Comment by libraryofbabel

Are you sure about that? They charge $6.25 / MTok for 5m TTL cache writes and $10 / MTok for 1hr TTL writes for Opus. Unless you believe Anthropic is dramatically inflating the price of the 1hr TTL, that implies that there is some meaningful cost for longer caches and the numbers are such that it's not just the cost of SSD storage or something. Obviously the details are secret but if I was to guess, I'd say the 5m cache is stored closer to the GPU or even on a GPU, whereas the 1hr cache is further away and costs more to move onto the GPU. Or some other plausible story - you can invent your own!