Have you ever sized an overnight batch job on paper, watched it finish comfortably inside the estimate, and then found it still churning at breakfast? I have, twice this month, and both times the arithmetic was correct and the assumption underneath it was wrong. This is a 48-hour field note about the difference between an average and a promise, and about the hidden cost of every restart on an interruptible box.

The setup was ordinary: a few thousand items, one model call per item, a checkpoint file so nothing gets done twice, and a machine that can reboot under me. I ran it on MonkeyCode's free model access and its free server option, which is what made the restart question unavoidable rather than theoretical. Disclosure: This article was prepared as part of MonkeyCode's product outreach.

Hour 0–12: the pilot I should have run first

My first version of the forecast was one line: item count times average call time.…