Hi,
It seems that Completion API and Assistant API differs here.
A workaround to play with JSON data from an assistant is to use function calling.
Example: Create a function storeMyData(json_schema) and use the function to manipulate and do whatever you need with the parameter of the function.
You can define the json schema using function definition.
Find official doc here: https://platform.openai.com/docs/api-reference/assistants/createAssistant
CreateAssistant > Tools > Function Tools > Function > parameters
The parameters the functions accepts, described as a JSON Schema object. See the guide for examples, and the JSON Schema reference for documentation about the format.