Originally published on hexisteme notes.
I was running a local capture proxy in front of the vendor's API — a small man-in-the-middle process I'd set up for an unrelated token audit — when my coding-agent CLI spawned two sub-agents back to back. Same task, same working directory, same assembly path. One sub-agent got routed to a mid-tier model, the other to a small model. Because the proxy was logging full request bodies, I could diff them byte for byte. That diff is the whole essay.
The accidental A/B
The two request bodies were 615,341 bytes and 617,134 bytes. That's a difference of 1,793 bytes, about 0.3% — for practical purposes, the same payload. Same system prompt scaffolding, same tool definitions, same conversation history, same task description. Nothing about the content should have made these two requests bill differently. If token count were a property of the bytes on the wire, these two numbers should have landed within a rounding error of each other.
They didn't.
