Building the cost model live

The take-home or whiteboard cost model is where this loop is won or lost. Cursor wants the artifact, not the ceremony - build the model, write the recommendation.

Under time pressure the failure mode is a sprawling spreadsheet with hidden assumptions and no conclusion. The winning move is a model that's legible: it starts at the unit, labels every input and ends with one recommendation and the tradeoff named. Structure beats polish here.

1. 1Start from the unit. Anchor on cost per inference or cost per active user. Everything else is a multiplier on or a rollup from that unit.
2. 2Label every input known vs. estimated. GPU hourly rate is known; requests per user per day is estimated. The interviewer trusts a labeled estimate far more than a confident guess.
3. 3Run sensitivity on the 2-3 dominant variables. Utilization, model mix and request volume swamp everything else. Show how the answer moves across a plausible range of each.
4. 4Show the bridge. Walk how a routing or batching change flows through the model to COGS, so the recommendation is traceable, not asserted.
5. 5Cross-check with back-of-envelope. Before trusting the sheet, sanity-check one cell against a napkin number. If they disagree by 10x, the sheet has a bug.
6. 6Land one recommendation, tradeoff named. Not a menu - a call, with the cost of being wrong stated.