Self-Trade Prevention

Current Behavior

Lighter imposes a self-trade prevention mechanism. Trades between the same account cancel the resting order (maker) instead of executing the respective trade. Without this mechanism, self-trades would result in no position change and fees being paid unnecessarily.

Upcoming Change

Default self-trade behavior on Lighter will change from reduce both to cancel maker:

• Current default (reduce both): When a user's order crosses their own resting order, both orders are reduced by the overlap amount. • New default (cancel maker): The resting order is cancelled in full, and the incoming order continues filling against the rest of the book.

This means that under the new default behavior, the taker order fills as intended against the rest of the book, instead of being silently reduced. The change only affects orders that don't explicitly specify a self-trade behavior. All four modes (cancel maker, cancel taker, cancel both, reduce both) will be allowed as explicit transaction attributes when placing the order.

The above changes will be effective starting May 31st, 2026 at 8:00 AM ET (timestamp: 1780228800)