At every daily close:

• Your positions are marked to market (valued at that day's close: the stock at its closing price, options at the model price).
• Cash earns a day of interest. Borrowed cash pays a day of interest, and short shares a day of fees.
• The margin check runs, on the last day too.

On the 21st trading day your options expire and are settled in cash (paid out in cash, not shares). A call pays how far the price ends above its strike, a put how far it ends below. Otherwise it pays nothing. If you wrote the option, you pay that amount. Then you decide again.

• Stake. $100,000, fixed. No deposits or withdrawals.
• The stock. Drawn at random from a pool with very different histories, including the S&P 500 itself. You can also pick one, which turns blind mode off. In blind mode the name and dates are hidden and the price starts at 100. Returns are unchanged.
• Window. The full challenge is the most recent 60 months (1,260 trading days), ending at the latest close. A quick round is a random 12-month slice of it. Each turn is 21 trading days.
• Stock. Set as a percent of equity (what the account is worth: cash plus positions, minus loans), from −100% (short) to +150%, which is long on margin (owning more stock than the account is worth, with borrowed cash). Each trade costs 0.05% of the amount traded. Leave it alone and your share count stays the same.
• Options. Calls (the right to buy the stock at a set price) and puts (the right to sell the stock at a set price) at strikes (the price an option lets you buy or sell at) of 90, 95, 100, 105 and 110% of the price. They are European (the right can be used only at expiry, not before): they expire at the end of the month and settle in cash. Size is a percent of equity: 100% means options on as many shares as your equity would buy. Up to four option trades a month, at most 200% of equity in total.
• Leverage includes options. Net exposure (your total bet on the stock, as a percent of equity), the stock plus each option's delta (how strongly an option moves with the stock: 0 to 1 for calls, 0 to −1 for puts), must stay between −100% and +150% when you place an order.
• Writing options. Written (sold to someone else: you collect the option's price and owe whatever it pays out) calls must be covered by shares you hold. Written puts need cash equal to the strike value after the whole order, not counting short-sale proceeds.
• Financing. Cash earns the 13-week T-bill (Treasury bill: short-term US government debt, the usual near-riskless return) rate. Borrowing pays the T-bill rate + 1.5%. Short positions pay 0.5% a year. An order can't take borrowing above 50% of equity.
• Drift and margin. Limits apply to orders. Between orders, positions drift with the market. If equity falls below 25% of the gross stock position at any daily close, a margin call closes every position. At zero equity the game ends and the account ranks last.

There are no free historical option quotes for every stock, so the game prices options with a model. Its implied volatility (the yearly size of price swings that an option's price assumes) comes from that day's market data:

IV_atm² = RV₆₃(stock)² + [ (0.88 × VIX/100)² − RV₆₃(S&P 500)² ]

IV(K) = IV_atm × (1 − slope × ln(K/S))

• Inputs. RV₆₃ is the trailing 63-day realized volatility (how much the price actually swung, measured from daily returns and put in yearly terms). K is the strike and S the stock price.
• The bracket is the market's volatility risk premium (the extra that option buyers pay over the volatility that actually happens) that day. The VIX (an index of the volatility priced into S&P 500 options for the next 30 days) is a variance-swap level that includes the skew, so at-the-money (with the strike at today's price) index volatility is taken as 0.88 × VIX. Adding the bracket once prices at-the-money S&P 500 options at 0.88 × VIX and gives single stocks a small premium over their own realized volatility.
• Smile. A smile (how implied volatility changes from one strike to the next) makes lower strikes richer. The index smile has slope 3.0 on both sides. Single stocks get 1.25 below the money and a flat call wing, because out-of-the-money (would pay nothing if it expired today) stock calls aren't cheap in real markets.
• Guard rails. Each smile is clipped to between 0.5 and 2 times at-the-money volatility. At-the-money volatility is floored at 75% of realized. Every IV stays between 8% and 250%.
• Bid and ask. You buy at the ask (the price you pay to buy) and sell at the bid (the price you get to sell): a half-spread of 2.5% of the premium (the option's price), at least 0.05% of the stock price.
• Rates and dividends. The risk-free rate is the T-bill yield. Prices include dividends, so the stock is treated as paying none.

Why this model. Three earlier versions were replaced after reality checks on real data. One made covered calls beat buy and hold on almost every stock. One let a 5%-out-of-the-money S&P 500 covered call beat the index by 46 points. One made out-of-the-money stock calls so cheap that call buyers profited even after delta hedging (offsetting an option's stock exposure with shares). With this version, delta-hedged buyers of out-of-the-money stock options lose on average, as the evidence says they should, and in our checks the S&P 500 buy-write (a covered call: own the index and write calls on it) finished within about 5 points of the index.

• Sharpe ratio. Mean daily return above the T-bill rate, divided by its standard deviation (the typical size of swings around the average), × √252 to put it in yearly terms.
• Maximum drawdown. The largest fall from a peak in the daily account value.
• IRR. The money-weighted return per year, computed as XIRR (IRR using the exact date of each cash flow). With a fixed stake it equals the compound annual growth rate (the steady yearly rate that turns the start value into the end value).
• Return per unit of drawdown. IRR above T-bills divided by max drawdown (floored at 1%).
• Overall place. Each strategy is ranked on Sharpe, return per unit of drawdown, and IRR, and ordered by the average rank. Equal averages go to the higher Sharpe, then the higher IRR, and only exact ties share a place. Drawdown enters per unit of return, so a tiny position can't win just by being small. A wiped-out account ranks last.
• Also shown. Sortino ratio (like Sharpe, but counting only the downside swings), which is excess return over downside deviation; volatility; and beta (how much the account tends to move for each 1% move in the S&P 500; 1 means 1%, on average) against the S&P 500.

• Buy and hold the stock. Buys 100% once and never trades again.
• Buy and hold the S&P 500. The same, on the S&P 500 ETF (a fund that trades like a stock).
• Covered call. Long 100%, writes 105% calls every month.
• Protective put. Long 100%, buys 95% puts every month.
• 200-day trend. Long 100% when the price is above its 200-day average (the average close of the last 200 trading days), otherwise in T-bills.
• T-bills only. All cash, earning the T-bill (Treasury bill: short-term US government debt, the usual near-riskless return) rate.

They run through the same engine as you, with the same rules and costs.

The link holds your stock, your window and every order you placed, so no server or login is involved. Your friend starts from month 1. When they finish, the engine replays your orders on the same prices and ranks you side by side, with the six benchmarks. They can challenge you back.

A scheduled GitHub Actions job downloads daily closes for every stock and the S&P 500 ETF (adjusted for dividends and splits), the VIX and the 13-week T-bill yield each weekday after the close. It validates every series and commits only real changes, and the site redeploys. Adding a stock is one entry in config/tickers.json. See the Data page for current status.

The engine has unit tests (Black-Scholes against textbook values and put-call parity, margin calls, settlement, the trading rules and every metric), and a separate pandas script recomputes every metric on real data and checks it against the game engine's own numbers.

• Model prices. The smile is a simple shape, and earnings days aren't modeled.
• Simple fills. Fractional shares are allowed, and market impact is ignored beyond the fixed costs.
• Ways to peek. A careful look at the chart's shape or the T-bill rate can hint at the stock or era. The S&P 500's steeper option smile (its out-of-the-money calls are cheaper) sets it apart from the stocks. The browser's developer tools can show the price file.
• Replays. Replay links reproduce a game exactly on the same data.