Anthropic Officially Files S1 – Shows $81B Net Loss

They issued convertible notes and other instruments that were priced as of an older valuation. As their valuation went up, those instruments are now more valuable and due to GAAP accounting rules they have to register that increase in valuation of their issued instruments as a loss. No money changed hands.

Matt Levine's column from Oct 22 explains it much better if someone has a gift link.