This am the US National security advisor, Jake Sullivan was asked about response from the US should Vlad go nuclear. He was understandably cagey in his response but twice said the response would “catastrophic” for Russia. He also said that if Vlad decides to use nuclear weapons, he will do so knowing full well what the US response will be.
So we know whet the US National Security advisor has said in public. There is also what messages have been passed through diplomatic channels. Also the NATO plans on how to respond to any use of WMD appropriate to the details of their use. And what Putin believes NATO will actually do.
It would be naïve to assume these are all identical.