On this page
What is an Expert Advisor?
An Expert Advisor (EA) is a small program written in MetaQuotes Language (MQL4 for MetaTrader 4, MQL5 for MetaTrader 5) that runs inside the trading platform and places trades autonomously based on coded rules. It is the MetaTrader equivalent of an algorithmic trading bot.
An EA can do anything a manual trader can do: open and close positions, modify stops, place pending orders, send alerts, run on multiple timeframes. The difference is that an EA does it without you. Once attached to a chart it will keep trading the rules you (or whoever wrote it) coded, 24/5 while the market is open.
How an EA actually works inside MetaTrader
When you attach an EA to a chart, MetaTrader runs the EA’s code on every new price tick. The EA reads the price, checks its conditions, and either does nothing or sends an order to the broker. That cycle repeats every tick — typically 5–50 times per minute on a busy forex pair.
MQL4 EAs are single-threaded: every chart with an EA attached uses one CPU core, and a slow EA can lag the chart’s tick processing. MQL5 EAs are multi-threaded and use the platform’s resources more efficiently. This is why MT5 EAs scale better to high-frequency strategies than MT4 EAs.
Why an EA needs a VPS
Because an EA only runs when MetaTrader is open and connected to the broker. If your home computer sleeps, restarts for Windows updates, loses Wi-Fi, or otherwise interrupts MetaTrader, the EA stops trading. Worse — if it stops while you have an open position, the position remains open at the broker, but the EA cannot manage it (no trailing stops, no break-even moves, no exits) until you reconnect.
A VPS keeps MetaTrader running continuously. The EA fires on every tick, manages every open position, and never misses an opportunity because the underlying computer was off. For any serious EA user, a VPS is the default infrastructure.
EAs for prop firm accounts
Most prop firms allow EAs but impose two restrictions. First, no copy-trading between accounts unless explicitly authorised — meaning you cannot run an EA that mirrors trades from one prop account to another. Second, no strategies that abuse internal pricing or execution differences ("HFT abuse", "latency arbitrage") — this rule targets EAs that look for delays in the broker’s pricing pipeline.
The common compliant EA categories are: trend-following bots, grid bots within reasonable risk limits, news bots that trade specific scheduled releases, and tape-reading scalpers. The non-compliant category is anything labelled "HFT" or "latency arbitrage" that exploits the broker’s feed rather than the market.
How to choose an EA
Two checks. First, the EA’s backtest must use real tick data (not 1-minute bars), at least 5 years of history, multiple pairs, and Strategy Tester’s "Every tick based on real ticks" mode. Modeling quality below 90% means the backtest is unreliable.
Second, the EA must be runnable in your live conditions. Live forward-test for at least 30 days on a demo account hosted on the same VPS you intend to use in production. If the forward-test results match the backtest, the EA is probably safe to live; if they diverge, the backtest was over-fit.
References & sources
- [1]Expert Advisors are MetaQuotes Language programs that execute inside MetaTrader and run on every price tick. MetaTrader 4 — Expert Advisors
- [2]MQL5 supports multi-threaded execution and 64-bit memory addressing. MQL5 documentation