Two different PoDs:
A) Stalin dies by accident or a lucky German bomb or shell before summer 1945. Subsequent Soviet collective leadership, focused on the tasks of reconstruction, and establishing the most secure perimeter in Europe with VE-Day, declines to participate in the war against Japan, judging it not worth the cost in lives and equipment and financial resources, even with prospective extension of Soviet security and economic influence and war booty. The Soviets will still reclaim southern Sakhalin island, but have confidence they can get it for free after the fighting ends by only diplomatic means without arousing diplomatic hostility. The Soviets understand they are not guaranteed a transfer of the Kurils or any restored or special rights in Manchuria or Korea.
Among the post-Stalin Soviet leadership, some argue that making strategic gains at a fraction of the cost of Great Patriotic War is an opportunity not to be passed up, and elements of the Soviet military say the mission is eminently doable. Plus, it is an opportunity to get revenge against Japan for the Russo-Japanese War, Japanese intervention in Siberia and several other lesser provocations.
However, without Stalin, the Politburo and collective leadership is dominated by men much younger than Stalin, who were small children or young teenagers during the Russo-Japanese War. With the exception of Voroshilov and the non-influential Kalinin, the Politburo members were all from 12 to 24 years younger than Stalin, and thus of a different generation, so the urge for revenge against Japan is much less strong among them.
Plus, the collective leadership knows there is no bottom-up pressure from Soviet society, nor much desire, to get into a war of choice against Japan after finishing the Great Patriotic War. In 1945 as in 1953 they care a bit more about domestic economic issues of reconstruction than about imperial agendas and are more concerned to maintain societal consensus.
So, the Soviets decline to declare war against Japan.
Does the Truman Administration and American military resent Soviet non participation and think of it in dark terms, as darkly as they were perceiving Soviet involvement by the final weeks of the war in OTL August 1945. [In other words, would Washington by summer of 1945 inevitably put a negative spin on Soviet decisions vis-a-vis the Far East, whether those decisions were aggressive or passive?]
More immediately, when does Japan surrender? What additional efforts, if any, do the WAllies have to make to compel Japanese surrender after the bombing of Nagasaki. Dropping of one or more additional atomic bombs? Invasion of Kyushu or Honshu?
Does the lack of Soviet participation cause any POWs held in Manchuria or Japan to not survive the war? If Japanese surrender is delayed, does this increase untimely civilian deaths in Japanese occupied areas?
Alternative PoD:
B) Stalin decides all along he is simply not inclined to go to war with Japan unless forced too. Despite the Russo-Japanese War being a major episode for his generation, he's just not interested, and there's nobody there interested in contradicting him. At most, the Generals in the east say "we could do it boss if you want"
So, the Soviets decline to declare war against Japan. Indeed, there is divergence in American expectations even earlier, as Stalin has declined to give or imply promises of eventual Soviet entry at Tehran or Yalta. How does that change American planning for the later months of the war, if at all?
Does the Truman Administration and American military resent Soviet non-participation and think of it in dark terms, as darkly as they were perceiving Soviet involvement by the final weeks of the war in OTL August 1945. [In other words, would Washington by summer of 1945 inevitably put a negative spin on Soviet decisions vis-a-vis the Far East, whether those decisions were aggressive or passive?]
More immediately, when does Japan surrender? What additional efforts, if any, do the Wallies have to make to compel Japanese surrender after the bombing of Nagasaki. Dropping of one or more additional atomic bombs? Invasion of Kyushu or Honshu?
Does the lack of Soviet participation cause any POWs held in Manchuria or Japan to not survive the war? If Japanese surrender is delayed, does this increase untimely civilian deaths in Japanese occupied areas?