Reference arithmetic for one to four proposed option legs: net debit or credit, payoff, breakevens and whole-position greeks. Long legs price at the ask and short legs at the bid; a leg without a usable quote falls back to its last trade, and `pricingIsExecutable` is then false. Legs in one expiration return a terminal payoff; legs in two return one result per path, since the back legs still carry time value at the front expiration; more than two is refused. This is a read, so legs travel as repeated `leg` values. Probability of profit is always null. Pro reads real-time quotes and Plus delayed indicative quotes; the Free plan covers end-of-day data only and receives 403. US listings only.