Paper trading: the bridge to live

Paper trading (also called demo or simulated trading) runs your strategy against live market data but with fake money. It is the indispensable bridge between a backtest and risking real capital.

Why it catches what backtests cannot

A backtest replays clean, finished history. Paper trading runs in real time and exposes problems a historical replay never will:

• Real latency — the gap between signal and fill, which a backtest often assumes away.
• Real spreads and liquidity — including how they widen around news, which historical bars hide.
• Execution bugs — your order logic, reconnection handling and error paths get exercised against a live feed.
• Data feed issues — a backtest uses one tidy file; live data arrives messy, late, or with gaps.

If a strategy that looked great in backtest falls apart in paper trading, you just saved real money — that divergence is information, not bad luck.

How to do it honestly

• Run it for a meaningful period — long enough to see varied conditions, not one good week.
• Match your intended live size and costs as closely as the demo allows.
• Compare paper results to the backtest over the same dates. Large gaps point to look-ahead bias, optimistic fills, or unmodelled costs in the backtest.
• Resist the urge to keep tweaking. Changing the strategy mid-test resets the clock and invites overfitting to the recent live data.

The limit to keep in mind

Even paper trading is not the real thing. Fills can be more favourable than a live account would get, and — crucially — there is no emotional pressure when no real money is at stake. That psychological gap is real and is the next, hardest step.