What a Vested Parimutuel actually does

There is a version of this mechanism that takes an hour to explain and a version that takes ninety seconds. This is the ninety-second one, written out.

A parimutuel is the oldest way to run a market that nobody is making. Everyone's money goes into a pot, the event happens, and the winners split the pot. No market maker, no quotes, no counterparty to find — it works from the first dollar, which is why racetracks, lotteries and every prediction market operating past its first few hundred listings end up running one.

It has exactly one flaw, and it is a bad one. A pool pays the same multiple to everyone who was right, regardless of when they were right. Someone who took a position an hour before the event and someone who took it three seconds before the whistle collect the identical return. The second person took almost no risk, contributed nothing anyone needed, and is paid out of the first person's winnings.

The Vested Parimutuel changes one thing about that. Not the pot, not the plumbing, not the payout arithmetic. One moment.

The change, in one sentence

In a classic pool, the losing money is divided among the winners at resolution. Here, it is assigned to them at the moment it arrives.

That is the entire idea. Everything else in the whitepaper is the consequences of that sentence and the one guardrail it needs.

Think about what it means to be on the other side of a bet. If you are on YES and someone stakes $100 on NO, that $100 is yours if YES wins — that is what having a counterparty is. The classic pool agrees, but waits until the end to say so, and by then more people have joined your side, so you share it with them. The vested rule says it immediately and never revisits it. The $100 is split among whoever was standing on YES at that instant, pro-rata by how much they had at stake, and no one who arrives afterwards can touch it.

Rule 1: money vests to the other side, on arrival

Formally, from §4.1:

When a stake arrives on an outcome, it is assigned — immediately and irrevocably — pro-rata by principal to the positions already standing on each of the other outcomes.

Two words in there are load-bearing.

Already. People who join your side after that moment share in nothing that came before them. They share only in what arrives after them. Your accrued claim is fixed the instant it accrues.

Irrevocably. Nothing in the mechanism ever decrements a claim. There is no rule anywhere that takes something back. The only condition is that the market actually resolves; if it voids, everyone refunds at principal (§12).

The direct consequence: a minimum payout that is set the moment you enter and can only go up. Not a guaranteed profit — if your outcome loses you get zero, exactly as in any market — but on your winning branch, a floor that ratchets. In the paper this is P2, and its proof is one line: a sum of non-negative increments, and nothing subtracts.

Rule 2: only as much as the other side can cover

Rule 1 alone would be broken, and it breaks in an entertaining way. If there is no limit on how much a book can absorb, then the first position on an empty book collects 100% of all opposing inflow until a second one joins. Make that position one cent and it earns an unbounded multiple. In the design-phase measurements a seeding straddle returned +155% at $50 a leg and +82,037% at one cent a leg (Appendix A.1). The optimal strategy would be to post dust on every new market and collect a toll while supplying no liquidity at all.

So there is a second rule:

Each position grants its own book κ · s of matching capacity. A stake is accepted only up to the capacity remaining in every opposing book. The rest is refused at entry and returned in the same transaction.

κ (kappa) is a single published number — a maximum-odds cap, the same object every betting exchange already publishes. That is all. No decay schedule, no bonus bucket, no parameters to tune.

What it does: it constrains acceptance, never distribution. The split stays pure pro-rata, which means return on capital is scale-invariant — a $1 position earns exactly the same multiple as a $10,000 position entering at the same moment, never more. Dust has nothing to dominate. And the multiple everyone shares is bounded at 1 + κ(1 + ln g), where g is the book's later growth.

The familiar consequence of a matching constraint is the partial fill, which is why the answer to "can I bet $500 here?" is sometimes "you can bet $400 of it."

What $500 into a fresh market actually looks like

Take the paper's reference parameters: a binary market, κ = 9, seeded with $50 on each side.

A symmetric seed of S per leg leaves each book exactly S(κ − n + 1) of headroom, so at S = 50, κ = 9, n = 2 that is $400.

You offer $500 on YES. The market accepts $400 and returns $100 in the same transaction. Nothing is held, nothing is escrowed, nothing is pending — the $100 is simply back in your wallet, and the refusal is telling you something true: there is only $50 of NO money in this market, and $400 at nine-to-one is everything that $50 can honestly stand behind.

Your $400 now sits on YES. Two things are true about it:

• If YES wins, you get at least your $400 back, plus your pro-rata share of every NO dollar that arrives from now on. Right this second that share is 100%, because you are the only non-seed position on YES.
• If NO wins, you get nothing, and your $400 belongs to the NO seed. Which is exactly the deal you took.

Then the market fills up. Every NO dollar that arrives vests partly to you, forever. Every YES dollar that arrives dilutes only your share of future NO flow — it cannot reduce a cent of what you have already accrued.

And if you had wanted the whole $500 in, you had two options: wait for someone to take the other side, or bring the other side yourself. Which brings us to the thing most people ask second.

The first bettor can be the seeder

A market opens when someone stakes every outcome, in a reserved slot the paper calls vintage 0. This is usually described as "the creator seeds it," which makes it sound like a chore imposed on whoever wrote the question.

It is better read as a directional first bet with a floor. The seed does not have to be balanced — §4.4 accepts any legs satisfying a_o ≤ κ · min_{w≠o} a_w, so at κ = 9 a $450 / $50 straddle is a perfectly valid opening. That is a nine-to-one directional bet that happens to leave on the table exactly what the second bettor needs in order to exist.

And whoever posts it recovers at least what they staked, in every branch, under every continuation (P6). The proof is trivial once you see it: immediately after seeding, you own every position in the market, and every branch pays the whole pool to its owners. Later flow only adds.

That property is what makes the seat worth taking, and the cold-start article is about why it matters more than it sounds.

The worked example

Here is the paper's §4.2 log — a $25/$25 seed, five ordinary stakes, $1,050 accepted, no fees, machine-checked as the first pair of conformance vectors.

If YES realizes, the classic parimutuel pays every YES holder a flat 1.680×. Here:

Read three things off it.

The point of the whole exercise is the last two rows. A carried the risk for 89% of the market's life. E arrived after the last opposing dollar. The classic column pays them the same. This one pays 3.071× and 1.000×, out of the identical pool.

Payouts still total $1,050 exactly, in both branches, in integer units. Nothing is created and nothing is skimmed. This is P1, and it is an accounting identity rather than a solvency claim someone tested — the conservation is a bijection between accepted units and payout units.

E is not being punished; E is being told the truth. Under classic rules E takes $136 of profit, every cent of it out of A, C and the creator, for arriving late and bearing no risk. Here, E gets its money back. That is not a penalty. That is what a bet with no counterparty is worth.

What follows for free

Because these are consequences of the two rules rather than features bolted on, they come with proofs rather than promises:

• No lock window. Because a stake arriving after the last opposing dollar pays exactly 1×, the buzzer-beating strategy that forces every parimutuel venue to close entry early has nothing to aim at. Measured across 20 seeds × 2,000 markets: unconditional expected value of the snipe is −1.05% here against +25.27% under classic rules. More on that here.
• Dilution protection is exact. Under a snipe worth 50% of the pool, early winners' total payouts fall 0.19% on average here, against 13.6% under classic rules.
• Settlement is O(1). The rule reads like a loop over every opposing position, but it collapses to one running accumulator per outcome — two scalars per position, updated in constant time. That is the difference between implementable on-chain and not (§6).
• Your payout depends on nobody else's record. It is a function of two numbers: your outcome's accumulator when you entered, and at resolution.

What it does not do

The paper is unusually direct about this, and an explainer that skipped it would be selling something.

• The floor is conditional on your outcome winning. It is not downside protection. Wrong side, zero.
• You cannot get out. Positions are transferable in principle, but the mechanism itself offers no exit, no cash-out, and no hedge. Something else has to provide that (§8).
• The late price gets worse. With rational late entry gone, the pool ratio stops tracking probability near the end: final-phase Brier 0.174, against 0.150 for a classic pool that simply locks. That is a measured cost, paid deliberately.
• A big enough early position squeezes everyone else. A seed three times the organic pool takes ordinary early winners from 1.70× to 1.09×. Nothing prevents it.
• There is no equilibrium theorem. The incentive arguments are arguments, and the paper labels them as such.

Next: The lock window, and what it costs to keep — why every pool venue closes early, and what deleting that rule is worth.