EditResponseMiddleware.cs

public class EditResponseMiddleware
{
    private readonly RequestDelegate _next;

    public EditResponseMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        var originBody = context.Response.Body;

        var newBody = new MemoryStream();

        context.Response.Body = newBody;

        await _next(context);

        newBody.Seek(0, SeekOrigin.Begin);

        string json = new StreamReader(newBody).ReadToEnd();

        context.Response.Body = originBody;

        await context.Response.WriteAsync("modifiedJson");
    }
}