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The EA deviation setting nobody checks

Expert advisors include a parameter for maximum deviation, or slippage tolerance. It tells the platform how far from the requested price a fill is acceptable.

Set too tight, orders get rejected whenever the market moves, and the EA misses trades. Set too loose, orders fill at poor prices.

The parameter's unit is points, not pips. A deviation of 3 on a five-digit broker is 0.3 pips. A value that seemed generous on a four-digit broker is tiny on a five-digit one.

There is also a gap between how the setting works with different execution modes. Under market execution, the deviation is often ignored altogether. A trader might believe the protection exists when it does not.

Reading the broker's documentation and testing with live orders in a quiet period reveal how the platform behaves.

Is the deviation parameter doing anything on the current account, or is it ignored by the execution mode?

Fast markets produce a specific error that shows up exactly when an order matters most: "off quotes," "price changed," "invalid price," or the platform just sits there thinking.

What's happening behind the screen is usually one of a few things. The server is receiving more orders than it can process in that instant. The price on the trader's screen is already stale compared with what the liquidity provider is quoting. Or the broker's risk system temporarily stops accepting orders at the old levels.

Re-sending the order repeatedly makes it worse, since each attempt can stack in a queue, and one of them might fill ten seconds later at a price nobody wants. Double fills from impatient clicking are a classic.

A calmer approach: after a rejection, check open positions and pending orders before doing anything else. The "failed" order may actually be sitting in the queue, about to execute.

Some platforms offer a deviation or slippage tolerance setting. Setting it too tight causes many of these rejections in volatile moments, while setting it loose invites worse fills.

Behind the scenes, the sequence matters. The platform sends the order, the server is busy, and the response takes several seconds. In that time, the trader sees nothing and clicks again. The server then processes the first order, then the second. Two fills, double the intended size, both at the worst moment.

A few platforms offer a setting to block repeat orders for a short interval. EAs can include a check that waits for confirmation before sending anything new. For manual trading, the discipline is a simple rule: after any error, look at the positions tab before touching the order button again.

Another habit that pays off is keeping a note of which error messages appeared at which times. Patterns tied to specific minutes, specific pairs, or the same server make a strong case when talking to support.

What deviation setting works best on volatile pairs, and has changing it made any difference?

Many EAs and the MT4 order window include a deviation or slippage setting, which tells the broker how far from the requested price an order may be filled. On instant execution accounts, if the price moves beyond that deviation before the order is processed, the broker rejects it with a requote. On market execution accounts, the setting is often ignored, because orders fill at the available price regardless. Many traders never check what their EA uses, and defaults vary widely, from zero to several pips. That can lead to constant requotes in one case or uncontrolled slippage in another.

I found out about this the hard way with an EA that ran on a market execution account. I had a deviation of 10 points set and assumed it was protecting me, and then one evening around a data release the EA filled a buy about 1.8 pips away from the price it requested. The broker's server simply ignored the setting, and nothing in the logs mentioned it.

After that I tested it properly. On a demo account with the same execution mode, I sent a batch of small orders with a deviation of 0 during a quiet hour and then again right after a news release, and compared the requested price with the fill price. In calm conditions the numbers were identical. In the news window two of ten orders were off by more than a pip even with a deviation of zero.

My rule now is to put my own check inside the EA. It compares the fill price with the intended price straight after the order is confirmed and closes the position at once if the difference is above my limit, whatever the broker's setting does.