Partial Assignment of Short Options

A partial assignment is what happens when only some of the short options in your position are exercised against you. You sold ten cash-secured puts on Friday. Monday morning three of them are gone from the account, 300 shares are sitting in their place, and seven contracts are still open. Nothing malfunctioned. The machinery that routes an exercise notice to a specific seller never looks at your ten contracts as a single block.

Why partial assignment of short options happens

Assignment travels through two stages, and neither stage works at the level of your account.

1. A long holder exercises. The Options Clearing Corporation, or OCC, the central counterparty sitting between every buyer and seller of a listed US option, totals the exercise notices filed in that contract series after the close.
2. OCC allocates those exercises to its clearing members, the brokerage firms carrying short positions in the series, on a random basis. OCC assigns to firms, never to individuals.
3. Each firm then allocates the exercises it received among its own customers short that series, using an exchange-approved method fixed in advance: random selection, or first in, first out by the date the short position was opened.
4. The assigned customer finds out the next morning. The short contracts leave the account and the stock arrives at the strike.

The unit moving through both stages is the contract, not the position. Picture a firm that receives 4,000 assignments against 50,000 customer short contracts in one series: roughly one contract in twelve gets drawn. A customer holding ten of them lands somewhere between zero and a handful, and almost never on exactly ten. Partial is the expected shape of the outcome. All ten at once is the special case.

Multi-contract positions are ordinary, which is what makes split outcomes ordinary. The panel below buckets one full session of AAPL option trades by the number of contracts printed on each trade.

SELECT
    multiIf(size = 1,   '1 contract',
            size <= 4,  '2 to 4',
            size <= 9,  '5 to 9',
            size <= 24, '10 to 24',
            size <= 99, '25 to 99',
                        '100 or more')             AS lot_size,
    count()                                        AS trades,
    round(100 * count() / sum(count()) OVER (), 1) AS pct_of_trades
FROM global_markets.options_trades
WHERE underlying_symbol = 'AAPL'
  AND sip_timestamp >= '2026-05-14 00:00:00'
  AND sip_timestamp <  '2026-05-15 00:00:00'
GROUP BY lot_size
ORDER BY min(size)

Single-contract prints accounted for 57.4% of trades that session. The rest arrived in larger clips, out to the 100 or more bucket at 0.6% of trades. Every clip above one contract is a position that allocation can split.

Which of your contracts are eligible in the first place

Only a contract that someone actually exercises can be assigned. At expiration that set is close to everything in the money: OCC's exercise-by-exception process automatically exercises long positions finishing in the money by at least a penny, unless the holder files contrary instructions. What happens when an option expires in the money walks that path end to end. Before expiration, exercise is a discretionary act by the long holder, and early assignment of short options covers when it becomes worth doing.

The pool of series that allocation can draw from is wider than most traders picture. The panel counts the contract lines that traded into the May 15, 2026 expiration across five household names, one line per strike and option type.

SELECT
    underlying_symbol        AS symbol,
    count()                  AS contract_lines,
    uniqExact(strike_price)  AS strikes
FROM global_markets.options_greeks
WHERE date = '2026-05-14'
  AND expiration_date = '2026-05-15'
  AND underlying_symbol IN ('AAPL', 'MSFT', 'NVDA', 'SPY', 'KO')
  AND volume > 0
GROUP BY symbol
ORDER BY contract_lines DESC

SPY went into that final session with 307 separate contract lines that traded, spread across 179 strikes. Each line is its own series, with its own crowd of short sellers and its own independent draw. Your ten contracts sit inside exactly one of those crowds.

Within a single name the eligibility line is sharp rather than gradual. A put with a strike above the closing stock price carries intrinsic value, and exercise-by-exception sweeps it. A put with a strike below the close carries none, and it expires unexercised. Short sellers on the first side of that line are in the allocation pool, and short sellers one strike away on the other side are not.

The stub: seven contracts and 300 shares

Delta measures how much an option's price moves for a $1 move in the underlying stock, quoted per share, and one contract covers 100 shares. A long put has a negative delta. Selling the put flips the sign, so a short put carries positive exposure to the stock.

The 300 assigned shares carry a flat 300 deltas that no longer change. The seven surviving short puts carry 700 times the contract's per-share delta, with the sign flipped.

Put an illustrative number on it, one that is not a quote from any particular contract. A surviving put showing a delta of -0.35 per share contributes 700 times 0.35, or 245 share-equivalents of positive exposure, on top of the 300 shares already sitting in the account. The stub reads 545 share-equivalents long. Move that same per-share delta to -0.60 and the stub reads 720. A partial assignment did not shrink the position. It converted part of the exposure into stock, and the stock half has no expiration date, no time decay, and no strike above which the risk stops.

What traders do with a stub

• Sell the shares. The account returns to seven short puts, and the sale settles on the standard T+1 timetable. This is the response that restores the original position shape.
• Write calls against the shares. A 300-share lot supports three covered calls, one per hundred shares, which adds a second short leg with its own allocation queue. How to calculate covered call returns lays out that arithmetic.
• Close the remaining contracts. Buying back the seven open puts ends any further allocation exposure in the series and leaves a plain stock position.

Leaving both legs untouched is a fourth choice rather than a neutral one. It is the version that carries the most stock exposure into the next session, which is the part of the stub that the assignment created without asking.

Where your broker discloses its allocation method

The method is disclosed, and it differs between firms. FINRA's options rule, Rule 2360, requires a member firm to establish a fixed procedure for allocating exercise assignment notices among customer accounts, to run that procedure on a random basis or first in, first out or another exchange-approved method, to describe the method in writing to every options customer at account opening, and to make the current method available on request. Firms restate it in the options account agreement, and usually again on a help page filed under assignment.

The practical difference is worth knowing. Under first in, first out, the oldest open short position in a series is assigned first, so a trader who sold early sits near the front of the queue. Under random allocation, every short contract in the series draws with equal odds on every allocation, and last night's outcome carries no information about tonight's. Neither method is predictable at the level of a single contract, and neither one is something a customer can opt into.

Cutoff times are a separate disclosure covering when your broker must receive your own exercise instructions, and broker exercise cutoff times collects those.

Cases that look similar

Pin risk describes uncertainty about a contract finishing at the strike, where the holder's exercise decision is genuinely in doubt. Partial assignment picks up after that question is settled: the contracts are in the money, exercise is happening, and the only open item is which contracts were drawn. Pin risk at options expiration handles the first situation.

A vertical spread where the stock settles between the two strikes has its own mechanics, with a short leg assigned and a long leg expiring worthless. When a spread expires between the strikes works that case on its own terms.

Early assignment on short calls clusters the night before an ex-dividend date, and the arithmetic behind it belongs to the dividend. Ex-dividend dates and options covers it.

FAQ

Why was I assigned on only three of my ten contracts?

Allocation runs in two stages, and the unit at both stages is the contract. OCC draws a clearing firm at random, and the firm then draws among its own customers short that series by random selection or first in, first out. Neither stage sees your ten contracts as one position.

Can I choose which of my short contracts get assigned?

No. The selection sits with OCC and with your broker's fixed allocation procedure. What you do control is position size and timing: a short contract closed before an assignment is allocated is out of the pool.

Does a partial assignment mean the rest are coming?

Not on its own. Under random allocation, each remaining contract draws fresh on every allocation. Under first in, first out, the remaining contracts do move up the queue as older positions are assigned, which changes their standing without guaranteeing anything.

How do I find out which allocation method my broker uses?

FINRA Rule 2360 requires the firm to state it in writing at account opening and to make the current method available on request. In practice it appears in the options account agreement or on the broker's help page about assignment, worded as random or first in, first out.

What happens to the shares that appear overnight?

They land in the account at the strike price of the assigned contracts, settle on the normal T+1 timetable, and carry full stock exposure from the moment they appear. A short put assignment is a purchase, with the cash or margin due like any other.

Every panel here carries the exact SQL beneath it. To count the contract lines at a different expiration, or bucket trade sizes on another session, ask the question in plain English on the Strasmore terminal.