Partial fills on market orders: when they break the trade idea

[FXS London Scalper]

On spot FX majors with normal size, you rarely see partial fills on market orders. But on some account types, larger size, or less liquid symbols, they do happen, and for a scalp they can ruin the logic of the trade.

Situations I've seen:

• Market order for 3 lots on XAUUSD during news: 1.2 lots filled at the requested price, 1.8 at a worse price, displayed as two separate deals.
• Exotic cross at Asia time: part filled, the rest rejected entirely, so the position was smaller than planned.
• Closing a position: partial close at one price, the rest a second later at a worse one.

Why it matters for a scalp:

1. Your average entry is worse, so the planned 1.5R might be 1.1R now.
2. If only part filled, your risk is lower than planned, but so is the reward. Not terrible, but your stats get messy.
3. On exit, partial fills mean you're still exposed in a fast market.

What I do: check which filling mode the symbol uses (FOK, IOC or Return), keep size below the level where the broker starts splitting, and review the deals tab rather than the positions tab after each trade.

Has anyone seen partial fills on majors at normal size? I'm curious whether some brokers are worse than others.