Two weeks ago we published a number: a Claude Code subagent costs ~436,000 tokens before it reads a single file. We built routing rules around it, wrote a break-even formula with it, and repeated it in two articles. Today we re-measured with a cleaner method and got 54,154 tokens. The old number was off by roughly 8×, and the way it was wrong is more useful than the number itself.
This post is the correction, the method that fooled us, and the break-even math redone.
The five-minute measurement we should have started with
The clean way to measure spawn cost is embarrassingly direct: spawn an agent that does nothing.
We gave a minimal subagent this prompt: "This is a measurement probe. Do not read anything, do not call any tools. Return the two characters 'ok'." Then we read the agent's transcript file — Claude Code writes one JSONL file per subagent, and every API call in it carries a usage block. The probe made exactly one request:
