A-Book vs B-Book vs Hybrid Execution: How FX Order Flow Is Handled

Definitions

A-book

In an A-book model, the firm passes client orders to one or more external liquidity providers. When a client buys EUR/USD, the firm places a matching buy with a provider. The firm's revenue typically comes from a commission, a markup on the spread, or both. Because the client trade is offset externally, the firm does not hold the market risk of that position, although it still carries credit, operational and provider risk. Labels such as STP (straight-through processing) and ECN-style execution are usually variations of A-book handling.

B-book

In a B-book model, the firm acts as the counterparty to the client trade and keeps the position on its own book. The firm's net result on that flow is broadly the opposite of its clients' net result, plus spreads and fees. B-book firms manage their exposure in aggregate: many client positions offset each other, and the firm may hedge any remaining net exposure externally once it passes internal limits. This is often called market making or internalisation.

Hybrid

A hybrid model combines the two. The firm routes some flow externally and internalises the rest. The split is usually rule-based: by account type, symbol, order size, client trading profile or overall exposure. Some firms also hedge part of a position externally and keep the rest. The term "C-book" is sometimes used for these variations, but its meaning is not consistent across the industry.

How each model handles order flow and risk

The models differ most clearly in what happens after an order reaches the firm's trading server:

• A-book: the order is sent through a bridge or FIX connection to a provider. The client's fill depends on the provider's execution report. Market risk sits with the provider; the firm's main concerns are execution quality, provider relationships and credit lines.
• B-book: the order is filled internally, usually against the firm's own price feed. Execution can be fast and consistent because there is no external round trip, but the firm now holds the position and must manage the risk across its whole client book.
• Hybrid: a routing layer decides per order, or per account, which path to use. The routing rules are the critical part. They determine both the firm's risk and the execution experience of each client.

In technical terms, all three models can run on the same platform. In MetaTrader 5, for example, the difference often comes down to routing configuration on the server and in the liquidity bridge, not to a different trading platform.

Conflicts of interest

The central issue with B-book execution is incentive. When a firm is the counterparty, a client's loss is, all else equal, the firm's gain. That does not mean B-book firms act improperly; many operate under strict rules and controls. But it creates a conflict that has to be managed, for example around:

• Pricing: whether the internal price feed tracks the wider market fairly, including during volatile periods.
• Slippage: whether price changes between request and fill are applied symmetrically, so clients receive favourable moves as well as unfavourable ones.
• Requotes and rejections: whether orders are rejected or delayed more often when they would be profitable for the client.
• Routing decisions: in hybrid models, whether moving a client from one book to another is based on clear, consistent rules.

A-book execution reduces this particular conflict, because the firm earns from volume rather than from client losses. It does not remove every conflict: firms still choose providers, set markups and decide how to handle rejections. Regulators in many jurisdictions address these issues through best-execution and conflicts-of-interest rules; in the EU, for instance, MiFID II requires investment firms to take sufficient steps to obtain the best possible result for clients and to maintain an order execution policy.

How to tell which model is in use

From the outside, there is no single test that proves which model a firm uses. A combination of documents and data gives a reasonable picture.

Read the documents

• Order execution policy: look for wording such as "we act as principal", "counterparty to your trades", "market maker" or "we may hedge our exposure". These indicate some degree of internalisation.
• Client agreement and conflicts-of-interest policy: these often describe whether and how the firm may profit from client trading.
• Note that many A-book firms are also legally the counterparty to the client, for example acting on a matched-principal basis. The legal structure alone does not show where the market risk ends up.

Look at execution data

• Slippage distribution: with external execution, slippage tends to occur in both directions. Consistently one-sided slippage deserves a question. See slippage measurement and control.
• Timestamps: compare request, acknowledgement and fill times. External routing usually adds a measurable round trip. See latency in execution.
• Rejections and requotes: note how often they occur and whether they cluster around fast markets or profitable entries.

Ask direct questions

Professional clients and firms evaluating a technology partner can ask directly: which liquidity providers are used, whether routing differs by account or client profile, whether execution reports are available per order, and whether routing decisions are logged and auditable.

Where Keystone FX fits

Keystone FX provides execution and connectivity technology for professional FX participants, including MT5 connectivity, FIX API access, liquidity routing and latency and slippage measurement. As described on our liquidity page, Keystone FX infrastructure supports aggregation of external venue quotes and does not internalise flow. For more detail, see the execution infrastructure overview or contact our team.

Frequently asked questions