Pin Risk: What Happens When a Stock Closes Right at Your Strike
The strike doesn't care that you're exactly on it
Options settle on a binary: in-the-money or out. A call one cent above its strike at expiration is exercised; one cent below, it expires worthless. That's clean everywhere except the one spot where it isn't — when the stock closes exactly at the strike, or close enough that a rounding difference between the official settlement price and what you can actually see on your screen leaves you unsure which side of the line you landed on. That ambiguity is pin risk: the stock appears "pinned" to the strike into the close, and you genuinely don't know your position's fate until assignment notices come through.
Why stocks actually pin to round strikes
Pinning isn't random. Market makers who are short a large amount of gamma at a heavily-traded strike need to delta-hedge that exposure continuously, and near expiration their hedging becomes extremely sensitive to small moves in the underlying. As the stock drifts toward a strike with dense open interest, the hedging flow required to stay delta-neutral can itself dampen movement away from that price — buying dips toward the strike and selling rallies away from it. The effect is strongest at strikes with the largest concentration of open contracts, which is exactly where the gamma exposure driving the hedging is largest. It's a feedback loop, not a coincidence, and it's most visible on names with heavy weekly options volume sitting at round-number strikes.
The actual risk isn't the pin — it's not knowing your assignment
If you're short a call or put that lands dead-on the strike at the close, you can't reliably tell whether you'll be assigned. The official settlement price used by the Options Clearing Corporation may differ by fractions of a cent from the last quoted price you saw, and individual option holders have until 5:30pm ET on expiration day to submit an exercise decision — meaning a holder can choose to exercise a technically out-of-the-money option, or let a technically in-the-money one lapse, for reasons that have nothing to do with the closing print (tax timing, an odd lot, simple error). You find out which happened Monday morning, in the form of either cash or a stock position you didn't plan for.
That uncertainty is the entire problem. A covered call assigned when you expected it to expire worthless just means you sell shares at the strike — mildly annoying, easily unwound. A naked short put assigned over a weekend when you were flat cash, or a spread where one leg gets assigned and the other doesn't, is a very different Monday.
Spreads are where pin risk actually bites
A single short option pinned at the strike is a manageable, known outcome either way. A vertical spread is where pin risk turns dangerous: if the stock closes between your two strikes, the short leg may get assigned while the long leg — being just out-of-the-money — expires worthless and provides no offsetting protection. You can wake up Monday with a stock position sized to the full notional of the short strike, with none of the defined-risk protection the spread was built to provide. This is precisely the scenario defined-risk traders assume can't happen, because on paper the spread's max loss is the width between strikes — but that math only holds if both legs settle the same way.
The fix is boring: don't let it stay ambiguous
Pin risk is one of the few options risks that's fully avoidable, because the ambiguity only exists if you let the position ride into the close. Closing (or rolling) any position that's within a few cents of its strike before expiration converts an unknown outcome into a known one — you pay or receive a small amount of bid-ask spread instead of gambling on which side of a coin a settlement print lands on. If you do intend to hold through expiration, most brokers let you submit a Do-Not-Exercise or explicit exercise instruction ahead of the 5:30pm ET cutoff, which removes the guesswork on your side even if the counterparty's decision is still unknown. Either way, the rule is the same: never let "I'm not sure if I'll be assigned" be the plan going into a weekend.
If you're holding a real position through an expiration where the strike and spot are close, the Position Analyzer can help you see the roll math and the resulting exposure of a same-day close versus letting the position ride, so the decision isn't made passively by the settlement print.