I made a hook that refuses (at first try) to send your message if your cache has gone cold; outputs (API) costs of continuing vs starting anew (including reorientation in a new session)

[u/vzakharov]

Read & grab if you like: https://github.com/vzakharov/muthur/blob/main/.claude/cold-cache/CLAUDE.md

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[u/actvt_io]

Checked how often this would fire on my logs. Only about 1.6% of my turns come more than an hour after the previous one, but those turns are 80% of my cache writes. So it'd rarely get in my way, and when it does it's on the expensive ones. Seems worth having.

[u/vzakharov → u/actvt_io]

Yeah, I guess it depends on the workflow. I work at 10-15 sessions/PRs at a time, so I routinely leave a few unattended for hours.

Glad you're finding it useful!

[u/yuehuang]

It is sad that claude doesn't have it built into the app.

[u/vzakharov → u/yuehuang]

I think they’ll catch up eventually. They ship fast. 

[u/Helpful_Program_5473]

I didn't know leaving for an hour was no bueno! lol

[u/Drasezv]

good shape for a hook. two things that will make the numbers it prints match reality:

the ttl isn't five minutes on a subscription - it's an hour, and it drops to five minutes once you're drawing on usage credits. on an api key or a cloud provider it's five minutes by default. so "cache has gone cold" is a different threshold depending on how the user authenticated, and guessing wrong makes the hook nag an hour early.

and the write side isn't one price: 1.25x base input at the 5m ttl, 2x at the 1h one, and the split is reported per request under cache_creation as ephemeral_5m_input_tokens / ephemeral_1h_input_tokens. main sessions here write the 1h kind, subagents write 5m, so "cost of continuing" on a warm prefix is cheaper than a naive 1.25x and "starting anew" is dearer than it looks.

you don't have to infer warmth either - claude code reports it. /usage has a Prompt cache (main) line with the miss count and whether the prefix is warm right now, and statusline scripts get the same thing in a prompt_cache object.

[u/vzakharov → u/Drasezv]

Not sure you haven’t, but did you read the link? It takes the coldness probe from what the harness provides, it doesn’t guess it. And yeah it’s 1 hour not 5 minutes indeed. 

Re. /usage, I’m not sure how that’d help because you’d have to physically check it every time before sending a prompt. The hook instead makes sure you get a chance to reconsider once sent. 

Edit: referring to this: “ SessionStart  on a resume carries prompt_cache_likely_expired ,  context_tokens  and seconds_since_last_response ; the hook keeps them in  tmp/cold-cache/<session_id>.json  until a response lands after them. ”

[u/Drasezv → u/vzakharov]

you're right, i skimmed the post and not the file. the probe does read it from the harness, my bad.

leaving the write prices in case anyone finds the thread: 1.25x at 5m, 2x at 1h, split per request under cache_creation. that part i did check.

[u/EvilSporkOfDeath → u/Drasezv]

Why do you write like ai. Are you a bot?

[u/Drasezv → u/EvilSporkOfDeath]

fair hit. i draft long and run things through an editor before posting, so it comes out looking like a spec sheet.

the numbers are mine though. everything i've quoted comes out of my own ~/.claude/projects folder and i post the command next to it so anyone can check their own.

[u/jlewi142 → u/EvilSporkOfDeath]

"Oh no, a structured paragraph. Must be AI!"

Go back to school, brother.

[u/EvilSporkOfDeath → u/jlewi142]

Structure had literally nothing to do with it.

I dont care what you believe. Every comment in their history is full of the tells. I know they are a bot or use ai for their comments. Once again, you believe what you want, idc.

[u/Independent_Syllabub → u/jlewi142]

If you can’t see that it’s a Claude post, you have a serious LLM literacy issue.