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Comment by s0ck_r4w | Hacker News Reader

How do a few extra requests with the same prefix make it more expensive for everyone else?

Keeping a conversation with a very large model active requires hundreds of GB of memory. If my conversation can never be swapped out, like when I go to lunch or take an afternoon walk, that's several hundred GB not available for other users. If everybody does that, Anthropic needs even more infrastructure than the quarter trillion dollars in infra they're already using.

The idea here is that your conversation is being swapped out for no good reason. The session is still active, as the subagents continue to make progress.

How is this comparable to going to lunch or taking a walk?

Is that actually happening, though?

Yes https://github.com/izeigerman/claude-thermos/blob/main/src%2...

CC actually prompts Claude about this by default in the ~20k system prompt and instructs it to avoid 300s timeout and to be mindful of the 300s cache expiration.

CC also will also block sleeps longer than eg 300s; the harness handholds the model quite a bit.

But you pay for cache writes, it's not like you're abusing anything, you're simply choosing to call a billed API for the explicitly intended purpose of writing/refreshing cache.

If you keep this running for hours without doing anything, it will drain your limits and API. The use case of keeping the main thread cache warm while subagents work is very genuine and legitimate.

The prefix cache is a resource shared by all users. This is basically a tragedy of the commons.

If true, I suspect Anthropic will begin suspending accounts that use it.

I highly doubt that.

Keeping your cache warm is a good thing, caching saves compute and electricity.

Cached input is cheap for a reason, it is in everyone’s mutual interests to maximise cache hit rates.

If that was true then Anthropic would have built this functionality in themselves.

They did. On any long running query with large context, tell Claude “give me a status report every 4 minutes to keep the cache warm”. It’s entirely happy to oblige and usually says “good idea, that will keep costs down”

Compute is also a resource shared by all users. When your prompt isn’t cached, your input needs to go through prefill; which is compute-bound.

The cache is discounted for a reason. They WANT you to use it.

They could always just extend the cache timeout beyond 5 minutes themselves. They don't do that because it is an expensive resource and there is a trade-off bewteen saving computation and reserving VRAM. Running a tool like this will force them to do one of the following:

1) Start charging for VRAM reservations.

2) Charge _other_ customers more.

3) Eat the cost themselves.

They charge you for cache writes already. There's a price assigned to it. Even if you're on subscription, it counts against your usage (very easy to observe with a big cache miss).

Anthropic (and now OpenAI too for 5.6) prompt caching is not free.

Holding your cache in VRAM for 5 minutes or 3 hours have very different costs to them.

"They already charge me to park my car, why can't I leave it there for a year for the same price as 1 week?"

So why is it 5 minutes at both peak and off peak hours? If they can guarantee 5 minutes at peak hours for everyone, why is it only 5 minutes at off peak?

I don’t think your understanding works.

you use more memory?

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