Megalightweight thermos

When I climb long routes, I try to skip anything un-necessary and heavy. Especially when it comes to very long and challenging climbs.... I am trying to find a container that would not be much heavier than a Nalgene, that would keep the liquids hot. Is there a thermos suitable for alpinism, or do you guys get your warm liquids when you stop and brew?

Zojirushi tuff slim

Klean Kanteen makes an insulated bottle, it's supposed to keep things hot for 6 hours vs. the longer time of the zoji but it weighs a little less. Still more than a nalgene.

Search for "glass vacuum insulated" products, they will be lighter than stainless steel. Keep in mind, they tend to break easily :(
Not sure what your capacity requirements are, this is one is 16oz Thermos

Hydro Flask is an option to consider as well. Little pricier, but have a lifetime warranty and claim they'll keep hot stuff hot for up to 12 hours (cold up to 24).

You can insulate your nalgene. I made a parka for my nalgene out of bubble wrap insulation and duct tape that works surprising well (although probably not as well as a thermos) and is super light. I use mine mostly to keep my water from freezing in the winter, but it also keeps hot drinks very hot for at least a couple hours. Outdoor Research also makes a water bottle parka that is 140g.

Hydro flask makes a great product

Granite Gear also makes a phenomenal insulating sleeve for Nalgene bottles. Similar to the OR version, but much lighter.