Just to contextualize this... https://lmcache.ai/kv_cache_calculator.html. They only have smaller open models, but for Qwen3-32B with 50k tokens it's coming up with 7.62GB for the KV cache. Imagining a 900k session with, say, Opus, I think it'd be pretty unreasonable to flush that to the client after being idle for an hour.

I wonder whether prompt caches would be the perfect use case of something like Optane.

It's kept for long enough that it's expensive to store in RAM, but short enough that the writes are frequent and will wear down SSD storage

Yes — encryption is the solution for client side caching.

But even if it’s not — I can’t build a scenario in my head where recalculating it on real GPUs is cheaper/faster than retrieving it from some kind of slower cache tier