Measurement
Token Counting for Fun and Profit
August 10, 2026
One skill invocation in a Claude Code session wrote 261,974 tokens into the prompt cache. The tokens replayed from cache on each subsequent request went from 93,786 to 355,760, and 93,786 + 261,974 = 355,760 exactly. Every turn after that point cost several times what the turns before it cost, for the rest of the session. That arithmetic is the mechanism confirming itself.
Model choice and reasoning effort both change how fast a session burns through a subscription usage limit, and both are settings you pick deliberately. Two more are not settings at all. They accumulate from how you work, turn by turn, mostly unobserved: what you put into the context, and whether the cache is still warm when you come back. Those two are the subject here.