It's not impossible we have internal documents from the time, the US was actively reading all of the diplomatic traffic and basically anything broadcast via radio anywhere in the Japanese government. One big sticking point that appears in a lot of discussions is the demand for unconditional surrender, a lot of effort diplomatically was spent around getting past the allied agreement to only accept unconditional surrender. One big factor in that from the primary sources was the possibility of the emperor being executed or completely dethroned.