n=1 note. This is the anatomy of one real session, not an average or benchmark. The specific numbers are this session's actual measured figures. The mechanic — re-sent context dominating long sessions — is general. Your numbers will differ with your workflow. The full dataset (percentile tables, methodology, charts) for n=66 sessions lives at the benchmark page; this article is about the one session that broke my mental model.
There's a session in my logs that cost $1,278.
Not $12.78. Not a typo. $1,278, across approximately 1,270 model turns, in a single Claude Code coding session. When I measured it properly, two things became obvious:
I had completely the wrong mental model of where the money goes.
Once you understand the mechanic, it stops being mysterious and becomes something you can actually control.
