A-book vs B-book broker: who is on the other side of your EA's trades

The one question most traders never ask

When your Expert Advisor sends a buy order, somebody sells to it. The A-book vs B-book broker distinction is about who that somebody is. An A-book forex broker passes your order on to a liquidity provider, a bank or non-bank market maker, and earns a mark-up on the spread or a commission. A B-book broker takes the other side itself: your loss is its profit, and your profit is its loss.

Neither label is printed on the account page, and most brokers run both at once. But the model changes the incentives behind every fill your EA gets, and fills are where an algo's real-world result is made or lost.

How the A-book works

In an A-book arrangement the broker is a middleman. Your order goes out to one or more liquidity providers, often through an aggregator that shows the best bid and offer from several of them. The broker's income is the difference between the price it got from the provider and the price it gave you, or a fixed commission such as $3 per side per standard lot on a raw-spread account.

The broker does not care whether you win or lose. It cares about volume. That is a healthy alignment for an EA that trades a lot, but it comes with a cost: the liquidity provider is a real counterparty with real inventory, so fills slip when the market is thin, and the spread you see is whatever the provider is quoting at that second. A news spike on an A-book account can take a 12-point EURUSD spread to 60 or more for a few seconds.

How the B-book works

A B-book broker internalises your trade. Nothing goes to the market; the broker simply records that you are long 1 lot of EURUSD and keeps the risk on its own book. Because most retail accounts lose over time, the B-book is very profitable, and the broker can afford things an A-book cannot: tighter or fixed spreads, no commission, fills at the requested price with no slippage, and generous bonuses.

The honest nuance is this. A B-book is not automatically against you. Most large brokers B-book small accounts because it is cheaper than hedging a 0.01-lot trade externally, and a well-run B-book gives you excellent execution because it is not waiting on anyone. The conflict of interest only bites if the broker starts acting on it: widening spreads at the moment your stop is near, adding a few points of slippage on the take-profit but not the stop-loss, rejecting orders when your EA is on a winning run, or delaying execution during fast markets.

Hybrid models: why you are probably both

Almost every retail forex broker runs a hybrid. New accounts usually start on the B-book. A risk engine watches each client; when one becomes consistently profitable, trades large, or trades in a style the broker does not want to be exposed to, that client is switched to the A-book and hedged out. Some brokers do this per trade rather than per client, sending only the part of the flow they do not want to hold.

For anyone running a forex robot this has a practical consequence. The execution you see in your first month on a small balance is not necessarily the execution you get after you scale up and start winning. Your fills can change the day you are moved across, and nobody will tell you it happened.

What it means for an Expert Advisor

Different strategies are exposed to the two models in different ways.

• Scalpers and high-frequency trading bots live on spread and slippage. A B-book with fixed spreads and no slippage can look ideal, right up until the broker decides the strategy is toxic flow and moves it, or adds a few points of friction. On an A-book, expect variable spreads but predictable behaviour.
• Swing and trend EAs that hold for days care far more about swap than about a point of slippage. The model matters less; the financing schedule matters more. Run the numbers through the swap cost calculator.
• News and breakout EAs are the worst case on both. A-book providers pull liquidity at the release and fills slip hard. B-book brokers often widen spreads or freeze execution for the same seconds. A strategy that only makes money on news in a backtest usually does not survive either model.
• Grid and martingale EAs are welcomed on B-books because the broker expects the eventual blow-up to land on its side of the ledger. Being welcomed is not the same as being safe.

Reading the model off a verified track record

You cannot see a broker's book directly, but its behaviour leaves marks in the data, and a live-synced account on ConfirmedTrades records them. The Execution quality section shows the spread profile per symbol in points, sampled from the live terminal, plus open and close slippage per trade in signed points. It is free to check execution quality on any verified account page.

A spread profile that is a flat line all day, including at the New York close and the Asian open, is typical of a fixed-spread B-book. A profile that is tight in London and New York and triples overnight is a variable-spread feed from a liquidity provider. Slippage that is always zero is either a demo server or a B-book filling at the requested price. Slippage that is consistently negative on take-profit closes but neutral on stop-loss closes is the pattern to walk away from, whichever book it came from. Broker cost audit walks through each of these numbers.

The account header tells you the broker and server name, whether the account is Demo, Real or Contest, the base currency and the leverage. Demo servers on either model fill better than real ones, so the account type is the first thing to check.

Questions worth asking a broker

Most brokers will not tell you which book you are on, but the answers to a few neutral questions narrow it down.

• Is the spread fixed or variable, and is there a commission? Fixed with no commission leans B-book.
• What is the maximum slippage policy, and is it symmetric (positive slippage passed on as well as negative)? Asymmetric policies are a warning.
• Are scalping, news trading and EAs allowed without restriction? A list of exceptions is a list of strategies the broker does not want to hold.
• Does the broker publish execution statistics? A real A-book with real providers usually can.

Related reading

The book model sits underneath the account types most brokers advertise. ECN vs STP vs market maker explains how those labels map onto it, and choosing a broker for an Expert Advisor turns all of this into a checklist. To see how different brokers fill the same strategy, browse verified accounts on Explore and compare their Execution sections.