The hard part of an AI feature is knowing where NOT to use AI

A payment decision has to be exact and repeatable. So in the product I built, the money logic is deterministic code, and the agent only touches the parts where judgement is genuinely open-ended. Every AI demo right now is an agent doing everything. Point it at the problem, let it reason end to end, marvel at the trace. It demos beautifully. Then you try to put it in front of a real workflow with real money and it falls apart, because the thing that makes a demo impressive, the model deciding freely, is exactly the thing you cannot allow when the output is a payment. I spent a while building a procure-to-pay product: a vendor invoice comes in, gets extracted, matched against a purchase order, routed through an approval workflow, and reconciled. It is the kind of thing everyone now wants to put an agent on. So I did, sort of. But the interesting decision, the one that took the longest to get right, was not where to add the agent. It was where to refuse to. The rule: a payment decision must be exact and repeatable A model is a probability distribution. Ask it the same question twice and you can get two answers. That is a feature when the task is fuzzy and a liability when the task is "does this $48,200 invoice match this purchase order". Matching, the approval engine, reconciliation: these have to be exact, auditable, and identical every run. So they are plain deterministic code. No model in the path. If a controller asks why this got approved, the answer is a code path they can read, not "the model felt it was fine". That sounds obvious written down. It is not how most people are building AI features right now. The default has become: agent first, and carve out the deterministic parts only when something breaks. I did the opposite. Deterministic by default, agent only where the trajectory is genuinely open-ended. The three places the agent actually earns its keep Once you hold that line, the places where AI belongs get very clear, because they are exactly the places a