the id field. I am using AssignValue = ResponseBody.id but it resolves to null. Is there a specific syntax for accessing nested properties in the Studio expression language, or do I need to manually parse the string first?