MT5 unsupported filling mode (error 10030)

Causes, most likely first

1. The EA requests a filling mode the symbol does not permit

MT5 supports Fill or Kill, Immediate or Cancel, and Return. Each symbol declares which it allows, and an EA that hardcodes one fails everywhere that mode is not offered. Read the permitted modes from the symbol specification and request one of those rather than assuming.

2. The broker changed execution type on the symbol

Market, instant and exchange execution permit different fill policies. A broker moving a symbol between them breaks EAs that worked the previous day, with no change on your side.

3. The EA was written for a hedging broker and moved to a netting one

Netting accounts — including US-regulated MetaTrader 5 — commonly permit a narrower set of fill policies, and an EA built against an offshore hedging account will not have been tested against them. It usually surfaces alongside other position-management problems rather than alone. See FIFO and netting rules.

4. Pending and market orders are treated the same

Permitted fill policies differ between market and pending orders on the same symbol. An EA using one code path for both works for one order type and fails for the other, which makes the error look intermittent.

Why this happens

This is a compatibility problem rather than a fault, which is why it confuses: the identical EA runs perfectly on one broker and fails on the first order at another. Nothing is wrong with the account, the balance or the strategy.

The correct fix is for the EA to read the symbol's permitted filling modes and choose one at runtime. Hardcoding a mode is the underlying bug, and it is common because it works fine until the software meets its second broker.

If you are evaluating any automated system for a broker other than the one it was built on, ask about this specifically. "It supports MT5" does not mean it negotiates fill policy.