A-Book vs B-Book Execution

How Prop Firms Execute Trades and Why It Matters

The distinction between A-book and B-book execution is the most misunderstood operational element in prop trading, and it has significant implications for both the firm's financial health and the trader's experience.

Almost all retail prop firms utilise a B-book model during the evaluation and early funded stages. Instead of passing trader orders to external markets or liquidity providers, the firm internalises these trades. The firm effectively becomes the counterparty to the trader's position. When a trader on a B-booked funded account places a buy order, the firm takes the opposite side internally. No trade goes to the live market. If the trader loses, the firm retains the capital instead of losing it to the market. If the trader wins, the firm must pay profits from its own funds, typically sourced from challenge fees paid by other traders.

The B-book model is financially efficient when the large majority of traders lose, which aligns with the observable pass rate data. The firm's risk materialises when too many traders win at the same time. B-book firms need continuous revenue from new traders to cover payouts. If payouts exceed incoming revenue from challenges, the firm risks operational collapse, because profits are paid from internal funds rather than market earnings.

A-book execution works differently. Instead of taking on all the trading risk internally, the firm hedges trades by passing them directly to the broader market through liquidity providers. When a trader places a successful trade, the firm sends the trade to a liquidity provider such as a bank or financial institution, which takes the opposite side. Some firms also receive liquidity rebates or earn a small margin from spreads on these trades. A-book execution is more expensive to operate. It requires proper brokerage infrastructure, relationships with institutional liquidity providers and significantly more regulatory compliance overhead. The trade-off is that the firm's risk is capped and does not accumulate with trader profitability.

In practice, the vast majority of prop firm income comes from challenge fees and account resets during phases when most traders lose. For profitable traders, firms transition them from B-book to A-book, using internal client matching and live market hedging to manage risk. The operational logic is clear: B-book during evaluation and early funded stages where failure rates are high, then transition consistent performers to A-book execution to cap the firm's growing exposure. This hybrid model is what most sustainable retail prop firms operate, even if they do not advertise it openly.