Source(s). Emphasis mine. The intercepted cables I see aren't going to the Soviet government but rather to a Japanese diplomat in Moscow, which is a little different.

Since September 1940, under the covername "Magic," U.S. military intelligence had been routinely decrypting the intercepted cable traffic of the Japanese Foreign Ministry. [...] This summary includes a report on a cable from Japanese Foreign Minister Shigenori Togo to Ambassador Naotake Sato in Moscow concerning the emperor's decision to seek Soviet help in ending the war. Not knowing that the Soviets had already made a commitment to its Allies to declare war on Japan, Tokyo fruitlessly pursued this option for several weeks. The "Magic" intercepts from mid-July have figured in Gar Alperovitz's argument that Truman and his advisers recognized that the emperor was ready to capitulate if the Allies showed more flexibility on the demand for unconditional surrender. This point is central to Alperovitz's thesis that top U.S. officials recognized a "two-step logic" that moderating unconditional surrender and a Soviet declaration of war would have been enough to induce Japan's surrender without the use of the bomb.[22]

Below, an opinion of the US Army intelligence chief:

The day after the Togo message was reported, Army intelligence chief Weckerling proposed several possible explanations of the Japanese diplomatic initiative. Robert J. Maddox has cited this document to support his argument that top U.S. officials recognized that Japan was not close to surrender because Japan was trying to “stave off defeat.” Having analyzed the document closely, Tsuyoshi Hasegawa argues that each of the three possibilities proposed by Weckerling “contained an element of truth, but none was entirely correct”

An then a plot twist?

Another intercept of a cable from Togo to Sato shows that the Foreign Minister rejected unconditional surrender and that the emperor was not “asking the Russian’s mediation in anything like unconditional surrender.”