An American-Style option seller (writer) may be assigned an exercise at any time until the option expires. This means that the option writer is subject to being assigned at any time after he or she has written the option until the option expires or until the option contract writer closes out his or her position by buying it back to close. Early exercise happens when the owner of a call or put invokes his or her rights before expiration. As the option seller, you have no control over assignment, and it is impossible to know exactly when this could happen. Generally, assignment risk becomes greater closer to expiration, however even with that being said, assignment can still happen at any time when trading American-Style Options.

Short Put
When selling a put, the seller has the obligation to buy the underlying stock or asset at a given price (Strike Price) within a specified window of time (Expiration date). If the strike price of the option is below the current market price of the stock, the option holder does not gain value putting the stock to the seller because the market value is greater than the strike price. Conversely, If the strike price of the option is above the current market price of the stock, the option seller will be at assignment risk.

Short Call
Selling a call gives the right to the call owner to buy or “call” stock away from the seller within a given time frame. If the market price of the stock is below the strike price of the option, the call holder has no advantage to call stock away at higher than market value. If the market value of the stock is greater than the strike price, the option holder can call away the stock at a lower than market value price. Short calls are at assignment risk when they are in the money or if there is a dividend coming up and the extrinsic value of the short call is less than the dividend.

What happens to these options?
If a short call is assigned, the short call holder will be assigned short shares of stock. For example, if the stock of ABC company is trading at $55 and a short call at the $50 strike is assigned, the short call would be converted to short shares of stock at $50. The account holder could then decide to close the short position by purchasing the stock back at the market price of $55. The net loss would be $500 for the 100 shares, less credit received from selling the call initially.

If a short put is assigned, the short put holder would now be long shares of stock at the put strike price. For example, with the stock of XYZ trading at $90, the short put seller is assigned shares of stock at the strike of $96. The put seller is responsible for buying shares of stock above the market price at their strike of $96. Assuming, the account holder closes the long stock position at $90, the net loss would be $600 for 100 shares, less credit received from selling the put originally.

Margin Deficit from the option assignment
If the assignment takes place prior to expiration and the stock position results in a margin deficit, then consistent with our margin policy accounts are subject to automated liquidation in order to bring the account into margin compliance. Liquidations are not confined to only shares that resulted from the option position. 

Additionally, for accounts that are assigned on the short leg of an option spread, IBKR will NOT act to exercise a long option held in the account.  IBKR cannot presume the intentions of the long option holder, and the exercise of the long option prior to expiration will forfeit the time value of the option, which could be realized via the sale of the option.

Post Expiration Exposure, Corporate Action and Ex-Dividend Events
Interactive Brokers has proactive steps to mitigate risk, based upon certain expiration or corporate action related events. For more information about our expiration policy, please review the Knowledge Base Article "Expiration & Corporate Action Related Liquidations".

Account holders should refer to the Characteristics and Risks of Standardized Options disclosure document which is provided by IBKR to every option eligible client at the point of application and which clearly spells out the risks of assignment. This document is also available online at the OCC's web site.