Self-Trade Prevention

How your resting orders are cancelled when an incoming order would trade with your own account, including across YES and NO.

Your orders cannot fill against another order from the same account. This applies to ordinary BUY/SELL trades and to cross-orderbook matching: BUY YES with BUY NO (MINT), and SELL YES with SELL NO (MERGE). Prevention is always enabled; there is no option to change which order is cancelled or to turn it off.

What happens on a self-match

When an incoming taker order would match a resting maker order owned by the same account:

The maker order's entire remaining quantity is cancelled, even if the incoming order is smaller. Any earlier fills remain completed.
A cancelled BUY releases its reserved buying capacity; a cancelled SELL unlocks its outcome tokens.
An account stream order_cancelled event is emitted with reason SELF_TRADE_PREVENTION.
The taker order continues matching against the next eligible resting order, if any. Self-trade prevention does not itself cancel the taker.

This means a taker order that would have matched only your own resting orders effectively cancels those resting orders and then rests on the book (if the time-in-force allows) or has its unfilled remainder cancelled.

Only eligible orders encountered while matching are cancelled. Orders outside the incoming limit, or orders not reached because the incoming order is already filled, remain in place. Orders are encountered by effective price, then arrival time, across both outcomes. A better-priced order from another account can fill your incoming order before it reaches your own quote.

Cross-outcome examples

Assume there is no other eligible liquidity and each incoming order is GTC:

Your resting order	Your incoming order	Result
BUY NO, 100 shares at 0.40	BUY YES, 50 shares at 0.70	The NO order is cancelled for its full remaining 100 shares; the YES order rests at 0.70
SELL NO, 100 shares at 0.60	SELL YES, 50 shares at 0.30	The NO order is cancelled and its 100 shares unlocked; the YES order rests at 0.30

Both orders in a potential self-match can be BUYs, or both can be SELLs. Having different outcome token IDs does not prevent a self-match. No pair is created or combined in these examples, and no trading fee is charged for the prevented match. Use split or merge to change your own paired inventory directly.

If another account has eligible liquidity behind your cancelled order, the incoming order can fill against it and normal fees apply to that actual trade.

FOK and post-only orders

A FOK order is checked against non-self liquidity before the book changes. If there is not enough to fill the complete order, the FOK is rejected and your resting orders remain in place. This check includes eligible liquidity on both outcomes when cross-orderbook matching is enabled; your own orders never count toward the required quantity.

Post-only checks also include eligible complementary liquidity from other accounts. For example, your post-only BUY YES at 0.70 would take another account's BUY NO at 0.40, so it is rejected even with no YES asks.

If the only eligible order belongs to you, it does not count as a taker fill. The post-only order can be accepted, cancel your older resting order through self-trade prevention, and then rest. Post-only protects you from taking liquidity; it does not preserve your older quotes.

Implications

A prevented self-match incurs no trading fee.
A self-match never produces a fill — it never appears in your trade history and never contributes to trading-volume metrics.
Track your quotes across both YES and NO. An aggressive order can cancel your resting order on either outcome, including one with the same BUY/SELL side as the incoming order.

How to detect it programmatically

The order_cancelled event on the account stream carries reason = "SELF_TRADE_PREVENTION". Treat this the same as any other cancellation: the resting order is gone, and you may re-place it if needed.