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The deviation setting on MT4 is not a slippage cap you can trust

The deviation field, or max slippage in points, looks like a safety net. Set it to 2 and surely no fill will land worse than 2 points away. That assumption fails more often than expected.

How the setting behaves depends on the broker server. With market execution, the field is often ignored entirely, because the order goes to market regardless. With instant execution, it controls whether the platform will accept a new price or send a requote. It never guarantees a better fill, only a rejection when the price drifts too far.

That distinction matters. A tight deviation on an instant account converts slippage into missed trades, and missed trades during momentum bursts are usually the best ones.

A smarter approach is treating deviation as a trade-off knob. Widen it slightly during liquid sessions, and narrow it only when spreads have already widened because a wide spread plus wide tolerance is how bad fills happen.

The only real protection is testing actual behavior on the specific account.

Has anyone seen the deviation field do anything at all on a market execution account?

On a market execution account at my broker, I tested it with deviation set to 0 and to 50 points on the same EA over a few hundred trades each. No difference in slippage distribution that I could see. Fills slipped by up to 1.5 pips on fast moves regardless of the setting. So on that account, the field does nothing. On their instant execution account, setting deviation to 0 caused frequent requotes during the London open, roughly one in eight orders. Know which account type you have before relying on it.

I tested the deviation field on two accounts from the same broker, one with instant execution and one with market execution, using about 150 orders on each over three weeks. On the market execution account, setting 0 or 30 points made no visible difference to the slippage, and the worst fill was 1.8 pips off during a quick London open move. The field simply did not matter there.

On the instant account, deviation 0 caused a requote on about one order in seven during the first half hour of London. I raised it to 3 points for EURUSD and the requotes mostly disappeared, with an average slippage of about 0.2 pips, which I could live with. Missing the trade cost me more than that.

I now check the execution type in the account details before I place any trust in the field. If it is market execution, I ignore it and protect myself with position size and by avoiding the moments when slippage is likely.