I measured one AI agent session token by token. 5 of 20 turns ate 94% of the cost.

Everyone talks about what AI agents can do. Almost nobody measures what they actually cost.

I run a small fleet of agents. Last week I pointed a simple guard at one session's transcript and read the usage per turn - not estimated, measured. In a single 20-turn session:

The expensive turns weren't the hard ones. They lined up with the tool list changing - an MCP server disconnecting and reconnecting invalidated the cached prefix, so the whole context got rewritten, and a write costs 2x a read.

Caveat: this is one session, on one setup. Don't generalize my numbers - measure your own. The point is the bill hides this and the transcript doesn't.

I packaged the doctrine, the lane templates, and the consumption guard (a small Python script that reads any session's transcript and prints this breakdown) into a kit: https://spataro1.gumroad.com/l/ghjrgi

How are you measuring your agents' cost? Curious what others are seeing.