Skip to main content
A binary market poses one question with two outcomes. A share of yes pays 100¢ if the answer turns out to be yes and 0¢ otherwise; a share of no pays 100¢ if it turns out no. Their prices always sum to 100 — if yes trades at 63¢, no is worth 37¢ by definition, not by coincidence. Prices are whole cents from 1 to 99. Zero and 100 are settlement values, not prices: a market at either has no uncertainty left to trade.

​
One market, two outcomes

This is the fact to internalise before writing any code against this API. A binary market is one market with one ticker and one book. yes and no are not two markets, and they are not two books. They are two ways of describing the same position: So a buy no order can trade against a buy yes order. They are the two sides of one trade.
A bot that treats yes and no as separate books sees half the liquidity that is actually there, and will quote against itself while believing it is quoting two independent markets.

​
What you paid, and what printed

When a buy no at 40¢ trades against a buy yes at 60¢, one trade happened at one price — but the two parties paid different numbers. Fills carry both, and picking the wrong one is the most expensive mistake available on this API: Reading trade_price where you meant outcome_price is wrong by 100 − 2p on every position — at 30¢ that is 40¢ per share — and it is wrong silently, in a direction that looks plausible. There is no error, just a P&L that drifts.

​
There are no shorts

You cannot sell what you do not hold. A sell beyond your inventory is 422 insufficient_shares — not a short position, a rejection. To take the other side of a market, buy the opposite outcome:
  • Bearish on yes? buy no.
  • Want out of a yes position you hold? sell yes, up to the quantity you hold.
Positions in both outcomes are non-negative, always. There is no borrow, no margin, and no synthetic short.

​
Covered pairs

Because yes and no pay 100¢ between them in every outcome, holding one of each is riskless: whichever way the market resolves, the pair pays exactly 100¢. You can end up holding both sides just by trading — buying yes on one order and no on another. That part of your book then carries no market risk: it pays 100¢ per pair at resolution no matter where the market trades in the meantime. Two consequences when you read your own state back:
  • A position is per market and per outcome. Both sides of one market is two entries in positions, not one net entry. Code that reads only the first drops a leg of its own hedge.
  • Netting is yours to do. Nothing on the venue collapses the two into a single number for you.

​
Time in force

Omitting time_in_force gives you gtc. That default is deliberate: an order that silently became ioc would die the moment it failed to cross, which is the opposite of what someone who omitted the field usually wants.

​
Fees

Fees are charged on the price you named, not on the canonical price. A buy no at 35¢ is charged against 35, not against 65. The curve is parabolic: zero at the extremes, largest at 50¢, and symmetric — 30¢ and 70¢ cost the same. Each market publishes its own maker_fee_rate and taker_fee_rate as decimal strings ("0.0700" is 7%). See Matching for the formula.