Polymarket Multi-Outcome Arbitrage Scanner

Finds Polymarket multi-outcome events where buying one share of every outcome costs less than the $1 the set pays out, after Polymarket's trading fees, at the size the live order books can actually fill.

How the gap works

A multi-outcome Polymarket event (for example "who wins the election") has several outcomes, and exactly one of them resolves to Yes. One share of every outcome therefore pays exactly $1 in total, whichever one wins. If buying one share of each costs less than $1, fees included, the difference is locked in until the event resolves.

What you get for every gap

• sharesPerOutcome: how many shares of each outcome the books can fill while one more set still costs under $1 including fees
• capitalUsd: the money you would put in, fees included
• feesUsd, grossProfitUsd, profitUsd: Polymarket's taker fees at that size, the profit before fees, and the profit after fees
• edgePct: profit after fees divided by capital
• daysToResolution and annualizedPct: how long your money is locked, and the simple annualised return. A 1% gap that closes in a year is not the same as a 1% gap that closes next week
• topAskSum and topAskSumWithFees: the sum of every outcome's best ask, before and after fees
• minOrderSize: the smallest order the books accept
• legs: every outcome with its best ask, the size at that price, the quoted price from the event list, its fee rate and exponent, the average price you would pay, and its fee. With these you can check the figures yourself
• warnings: anything that weakens the gap, such as an outcome named like "Other" or a missing end date
• outcomesCanBeAdded: true only when you turned on includeAugmented and this event can still add outcomes. Such a row always carries a warning: if an outcome that is not listed yet wins, every share bought pays nothing

Fees follow Polymarket's own formula from its official trading client: fee = shares × rate × (price × (1 − price))^exponent, with the rate and exponent read for each market from Polymarket's order-book service in the same run. Markets without a fee schedule are charged nothing, and feeInfoPresent says so for each leg. If a market's fee schedule cannot be read, that event is not reported; an unknown fee is never treated as zero.

How it scans

1. It reads open events, the most traded in the last 24 hours first, and keeps the multi-outcome ones.
2. It screens them on each event's quoted best prices.
3. It checks the tightest ones, up to the number you set, against the full order books and fee schedules, two requests per outcome.

The run summary lists how many events were read and skipped and why, and how many were checked. A run that finds no gap is a normal result; the free NEAR_MISS rows show how near the market came.

What is skipped, and why

• Events past their end date. An event that should have ended but is still open usually has a resolution problem: an annulled election, a delayed result, a dispute. Prices there can add up to well under $1 because the market expects that none of the listed outcomes will win, so it is not a locked-in gap. These are skipped and counted.
• Events that can add outcomes later or hold placeholder names. If a new outcome is added and wins, the set you bought pays nothing. These are skipped unless you turn on includeAugmented.
• Gaps smaller than the minimum order size. If the books can only fill fewer shares than the smallest accepted order, the gap is listed as a near miss with the reason.

What the numbers still leave out

• Moving the price. Placing several orders one after another can move the prices. The depth shown is what the books held at the moment each was read.
• Time. Order books change while the run happens. A gap seen here can be gone when you act.
• Lock-up. Your money is tied up until the event resolves.
• Gas, deposits and conversion. Only Polymarket's trading fee is counted.
• Resolution risk. The gap is only locked in if exactly one listed outcome resolves Yes.

This is a calculation on public order-book data, not a recommendation to trade.

Input

Field	What it does
minEdgePct	Minimum profit after fees divided by capital, in percent. Default 0.5
minProfitUsd	Minimum profit after fees in dollars at the fillable size. Default 1
maxEvents	How many open events to read, most traded first, 100 per request, up to 1,000. Default 500
maxVerify	How many of the tightest events to check against order books. Default 40
screenSlack	Events whose quoted prices add up to under 1 plus this are still checked, since quotes can be slightly stale. Default 0.03
includeAugmented	Include events that can add outcomes or hold placeholders. Default off

Output

Two kinds of dataset rows:

• ARBITRAGE: a gap that survives fees, the minimum order size and your thresholds, sorted by edgePct. Only these rows are charged.
• NEAR_MISS: every other event that was checked against the order books, closest first, with reason saying why it is not a gap: NO_GAP_AFTER_FEES, BELOW_MIN_ORDER_SIZE or BELOW_YOUR_MINIMUM, plus the same legs and sums so you can check it. These rows are free. They are there so that a run that finds nothing still shows how close the market came.

The run summary, including counts, the closest misses and the caveats, is stored under the key SUMMARY in the key-value store.

Fields that cannot be known are empty rather than zero: a missing end date gives an empty annualizedPct.

If something goes wrong

If Polymarket answers with a shape this Actor does not recognise, an order book holds an impossible price, or a fee schedule is unreadable, the run fails and names the problem. It does not guess. If many order-book or fee requests fail, the run fails instead of reporting from the few that worked.

Data and fair use

The Actor reads public, read-only Polymarket endpoints with a descriptive User-Agent. It does not log in, trade, hold keys or work around any blocking, and it paces its requests.