Generally speaking, IDK about Anthropic specifically, they don't train purely from scratch though. A good chunk of the setup is reused previous models can serve as a basis for the next model. Plus there are methods that also use the previous model as a warm start.

Just taking a wild guess, but I'd assume the .5 releases are built on the previous and the Majors (3, 4, 5) are more extensive retrains?

That’s maybe a reasonable argument but it’s not one Anthropic is making.

If the article is to be believed they aren’t including their training costs.

Also lol at reporting it as above 80% without accounting for the revenue sharing as well.

I bet anyone’s finances look great if you just start ignoring all the money they owe.