MT5 Order Filling Modes Explained: FOK, IOC and Return, and How to Fix an Invalid Fill Error

What a filling mode is

When an order reaches the broker, the full volume may not be available at the price you asked for. The filling mode is an instruction that tells the server what to do about the part that cannot be filled. MetaTrader 5 defines the options in the order request, and the MQL5 documentation lists the constants and the return codes on the page about trade server return codes.

For a small retail order, the difference rarely matters in practice, because the full volume is almost always available. It matters in a fast market or with a large lot, when the best price can supply only part of the volume. In that case FOK rejects the order, IOC gives you a partial fill and Return leaves an order resting at the broker.

Execution type and why it matters

The filling mode works together with the symbol's execution type: market, instant, request or exchange. In market execution, the broker fills at the market price when the order arrives, and the filling mode controls partial fills. In instant execution, you send a price and a deviation, and the broker fills if the price is within it. You can read the execution type and the allowed filling modes in the symbol specification: right-click the symbol in Market Watch and choose Specification.

The invalid filling type error

If the EA asks for a mode that the symbol does not support, the server refuses the order and the terminal reports return code 10030, an unsupported filling type, with a message such as invalid fill. Because the symbol list and settings differ by broker and account type, an EA that works on one account can fail on another.

How to fix it

1. 1

   Step 1: Check the EA's inputs

   Many EAs have a filling mode input, sometimes called filling type, order filling or type filling. Try the other values: FOK, IOC and Return.

2. 2

   Step 2: Read the symbol specification

   In Market Watch, right-click the symbol, choose Specification, and read the execution mode and the filling modes allowed. Choose one from that list.

3. 3

   Step 3: Try an auto setting

   Some EAs offer an automatic option that picks the mode from the symbol. Use it if the manual says it is supported.

4. 4

   Step 4: Check the symbol

   A suffixed symbol such as XAUUSD.m can have a different execution mode from the plain one. Use the symbol your EA is meant for.

5. 5

   Step 5: Check the account type

   Standard, raw-spread and cent accounts at the same broker can have different modes. Ask support if the specification is unclear. See account types for EAs.

6. 6

   Step 6: Contact the seller with the journal

   If no input exists, send the seller the error text and the broker name. They may offer a build for your broker.

Which mode should you choose?

Be careful with IOC and Return in a strategy that depends on a full-size fill. A partial fill leaves you with a smaller position than the EA expected, and its stop and take profit logic may assume the full volume. Test on demo, then check the filled volume in the History tab.

Filling modes, slippage and news

In a fast market the price can move between sending the order and its arrival. The deviation setting tells the broker how far from the requested price you will accept a fill. A tight deviation with FOK can mean many rejected orders around news, while a loose one gives fills at poor prices. Our spread and slippage guide explains the trade-off, and the gold news guide covers what happens at releases.

A worked example: five lots, three available

Suppose an EA sends a market order to buy 5 lots of gold, and at the moment it arrives the broker can fill only 3 lots at your price or better. With Fill or Kill the whole order is cancelled and the EA receives a refusal; no position opens. With Immediate or Cancel the broker fills 3 lots and cancels the remaining 2, so you hold 3 lots and the EA must cope with a smaller position than it planned. With Return the broker fills 3 lots and leaves a pending order for the other 2 resting in the market, which can fill later at a worse price, or never.

For small lots this almost never happens, which is why most retail traders never notice. It matters in fast markets and with large sizes, and it matters for an EA that assumes each order is fully filled. A robust EA reads the filled volume after sending the order and sizes its stop and take profit to what it actually holds. If yours does not, prefer Fill or Kill where the broker allows it, and keep lot sizes small.

What a well-built EA does with filling modes

• Reads which modes the symbol allows and chooses one that is supported, instead of using a fixed default.
• Checks the result code of every order request, not only whether the call returned.
• Handles a partial fill by adjusting protective orders to the filled volume.
• Does not retry endlessly after a refusal, so it does not hit the broker's request limit.
• Logs the mode and result so you can see what happened.