What is an Expert Advisor?

What is an Expert Advisor (EA)?

An Expert Advisor (EA) is a program written in MQL5 that runs inside MetaTrader 5, the industry-standard retail trading platform. The EA reads price data tick-by-tick, applies whatever logic the developer coded (indicators, pattern detection, risk rules, news filters) and, when conditions align, fires actual orders through the broker's MT5 terminal.

In plain English: an EA is a robot that trades your account 24/5 without you sitting in front of the screen. It doesn't get tired, doesn't fear a drawdown, and doesn't revenge-trade after a loss. It also doesn't have judgment — if your logic is bad, it'll lose money faster and more consistently than you ever could manually.

How EAs actually work

Every EA has four layers. Well-built ones keep them clean and separate; amateur ones smear them together into one giant OnTick() function and break quietly when the market regime shifts.

1. Signal layer — what tells the EA a trade exists. Supertrend cross, Kalman-filter deviation, London-open breakout, ML classifier output.
2. Filter layer — what blocks trades. Session filter, news blackout, spread gate, correlation cap.
3. Risk layer — how big. Fixed-fractional sizing (0.25-1% of equity), ATR-based stop distance, daily loss cap, max drawdown kill-switch.
4. Execution layer — order placement, slippage handling, partial closes, trailing stops.

An EA that looks impressive in a backtest but fails live almost always fails at the risk layer — ignoring spread widening, missing news blackouts, or hidden martingale patterns that blow up once the wrong sequence of losses hits.

MT4 vs MT5 — why new EAs are MT5-native

MetaTrader 5 has been out since 2010 but only hit majority share around 2022. It's objectively better than MT4 for algo trading:

• Multi-currency Strategy Tester — MT4 could only backtest one symbol at a time. MT5 handles portfolios.
• More order types — MT5 supports stop-limit orders and proper partial position close.
• Better execution model — netting (one position per symbol) simplifies accounting; hedging is still available for brokers that support it.
• True multi-threaded tester — optimization runs 10-20x faster.
• Modern language — MQL5 has proper OOP, structures, namespaces. MQL4 is a C-with-arrays relic.

If you're still on MT4, migrate. Every serious EA developer shipped MT5 as default by 2023.

Are EAs profitable?

Some are. Most aren't. The distribution is long-tail — a few EAs (including ours) have verified multi-year live track records; the vast majority of MQL5 Market products are either newly launched (no live data) or silently discontinued after 6-12 months.

If you evaluate EAs like you'd evaluate hedge-fund strategies — real tick-data backtest, out-of-sample test, walk-forward, verified live signal — 5-10% of what's sold to retail meets the bar. The rest is marketing.

Read the detailed answer in Are forex EAs profitable?

How to pick one

• Verified live track record ≥ 6 months — ideally on Myfxbook or a MQL5.com verified signal.
• Real tick backtests — 99% modeling quality, realistic spread, slippage, and commission.
• No martingale, no grid — ever. These are marketing devices, not risk strategies.
• Transparent stats — published CAGR, max DD, win rate, profit factor, number of trades.
• Active developer — updates shipping within 30-60 days when brokers update symbols.

Or take the shortcut and run our EA picker quiz.

Frequently asked questions