It is quite simply impossible to have "reliable intelligence" that Russia will not use nuclear weapons because the decision to do so will be based on the situation at the time as viewed by Putin. Even if today people close to him (in so far as they exist) are "certain" he won't use such weapons, and even if such people were in contact with our intelligence agencies, Putin could wake up tomorrow and change his mind, especially if "the situation" changes. The question would then become to what extent his orders would be followed, and again, that is something we can't know with any confidence. Even if we had been "assured" that none of the "official" channels would act (pretty much a zero probability event even if in fact they would not), there are always "unofficial" channels in Russia.
The actions of the US are based on an estimate of risk vs benefit. To what degree that assessment is based on an evaluation of Russian "intention" is unknown to me, but I suspect not very much. The risk that Russia would start a nuclear war with the US is always there, but is not greatly increased by what happens in Ukraine. If Russia comes to believe that using a nuclear weapon in Ukraine "makes sense" in their risk vs reward calculation, they will do it. The US is relying on the fact that it is unclear what benefit Russia would achieve. If Russia comes to believe that such action would make Ukraine quit or would make NATO quit helping Ukraine, they will, IMO, use a tactical nuclear weapon. I suspect Russia is quite aware that such use would have the opposite effect and could result in an escalation that would be highly unfavorable to Russia. If Putin becomes convinced that evaluation is wrong and that the US/NATO (or Ukraine) would fold, a "miscalculation" is possible. The US is operating under the assumption Putin is still getting enough "real" information to prevent such a decision.
The other (and IMO more probable) reason Russia might take such an action would be to stop a Ukrainian attack that is overwhelming the Russian military and that may inflict an outright defeat on Russia. I think Russia would make sure our intelligence agencies observed them making preparations for such an action. The hope would be we would restrain Ukraine. If we can't/don't, Russia would use the weapon. Hopefully before that point is reached the Russian government/army/people would remove Putin.
In any case, both sides are well aware that the use of nuclear weapons in Ukraine can have unpredictable consequences and that most of those consequences are "bad", some actually "catastrophic". That uncertainty is actually what makes the use unlikely. What could cause it to happen is a false sense of "knowing" the outcome on the part of either side.
The actions of the US are based on an estimate of risk vs benefit. To what degree that assessment is based on an evaluation of Russian "intention" is unknown to me, but I suspect not very much. The risk that Russia would start a nuclear war with the US is always there, but is not greatly increased by what happens in Ukraine. If Russia comes to believe that using a nuclear weapon in Ukraine "makes sense" in their risk vs reward calculation, they will do it. The US is relying on the fact that it is unclear what benefit Russia would achieve. If Russia comes to believe that such action would make Ukraine quit or would make NATO quit helping Ukraine, they will, IMO, use a tactical nuclear weapon. I suspect Russia is quite aware that such use would have the opposite effect and could result in an escalation that would be highly unfavorable to Russia. If Putin becomes convinced that evaluation is wrong and that the US/NATO (or Ukraine) would fold, a "miscalculation" is possible. The US is operating under the assumption Putin is still getting enough "real" information to prevent such a decision.
The other (and IMO more probable) reason Russia might take such an action would be to stop a Ukrainian attack that is overwhelming the Russian military and that may inflict an outright defeat on Russia. I think Russia would make sure our intelligence agencies observed them making preparations for such an action. The hope would be we would restrain Ukraine. If we can't/don't, Russia would use the weapon. Hopefully before that point is reached the Russian government/army/people would remove Putin.
In any case, both sides are well aware that the use of nuclear weapons in Ukraine can have unpredictable consequences and that most of those consequences are "bad", some actually "catastrophic". That uncertainty is actually what makes the use unlikely. What could cause it to happen is a false sense of "knowing" the outcome on the part of either side.