FOMC Three-Way: Why the Price Gap Is Not Arbitrage

The September FOMC is already trading like a simple choice: cut, hold, or hike. Yet the prices do not line up neatly. Kalshi’s listed September contracts show a 62¢ last price for a hold and 39¢ for a 25-basis-point hike. Its separate cut contracts last traded at 2¢ for 25 basis points and 1¢ for more than 25 basis points. Polymarket’s live event data shows 60.5¢ for no change, 38.5¢ for a 25-basis-point hike, and small prices across its cut buckets. Those totals can exceed a dollar.

That does not create a free trade. It reveals the real object being priced: separate contracts, with separate order books, settlement rules, access constraints, and bid–ask spreads. Fed funds futures add a third lens. CME FedWatch derives its probabilities from 30-Day Fed Funds futures prices, not from prediction-market order books. The difference between those lenses is the analysis.

Oddup’s Yes/No/Maybe design provides a useful refresher. Yes and No split 90% of the pool reserve, while Maybe receives 10% of that reserve. That is a platform-specific payoff mechanic, not a forecast that an FOMC decision has three equal paths. For macro traders, the useful habit is similar: separate the event outcome from the contract structure before assigning meaning to a price.

September’s three-way is really a five-bucket book

The Federal Reserve’s next scheduled meeting is 15–16 September. The policy decision will follow a July meeting that left the target range at 3.50%–3.75%. The July decision passed 9–3. Three dissenters preferred a 25-basis-point increase. The statement also said inflation remained elevated relative to the 2% goal, while job gains kept pace with the workforce. Those details explain why the hike tail is not negligible.

The headline “cut, hold, or hike” compresses the actual contracts. Kalshi lists hold, a 25-basis-point hike, a larger hike, a 25-basis-point cut, and a larger cut. Polymarket uses the same broad buckets. Each contract is binary. A trader is not buying one exchange-traded, three-state security.

That distinction matters because the displayed contract price is not a universal probability. A 62¢ Kalshi last price means the most recent matched hold trade was at 62¢. It does not mean a new buyer can necessarily enter at 62¢. At the research snapshot, the hold best bid was 61¢ and the best ask was 62¢. The 25-basis-point hike contract showed a 39¢ bid and 40¢ ask. The cheap tails were wider still.

Kalshi’s contract rules say these buckets are mutually exclusive. If a 50-basis-point hike occurs, the 25-basis-point hike contract resolves No. The market is logically a partition. The executable book can still be untidy. That is where a superficial probability calculation goes wrong.

The apparent overround is an execution fact, not a forecast

Start with a deliberately mechanical exercise. Take the Kalshi displayed asks for every listed September outcome: 62¢ for hold, 40¢ for a 25-basis-point hike, 1¢ for a larger hike, 2¢ for a 25-basis-point cut, and 1¢ for a larger cut. The basket costs 106¢ before any other trading costs.

Only one mutually exclusive contract can pay $1. The other four settle at zero. A trader buying the complete basket at those asks locks in a maximum $1 payout against a $1.06 outlay. This is the opposite of arbitrage. It is the cost of crossing several small books.

Now reverse the thought experiment. A trader who wants to sell every outcome needs accessible borrow, adequate collateral, and executable bids. The posted bids also do not form a clean theoretical strip. The result is a familiar market-microstructure message: last trades, bids, asks, and mid-prices answer different questions.

This is not a defect unique to rate contracts. It is more visible here because five price tiles invite a quick sum. In a deep, centralised, complete state-price market, claims on all possible outcomes should aggregate near $1 before fees. In a collection of retail-facing binary contracts, the distance from $1 contains information about friction. It should not be treated as a directional call on Jerome Powell or the Committee.

Kalshi and Polymarket agree on direction, not on a single number

Polymarket’s September event data prices no change at 60.5¢ and a 25-basis-point hike at 38.5¢. Its 25-basis-point and 50+-basis-point cut contracts show 1.25¢ and 0.65¢. Its 50+-basis-point hike contract shows 0.55¢. The five displayed Yes prices total 101.45¢. That is much closer to a dollar than the Kalshi ask basket, but it is still not exactly a dollar.

There is a common message across venues. Hold remains the modal path. A single quarter-point hike is the meaningful alternative. Cuts are a remote tail. The disagreement is in the margin: Kalshi’s latest trades imply a 62¢ hold and 39¢ 25-basis-point hike, while Polymarket shows 60.5¢ and 38.5¢ respectively. Those are not identical probabilities. They are close snapshots taken through different contract and liquidity structures.

The right comparison is therefore a range, not a false point estimate. Treat the hold band around 60.5¢–62¢ as the liquid consensus zone. Treat the one-to-two cent cut prices cautiously. In thin tails, one extra cent can double a quoted probability. A move from 1¢ to 2¢ looks dramatic in percentage terms. It may not change the core Fed thesis at all.

Volume should receive the same discipline. The Kalshi September hold contract reports more than $2.6 million in cumulative volume. Polymarket’s no-change market reports more than $6.1 million. These totals show attention and trading activity. They do not prove that either price is the best forecast. They also cannot be netted together, because venue participants, timestamps, currency rails, fee schedules, and trade histories differ.

Fed funds futures answer a different question

FedWatch is often treated as the referee in an argument between prediction platforms. That overstates its role. CME describes FedWatch as probabilities implied by 30-Day Fed Funds futures prices. Those futures embed expected average effective fed funds rates over a contract month. The mapping from a monthly average to a meeting-by-meeting decision requires calendar arithmetic and an assumption about the policy path around the meeting date.

Prediction contracts instead settle on a discrete policy statement. Polymarket’s rules use the upper bound of the target range and specify its September FOMC statement as the resolution source. Kalshi’s rules specify the exact size of the decision at the 16 September meeting. These are related exposures. They are not interchangeable instruments.

That makes a cross-market gap a research prompt. Ask whether the difference comes from the futures’ averaging window, the target-bound definition, a specific contract’s settlement language, or simply the price to cross the spread. The answer changes the trade thesis. A rate-sensitive futures position and a binary event contract can react differently to the same statement.

Worked example: stress-test the “buy every outcome” thesis

Suppose a trader sees the Kalshi tiles and argues that a hold, a 25-basis-point hike, and a 25-basis-point cut are “basically” the whole world. They buy one Yes contract in each at the displayed asks: 62¢, 40¢, and 2¢. The outlay is $1.04. If the Fed holds, the payout is $1. If it hikes 25 basis points, the payout is $1. If it cuts 25 basis points, the payout is $1. The position loses 4¢ in every one of those outcomes.

It is worse than it first appears. A larger move is possible under the contract rules. In that case the three-contract basket pays nothing. The trader omitted the two tail contracts. Even after adding both at 1¢ each, the cost becomes $1.06 and the maximum payout remains $1.

The correct trade thesis is not “the prices add to more than 100, so sell the expensive side.” A testable thesis has three layers:

• Outcome: What macro data or FOMC communication would change the relative odds of hold and a 25-basis-point hike?
• Instrument: Which contract has the cleanest resolution wording for that view?
• Execution: Can the position enter and exit near the visible bid or ask, at a size that matters?

For September, the official baseline is not a blank page. July’s statement paired solid activity with elevated inflation. It also recorded three dissents for a hike. A trader who thinks this combination is being underweighted must explain why the 25-basis-point hike price is too low relative to the hold price. A trader who thinks weak data will dominate must explain why the cut tail deserves more than its current small allocation. Neither argument follows automatically from an over-100¢ basket.

Why this matters for prediction traders

Three-way thinking is valuable because it forces traders to name the middle path. For the September FOMC, hold is not the absence of a view. It is the central contract. But three-way thinking fails when it turns separate binary quotes into a synthetic, frictionless market that does not exist.

Read a rate market in this order: official decision framework, exact resolution rule, executable bid and ask, then the headline probability. Compare Kalshi and Polymarket for corroboration. Use CME FedWatch to understand what fed funds futures imply. Do not collapse the three into one magic number.

The useful edge is not spotting that prices sometimes sum above 100¢. Everyone can add five tiles. The edge is recognising whether the gap survives contract definitions, tails, spreads, fees, and the ability to execute. In September’s Fed complex, that discipline is more informative than a tempting arbitrage headline.

Disclaimer: This article is for general information and market education only. It is not financial, investment, legal, or tax advice. Prediction-market prices can change quickly, and contracts may have different rules, liquidity, fees, eligibility requirements, and settlement processes. Review the official rules and assess risk independently before taking any position.