Paper trading validates process: whether entry rules fire as designed, whether exits execute, whether the discipline survives contact with real market hours. It systematically flatters economics, because simulated fills skip the bid-ask cost every real order pays. Both facts are useful as long as neither is mistaken for the other.
Mechanics and behavior. A few weeks of simulated trading answers real questions: do the entry rules produce candidates at the expected rate, do exits fire when their conditions are met, does the position count stay within sizing caps, and, honestly the most valuable, does the human overseeing it interfere with the rules when a position goes red? Process failures found on paper cost nothing; every one of them found live costs money. That is why a paper-first sequence is the standard onboarding for any rules-based discipline, automated or manual.
Fills. A simulator that fills at the mid price collects an edge real orders pay away: a real four-leg condor order crosses some fraction of four bid-ask spreads on the way in and again on the way out, and on marketable orders the fill sits closer to the far side than the mid. Over dozens of trades that difference compounds into percentage points. The honest uses of a simulated track record survive this (rule firing rates, discipline adherence, relative comparisons between rule variants); the dishonest use, projecting simulated returns onto real capital, does not. This is also why Options Scanner excludes simulated fills from its outcome-model training data: mid-price fills would teach the model an edge that does not exist.
Time and evidence rather than enthusiasm: enough simulated cycles to see each rule fire (including the loss stop, which quiet markets can hide for weeks), a journal showing the rules were followed rather than overridden, and realistic-fill accounting if the simulator offers it. The transition itself scales rather than switches: smallest real size first, because the remaining unknowns (fill quality, assignment mechanics, one's own behavior with real money at stake) only surface live, and surfacing them at one contract is tuition rather than loss.