Expert Advisor

In plain English

A program that trades for you inside MetaTrader. It has no judgement and no memory beyond what you gave it: it wakes when the platform calls it, looks at the few things it was written to look at, and acts.

Why it matters

Backtests, drawdown figures, magic numbers, VPS uptime — all of it exists because a program, not a person, places the orders. Knowing what the platform hands an EA, and what it withholds, is the difference between reading a result correctly and being surprised by it.

• ✓It executes without hesitation, which is its whole value and its whole risk. The rule set that never skips a valid signal also never notices that today is different.
• ✓It sees a narrow slice: prices for its symbol, indicator values it requests, the account's own positions and balance. News, other brokers' quotes and your other accounts sit outside that window unless the code fetches them.
• ✓It is distributed compiled. A .ex5 file runs but cannot be read, so a buyer's confidence has to come from published test evidence rather than the source.
• ✓It runs only while the terminal runs. An EA on a sleeping laptop is an EA that is not trading — the entire argument for a VPS.

In MetaTrader 5

Where it appears in MT5

• Navigator → Expert Advisors — the compiled EAs the terminal can see
• File → Open Data Folder → MQL5/Experts — where the .ex5 file has to sit for it to appear
• Toolbar → Algo Trading — the master switch; an EA with a sad face on the chart is one this button is blocking
• Chart → EA properties (F7) → Inputs / Common — parameters and per-EA permissions
• View → Strategy Tester (Ctrl+R) — the same EA run over historical data

How EAs use it

• The terminal calls OnInit() once at attach, OnTick() on every incoming quote for the chart's symbol, and OnDeinit() at removal.
• Work usually happens on closed bars, not every tick: the EA compares the current bar's open time with the one it stored and returns immediately when nothing new has formed.
• Orders go out as a filled-in MqlTradeRequest, or through the CTrade wrapper that fills most of it in for you.
• Each instance tags its orders with a magic number so it manages only its own positions — without it, two EAs fight over the same tickets.

Typical settings

Common operational problems

• The terminal is closed, asleep or restarting for updates — the EA is simply not running, and no error is recorded.
• A symbol-name mismatch between broker and settings (EURUSD against EURUSD.m) leaves the EA attached and silent.
• Insufficient free margin: OrderSend fails per attempt and the EA keeps trying, so the log fills while the account does nothing.
• A trade context is busy when several EAs send at once — intermittent, hard-to-reproduce failures.
• The EA was tested on a broker with a different execution model or contract size, and behaves differently live with no code change.

Related MT5 functions

OnInit() / OnDeinit()

Called once at attach and once at removal — where inputs are validated and handles created.

OnTick()

Called on every incoming quote for the chart symbol; the main entry point.

OnTimer()

Called on a fixed interval set by EventSetTimer(), for work that should not depend on ticks arriving.

OrderSend(request, result)

The single call that sends every trade operation; the request struct decides what kind.

CTrade (Trade/Trade.mqh)

The standard-library wrapper most EAs use instead of filling MqlTradeRequest by hand.

Example

Almost every EA, reduced to the order the platform calls it in. None of this is strategy — it is the frame a strategy sits inside.

Removing an EA from a chart does not close what it opened. The positions remain, with nothing managing them.

How it is used

Judging an EA you did not write is a matter of evidence: the compiled file tells you nothing. Below is what to ask for.

• Ask what data the backtest used and how it was modelled. Real ticks and a modelling quality figure mean something; an unlabelled equity curve does not.
• Ask for the trade count and the window. The median EA published here closed 362 trades over several years; forty trades over four months is an anecdote.
• Check whether the strategy holds losers open. Averaging down and grid recovery produce beautiful curves until the basket that does not recover, and no ratio from closed trades warns you.
• Demo it on your own broker before funding it. Execution model, spread, contract size and symbol naming all differ, and all change results.
• Give every instance its own magic number, and record which is which before more than two run.

Every EA listed here publishes its tested window, data source, modelling quality and full trade ledger, so this evidence can be checked rather than requested.

Common mistakes

Frequently asked questions

What is an Expert Advisor in MetaTrader 5?

A program written in MQL5 and compiled to a .ex5 file that runs inside the terminal and trades by its rules. The platform calls it on every incoming quote for the chart it is attached to, and it opens, modifies and closes orders with nobody at the keyboard.

Do I need to leave my computer on for an EA to work?

Yes — an EA runs inside the terminal, so it trades only while the terminal runs and is connected. That is why unattended EAs usually live on a VPS rather than a computer that sleeps or restarts.

Can I read the strategy inside an EA I bought?

Not from the .ex5 file. Compiled EAs run without exposing their logic, which is why sellers ship them that way. The strategy has to be described honestly in the listing and backed by published test evidence.

Can I run more than one EA on the same account?

Yes, and it is common. Each instance needs its own magic number so it manages only the positions it opened, and the account needs free margin for all of them at once — the strategies do not know about each other.

Related terms

• Magic number An integer an Expert Advisor stamps on every order it sends, so it can recognise its own positions and leave everything else on the account alone.
• Virtual Private Server A rented remote computer that keeps MetaTrader open around the clock — so an EA keeps trading when your own machine sleeps or drops its connection, and sits nearer the broker's servers.
• Backtest A simulation of an EA's trading rules applied to historical price data to estimate how the strategy would have performed in the past.
• Lot size The unit of trade volume in MT5. One standard lot is 100,000 units of the base currency; a micro lot is 1,000 units, entered as 0.01.
• Slippage The difference between the price an order asked for and the price it was filled at. It can fall either way, but for a strategy that trades on speed it is a cost that backtests rarely model.

Related resources

• How to Build a MACD EA in MT5 Without Coding (and Why Its Best Result Was the Most Dangerous One)Guides
• How to Build an RSI EA in MT5 Without Coding (and What the Shipped Template Actually Scores)Guides
• How to Create an MT5 EA Without Coding (and What the Templates Actually Score)Guides