1.98×
what the same work costs in a session that grew past 350,000 tokens, compared with one that stopped at 200,000.
Cost per unit of work, by the session's peak context
Each bar is the whole session's total cost divided by that session's actual output. So what counts is not what a single turn cost, but what the finished work cost.
Why — two opposing taxes
The cost breakdown explains everything. In a short session the money goes into building the cache. In a long one the entire conversation is re-read from cache on every single turn — and less and less is left for output.
- building the cache
- reading the cache
- output, i.e. actual work
- 3% workbuilding the cache 96, reading the cache 1, output, i.e. actual work 3
- 22% workbuilding the cache 23, reading the cache 55, output, i.e. actual work 22
- 20% workbuilding the cache 12, reading the cache 67, output, i.e. actual work 20
- 9% workbuilding the cache 5, reading the cache 86, output, i.e. actual work 9
- 7% workbuilding the cache 4, reading the cache 89, output, i.e. actual work 7
In a fresh session 96% of the cost is building the cache — that's the restart tax, and it only pays for itself after a few dozen turns. In a large session 89% of the cost is re-reading old context and 7% goes to the actual work. Across my total spend, 64% went to cache reads alone.
The numbers
| Session grew to | Sessions | Cost per unit of work |
|---|---|---|
| 50–100k | 12 | 1.05× |
| 100–200k | 54 | 1.00× |
| 200–350k | 54 | 1.45× |
| over 350k | 23 | 1.98× |
What I do about it
- I let a session grow to 150–200k tokens, then restart — at a natural task boundary, not mid-work.
- I don't restart for something small. Under ~30 turns the cache build never pays for itself, so you make it more expensive, not cheaper.
- I put context size on the dashboard so I don't have to guess. The number comes from the last
cache_readfield in the transcript.
One clarification: on a subscription this isn't a money bill, it's a rate-limit cost. Restarting does not reset the 5-hour window — that one is wall-clock. But a lower cost per turn means more work fits inside the same window.