Get EC2 DescribeInstances API response in JSON

Hi, I am trying to get the API response of EC2 DescribeInstances API in JSON format. specifying Accept=application/json header works for other APIs but not in DescribeInstances API. Is there any workaround to get the response in JSON?

Edit: I am not using SDK/CLI. I am making the API request separately using JAVA by deriving v4 sign key.