They obviously did spot the build up because they were able to withdraw.Idk why Putin insists on fighting this war with his hands ziptied to his legs. He seems to care more about domestic approval, when the direction of the entire country is resting on this campaign. Yet he can afford 50k troops to participate in Vostok 2022 instead of bolstering defenses. We can all see the result of that here. He wont strike Kyiv, I'm not sure if they even have the capability to strike bridges near the UA frontlines or the Dnieper anymore. It should be clear there's no way to win the war with current force dispositions, but no mobilization is underway yet... Frankly if Russia is struggling this hard against Ukraine, I wouldn't even want to imagine how'd they do against the US. Which reminds me, how did Russia not spot the buildup near Kharkiv? This wasn't a small-time, local counterattack. I wont even go into the performance of the RuAF.
Up until now all fronts have been effective stalemates. Russians weren't prepared to do what was necessary to end the war (mass destruction of Ukrainian forces and cities). The Ukrainians were too afraid to leave their cities.
This Ukrainian offensive reminds me of the Battle of the Bulge. A big German counter offensive against allied positions. The Allies withdrew until the Germans overextended and then they were attacked. It lead to the collapse of the Siegfried line. I'll give the Ukrainians some credit for changing the status quo, but it seems suicidal to me.