Tons. It wasn't a secret at all. We have communications between the Soviet and the Allies and the Americans especially had asked Stalin for a long time to attack Japan in order to lessen the burden of the Pacific front. The answer was always "yes, when we're done with Germany". And so when they were done with Germany in may, they put a lot of troops in the east and attacked.

It wasn't a secret it was a military plan coordinated with the rest of the Allies.

As for their plans for Japan specifically it's a bit harder to determine, true. But they still got Korea and tried to took the whole peninsula in 1950 (Korean war and all that).