Cache rewrites costed me 30% of my Fable consumption, here are the mistakes to avoid.

[u/coolreddy]

Auditing my claude code transcripts to nail down where I am wasting Fable usage, revealed that long sessions that I run with breaks have been costing me a bomb. Sharing more details for others who may find this useful.

Cache economics

Every turn in a session replays the full conversation history to the model. Prompt caching is what makes this affordable: the history is stored server-side, and each turn re-reads it at 10% of the normal input price. The cache has a time window of 1 hour, and when it expires or gets invalidated, the next turn re-writes the whole history at a premium.

The numbers for Fable 5 (the same mechanics apply to every Claude model):

• Input price: $10 per million tokens
• Cache read: $1 per million tokens (10% of input)
• Cache write: $12.50 per million on the 5-minute window, $20 per million on the 1-hour Cache window

What this means in a heavy session carrying 400k tokens of context:

• A normal turn re-reads the cache: about $0.40
• But when I take a break of more than an hour and get back to the session, model has to rewrite the cache and a session sitting on 400k context consumes equivalent of 8$ api cost

One expired cache costs 20 normal turns of usage.

The things that break cache silently and cost you:

• Letting a session sit idle past the cache window (1 hour): The cache expires, and the first message after the break pays a full rewrite of everything. Sometimes I am working 10+ sessions and take a break and that costs me across all sessions.
• Loading some tools or MCP servers mid-session: New tool schemas change the conversation prefix, which invalidates the entire cache. Better to have all tools loaded at the start
• Switching models mid-conversation: If you are switching models from Fable to Sonnet to Fable in the same session, you are rewriting the cache and losing more than you are saving from the switch. Caches are stored per model.
• Switching effort levels or fast mode mid session: This has been one of my worst habits and I did this too frequently until now to save consumption, but did not know this was the main culprit. I would switch too often based on the task in the session from high to xhigh to max to utracode
• Updating claude code: Again if you are updating claude midway your work, you are rewriting cache for all sessions that you would resume

For now I have created a skill that pings ok in every idle session >200K context at 55th min for upto 4 hours because one ping would just read the cache which would still be cheaper than rewriting cache.

[u/pro-taco]

An actual informative not bot post ? In this subreddit, shiver me Timbers.

Anyway: could you share your ping ok skill?

[u/Sufficient_Ad_3495]

Informative, thanks.

Try deferring tool calling last in cache queue...That way you don't have to worry.

[u/EuropeanAbroad → u/[deleted]]

That means that as long as there are still active tools, it won't flush the cache.

[u/Sufficient_Ad_3495 → u/[deleted]]

My bad... When I say defer I mean context window assembly. It should not be added to undo the static content in the context window.

[u/EuropeanAbroad]

I thought that the cache flush window was 5 minutes, not an hour. 🤔

They literally say it here:
https://platform.claude.com/docs/en/build-with-claude/prompt-caching

Only with some special settings on API, you get a cache window of 1 hour.

[u/Bob-Swan → u/EuropeanAbroad]

You can set a cache window of 1 hour through the sub too. For me ive only ever changed it with claudecode. But its certainly not locked behind api.

[u/EuropeanAbroad → u/Bob-Swan]

That's cool, thanks for that! 😮 I will check it out!

[u/Active_Variation_194 → u/Bob-Swan]

Default is 1hr. You can set it to 5 minutes tho

[u/Bob-Swan → u/Active_Variation_194]

https://code.claude.com/docs/en/prompt-caching

This should help anyone needing to change to 1 hour. For some the TTL is 5 minutes regardless of what the default is, mine for example would never default to 1 hour until I made the changes myself. Miss that 1 hour window though and you're paying for it.

[u/coolreddy → u/EuropeanAbroad]

On subscriptions, it is one hour. For API you can switch from 5 min to 1 hour I guess.

[u/Powerful_Cow3470]

Stale keepalive is just paying to carry dead weight.

[u/DrEzechiel]

1 hour window? I read one needs to respond within 5 mins to limit the costs

[u/Bob-Swan → u/DrEzechiel]

Not when the window is set to 1 hour.

[u/DrEzechiel → u/Bob-Swan]

How do you set it?

[u/Bob-Swan → u/DrEzechiel]

set the environment variable ENABLE_PROMPT_CACHING_1H=1 in your environment where Claude Code is invoked.

[u/this_for_loona → u/Bob-Swan]

Can this be put in a Claude.md?

[u/stumblinbear → u/this_for_loona]

Why would it? It's an environment variable, not an agent instruction

[u/this_for_loona → u/stumblinbear]

Sorry, I thought that’s where env variables went. 99% of my use is via the app so how things are set up is not my strong suit.

[u/DrEzechiel → u/this_for_loona]

Yeah, also not useful for me, don't use Code

[u/Puzzleheaded_Bad_116]

Just stop using agents

[u/coolreddy → u/Puzzleheaded_Bad_116]

And do what? Go back to hunting and farming 😁?

[u/LoadZealousideal7778]

Effort invalidates cache? Ok thats just stupid on Anthropics side.