Guide · Updated 2 November 2026

How Brokers Detect Latency Arbitrage and How to Trade Within the Rules

By Sergey Berezhnov, Founder, CEO and Lead Developer of Westernpips · Published 2 November 2026 · Updated 2 November 2026 · About 8 minutes to read

Brokers detect latency arbitrage mainly by looking at what happens to the price after a client’s trade: if a client keeps buying just before the quote rises and selling just before it falls, mostly on very short trades, the pattern stands out in the broker’s reports. Brokers then respond with last look, wider spreads, minimum holding times, moving the account to another execution model, or, under their client agreement, closing it. The way to avoid these problems is to choose a broker whose terms allow your strategy and to trade within them.

Key takeaways

Markouts are the main signal: brokers measure how far the price moves in the client’s favour seconds after each fill.
Short trades with a high win rate and trades that cluster at sharp price moves are the other typical signs.
Responses range from last look to account closure, and client agreements often allow profits from “abusive” trading to be cancelled.
No software makes a prohibited strategy permitted. The safe route is a broker that allows arbitrage, confirmed in writing.

Why Brokers Care About Latency Arbitrage

Brokers care because latency arbitrage profits come from trading at prices that are already out of date, and someone pays for that: the broker itself if it takes the other side of client trades, or its liquidity provider if it passes them on. Either way the flow is costly to fill, which is why it is often called toxic flow.

A broker that keeps client trades on its own book (often called B-book) loses directly when a client trades a stale quote. A broker that passes trades to liquidity providers (A-book or STP) may see its providers reject orders, widen prices or stop quoting it. Research on exchanges shows the same economics: Budish, Cramton and Shim (2015) describe stale-quote sniping as a cost borne by whoever is quoting.

What Brokers Look at to Detect Arbitrage

Brokers look at markouts (how the price moves after each fill), holding times, win rate on short trades, whether trades cluster at sharp price moves, and whether several accounts trade the same way at the same moments. Most of this comes from standard trade reports, not from inspecting the trader’s software.

Signal	What the broker sees	Why it points to arbitrage
Markouts	Price moves in the client’s favour within milliseconds to seconds after fills, trade after trade	The client is trading before the quote updates, not predicting the market
Holding time	Many trades closed within seconds	One-leg latency arbitrage closes when the quote catches up
Win rate on short trades	Very high share of small winners	Ordinary short-term trading does not win that consistently
Timing	Entries cluster at sharp moves in a fast market and at news releases	That is when broker quotes lag most
Linked accounts	Opposite or identical positions on several accounts at the same moments	Points to lock or hedge arbitrage across accounts

How Brokers Respond: Last Look, Spreads, Hold Times and Closure

Brokers respond in steps: last look or price tolerance checks that reject orders at stale prices, slower or delayed execution, wider spreads on the account, minimum holding times, moving the account to a different execution model, and, under the client agreement, cancelling trades or closing the account.

Last look: a short hold during which the price and the order are checked and the order may be rejected. The FX Global Code allows it under conditions, including transparency to clients (see the GFXC guidance); Oomen (2017) analyses its costs for clients.
Price tolerance and slippage: orders fill at the current price rather than the stale one, which removes the edge.
Spread and execution changes: wider spreads or added delay on a specific account or group.
Minimum holding time: trades must stay open for a set period; this is why lock and hedge strategies exist.
Trade cancellation and closure: some client agreements allow profits from trades classified as abusive to be cancelled.

What Broker Client Agreements Say About Arbitrage

Client agreements usually address arbitrage in a clause on abusive or prohibited trading. Typical wording covers exploiting price delays or errors, trading on stale quotes, using two accounts to hedge against each other, and very short trades, and it gives the broker the right to cancel trades or close the account.

The wording varies a great deal. Some brokers prohibit latency arbitrage outright; some allow algorithmic trading but set a minimum holding time; some ECN or STP accounts accept fast trading because every trade goes to a liquidity provider. The only reliable answer is the current agreement plus a written reply from the broker to a specific question.

How to Trade Arbitrage Within Broker Rules

To trade within the rules, read the client agreement before funding, ask the broker in writing whether your specific strategy is allowed, match the strategy to the terms, for example a holding time that meets the broker’s minimum, and keep records of fills and correspondence. If the answer is no, choose another broker rather than adjusting settings.

Pick the account type deliberately: ECN or STP accounts at a regulated broker are more likely to accept fast trading than standard accounts.
Respect holding times: if a minimum duration is required, use a strategy built for it, such as a two-leg lock, where the agreement allows two accounts. See two-leg arbitrage.
Start at the minimum lot and compare real fills with the recorded gaps before adding size.
Keep records: the broker’s written answer, statements and execution logs.

The legal side, including regulators and prop-firm rules, is covered in the risks section of our latency arbitrage guide.

Questions to Ask a Broker Before Trading Arbitrage

Ask the broker five questions in writing: whether latency or other arbitrage is allowed on your account type, whether there is a minimum holding time, whether orders are subject to last look and how long it is, whether two accounts may hold opposite positions, and under what conditions trades can be cancelled.

Is latency arbitrage, or algorithmic trading using an external price feed, allowed on this account type?
Is there a minimum holding time for trades?
Are orders subject to last look? What is the maximum hold and is it symmetric?
May I hold opposite positions on two accounts, at your firm or another?
Under which conditions can you cancel trades or adjust prices after execution?

Glossary

Markout: The price move after a fill, measured at set intervals. Consistently favourable markouts suggest trading on stale quotes.
Toxic flow: Client orders that are costly to fill because they trade on prices that are about to change.
B-book: A broker execution model in which the broker takes the other side of client trades itself.
A-book (STP): A broker execution model in which client trades are passed to liquidity providers.
Last look: A short hold in which a liquidity provider checks price and validity and may reject the order.

Broker Arbitrage Restrictions FAQ

How do brokers detect latency arbitrage?

Mainly through markouts: they measure how the price moves after each fill. A client whose trades are consistently followed by price moves in their favour, mostly on very short trades, stands out. Holding times, win rate, timing and linked accounts are the other signals.

Can a broker cancel arbitrage profits?

Some client agreements allow the broker to cancel trades it classifies as abusive, including trades on stale prices. Read the agreement before funding and ask the broker in writing whether your strategy is allowed.

Is latency arbitrage illegal?

It is generally not a criminal offence, but it can breach a broker's terms, and the broker can then act under the contract. The legal side is covered in our latency arbitrage guide.

Can software make arbitrage undetectable?

No. Brokers see the results of trades in their own reports, whatever software sends them. Software can match a strategy to a broker's rules, such as a minimum holding time, but it cannot make a prohibited strategy permitted.

Which brokers allow arbitrage?

Policies differ and change, so we do not keep a list. Ask the broker in writing about your account type, minimum holding time, last look and opposite positions on two accounts.

Disclosure and risk

This page is published by Westernpips, which develops and sells latency arbitrage software, including Westernpips Private 7. Leveraged trading can result in the loss of your capital, and brokers can restrict accounts under their client agreements. Worked examples use hypothetical prices. This is general information, not investment advice.

Sources

Global Foreign Exchange Committee (2021). Guidance paper on last look, published alongside the FX Global Code (Principle 17). www.globalfxc.org/docs/gfxc_report_last_look.pdf
Oomen, R. (2017). Last look. Quantitative Finance, 17(7), 1057–1070. econpapers.repec.org/RePEc:taf:quantf:v:17:y:2017:i:7:p:1057-1070
Budish, E., Cramton, P. and Shim, J. (2015). The High-Frequency Trading Arms Race: Frequent Batch Auctions as a Market Design Response. Quarterly Journal of Economics, 130(4), 1547–1621. doi.org/10.1093/qje/qjv027
Westernpips (2026). Latency Arbitrage: How It Works and How to Test It. Guide with costed examples and a broker test. westernpips.com/latency-arbitrage.html
Financial Conduct Authority (2019). FCA confirms permanent restrictions on the sale of CFDs and CFD-like options to retail consumers (rules apply from 1 August 2019): leverage limits of 30:1 to 2:1, 50% margin close-out, negative balance protection and a standardised risk warning. www.fca.org.uk/news/press-releases/fca-confirms-permanent-restrictions-sale-cfds-and-cfd-options-retail-consumers
European Securities and Markets Authority (2018). ESMA adopts final product intervention measures on CFDs and binary options: leverage limits from 30:1 (major currency pairs) to 2:1, 50% margin close-out, negative balance protection and a standardised risk warning; CFD measures applied from 1 August 2018. www.esma.europa.eu/press-news/esma-news/esma-adopts-final-product-intervention-measures-cfds-and-binary-options

About the author: Sergey Berezhnov, Founder, CEO and Lead Developer of Westernpips

Trader and developer of software for latency arbitrage and HFT trading with more than 19 years of experience. He founded Westernpips in 2007 and leads the development of Westernpips Private 7, its FIX and API connectors and the tools used to record and analyse quotes and fills.

Risk warning: leveraged forex and CFD trading can result in the loss of your capital, and retail providers must disclose the share of their accounts that lose money (FCA, ESMA). Latency arbitrage depends on execution you cannot guarantee: orders can be late, rejected or restricted by the broker. Worked examples use hypothetical prices; the gold figures are displayed quotes from three single sessions, not executed trades; research figures are quoted from the named studies. Westernpips publishes this page and sells the software described on it. This is not investment or legal advice, and past or simulated results do not guarantee future results.