# Under the Hood

By [DYLIT Chronicles](https://dylit.info/user/dylitmediabuzz)

[Everything AI - beyond the hype](https://dylit.info/pr/everything-ai-beyond-the-hype/6a9efac02e92664f4d50cf9d) > [Under the Hood](https://dylit.info/ch/under-the-hood/6ab35f8099638e119f592d1f)

When a Spreadsheet Beats an AI Tool If a task has one correct answer produced by a fixed rule, use something that follows rules. Arithmetic is the clearest case. A language model predicts plausible text, and the text of a correct sum looks very much like the text of an incorrect one. Ask it to add a column of forty figures and you will usually get the right answer, which is worse than always getting it wrong, because you stop checking. Many tools now handle this by writing and running actual code when they detect a calculation. That genuinely fixes the problem, but you need to know whether yours does it, and whether it did it on this particular request. If you cannot see the calculation, do not trust the number. The broader category Arithmetic is one instance of a general rule. Reach for deterministic tools when: The answer is computed, not composed. Totals, conversions, date arithmetic, percentages. The rule is fixed and known. Tax bands, shipping tiers, commission structures, eligibility criteria. A formula does this correctly every time and shows its working. You need the same input to give the same output. Reconciliation, compliance checks, anything auditable. Built-in variation is a feature in writing and a defect here. The data is structured and large. Ten thousand rows belong in a database or a spreadsheet, not a prompt. Where the line actually sits The useful split is not "numbers bad, words good." It is closer to this: Computing a figure is a job for a spreadsheet. Explaining what the figure means, writing the commentary, spotting which variances deserve attention: those are good AI tasks, working from numbers you calculated properly. That combination is where most finance teams land, and it works well. The spreadsheet produces the truth. The model produces the narrative around it, from figures you supplied rather than figures it generated. The test Before opening an AI tool, ask whether the task has a single right answer that a rule would produce. If yes, use the rule. You will get the correct answer, the same answer every time, and a record of how it was derived. If the answer depends on judgment, phrasing, or synthesis, you are in the right place. Using a language model for arithmetic is not a small inefficiency. It is choosing a probabilistic tool for a deterministic problem, and the errors it produces are the kind that look completely normal.