Can an AI agent beat Kalshi's Chicago high temperature market?

From a rule to a record

A backtest of a Chicago temperature rule tells you what it would have done against prices already known. This page wants the other thing: how the idea that the forecast beats the crowd when the two disagree fares against the KXHIGHCHI book as it stands, each day at 10:00 AM Central, filled at the ask, charged Kalshi's fee (about 1.7¢ a contract at 43¢), and settled when the CLIMDW report says so. So the Chicago agent runs on Windmill like any user's agent, and each bet it places on KXHIGHCHI is written here as it settles, wins and losses alike.

How the agent works

The agent is the rule below in plain English, on Claude Haiku 5.5, scheduled once a day, at 10:00 AM Central. It builds its record in four steps.

1. Read the station

Every day at 10:00 AM Central, it reads today's daytime high from the NWS forecast for Midway (api.weather.gov/gridpoints/LOT/72,69/forecast), then reads today's KXHIGHCHI ladder, bracket by bracket, with the live ask on each. A KXHIGHCHI day in the facts below had 6 brackets, listed about 40 hours before the ladder closes at 1:00 AM Central time.

2. Filter for the setup

It places nothing unless both of these hold:

• the highest-priced bracket does not hold that high
• its YES bid is 45–70¢

Why this setup: Midway's official highs below moved a median of 4°F from one day to the next, so the morning favorite is a guess; when the NWS forecast for Midway, not O'Hare, puts the high in another bracket, the rule bets against that favorite.

3. Size and place

It buys 5 NO on it at the NO ask (100 − YES bid), on paper against the live KXHIGHCHI book: a paper order fills only against what the book shows at that price. At about 43¢ a contract, Kalshi's taker fee at KXHIGHCHI's settings (fee type "quadratic", multiplier 1) is about 1.7¢; the 40–59¢ favorites in the price table below paid 1.7¢ on average. So the five contracts cost about $2.24 all in, and pay $5.00 if they win.

4. Hold to settlement and record it

Every position is held until Kalshi settles the day on Chicago Midway Airport's official reading, the National Weather Service report CLIMDW as The Weather Company reports it, a median 6.2 hours after the ladder closes. The price paid, the fee, the result and the net go into the record below as each bet settles. Most of the daily runs end with no order, and the agent's log keeps those too.

The record: October 30, 2026

Started October 29, 2026; no settled bets yet.

Nothing is read into the record before thirty settled bets.

It runs this page's starter rule, word for word:

“Every day at 10:00 AM Central, read today's daytime high from the NWS forecast for Midway (api.weather.gov/gridpoints/LOT/72,69/forecast); on today's KXHIGHCHI market, if the highest-priced bracket does not hold that high and its YES bid is 45–70¢, buy 5 NO on it at the NO ask (100 − YES bid). Otherwise no order. Hold to settlement.”

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The market the rule is betting into

Kalshi's API listed 67 settled Chicago days, from Aug 24, 2026 to Oct 29, 2026: 402 brackets, 6 a day in the median, of which 67 won. The official highs ran from 56°F (Sat, Oct 17) to 96°F (Wed, Sep 2), with a median of 74°F, and on 4 of the 67 days the winner was one of the two open-ended brackets at the ends of the ladder. A median day traded 53,855 contracts across its brackets over the last 7 days.

Each day's ladder was listed about 40 hours before it closed, closed at 1:00 AM Central time after the day it is about, and settled a median 6.2 hours after the close, once the official reading was in. Trading runs through the afternoon, while the high is still being set: a rule that buys late in the day knows more, and pays for it in the price.

Chicago's official highs

The ten newest days:

The daily low (KXLOWTCHI)

Kalshi also lists Chicago's daily low as its own series, KXLOWTCHI, on the same station. It settled 67 days from Sun, Aug 23 to Wed, Oct 28, with official lows from 40°F to 79°F. A low is often set in the early morning, so its ladder is a different question from the high's, on the same report.

How the 1 PM favorite did

For each of the 48 newest days up to Oct 29, 2026, we read Kalshi's quote 12 hours before the close and took the favorite, the bracket priced highest, at its ask. 47 had a quote at that moment, and the favorite won 33 of them. Breakeven is the price plus Kalshi's taker fee, as a share of the $1 a winning contract pays.

Three notes on that table. No band won more often than its breakeven. Three of the three bands fell short of breakeven, the 80–99¢ band by the most: 87% against 92.3%, where at an average 91.8¢ one loss costs what 12 wins earn. And every band's interval includes breakeven, which is why the last column reads the same three times. In our read of Kalshi's prices, the Chicago favorite 12 hours before the close is priced about right. Nothing on this page says a buyer of the favorite has an edge.

The rule under test pays 30–55¢ a contract, which puts it in the 40–59¢ row: there the favorite won 4 of 8 days, against a breakeven of 53.8%. The rule's own conditions are what the record above tests; these rows are only the market's baseline at its price.

Kalshi's rules, in Kalshi's words

This is the rule of the market that was open (KXHIGHCHI-26OCT30-T67) when we read Kalshi's API on October 30, 2026, quoted from Kalshi; the series and its full rules are on Kalshi's site. The day and the bracket's bounds change with every market:

“If the maximum temperature recorded at Chicago (CLIMDW) for Oct 30, 2026, is greater than 67° fahrenheit according to The Weather Company, then the market resolves to Yes.”

Kalshi adds, in the same market's rules:

“Not all weather data is the same. While checking a source like AccuWeather or Google Weather may help guide your decision, the official and final value used to determine this market is the maximum/minimum temperature as reported by the Weather Company.”

The series names The Weather Company as its settlement source. Kalshi lists the series with the fee type "quadratic" at a multiplier of 1: a taker pays 1.8¢ a contract at 50¢ before rounding, less toward either end, and a resting order that fills later pays nothing. The fee calculator works out the fee and the breakeven for any price.

The rules name the station "Chicago (CLIMDW)": the National Weather Service report headed "The Chicago-Midway climate summary", issued by its Chicago office for Midway Airport (KMDW). Not O'Hare: a rule that reads O'Hare's forecast or observations is reading a different station. (NWS Chicago, climate report CLIMDW, checked October 30, 2026.)

What success looks like

A positive net per contract after Kalshi's fees over thirty settled bets, with the confidence interval clear of zero, would be a result.

A rule that only matches its price is not one. A contract bought near 43¢ that wins as often as that price says, 43 times in a hundred, loses about 1.7¢ a contract to the fee alone (the 40–59¢ favorites in the price table paid 1.7¢), so the line to beat on KXHIGHCHI is a win rate of about 44.7%, not 43%. Each win at that price nets about 55.3¢ a contract after the fee and each loss costs 44.7¢, so one win covers 1.2 losses.

The rule runs once a day, so thirty settled bets take at least 30 days, and longer when the setup is rare. If the record climbs, the idea that the forecast beats the crowd when the two disagree holds at these prices. If it falls, the idea goes, and the record stays on this page.

Follow along

• This page refreshes every hour from Kalshi's settled data and the agent's own log.
• Copy the Chicago agent: the button below opens the builder with this KXHIGHCHI rule, to run as written on paper under your own account, or with your own numbers in place of 30–55¢.
• The fee calculator shows the fee and the breakeven at any price on KXHIGHCHI.
• The 90-strategy test is the longer version of this exercise: about 90 rules and 4,200 paper bets across ten days.
• The Kalshi markets hub lists the other weather cities and every other market page.

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Questions

What does Kalshi's Chicago high temperature market settle on?

On the day's maximum temperature at Chicago Midway Airport, as The Weather Company reports it; the rules name the station MDW report (CLIMDW). Exactly one bracket of the day's ladder wins.

What is the fee at 50¢?

A taker pays 1.8¢ a contract before rounding at 50¢ on this series (fee type "quadratic", multiplier 1), less toward either end. The fee calculator gives the fee and breakeven at any price.

Is a bot here profitable?

Nothing on this page says so. The favorite 12 hours before the close won 33 of 47 days in our read of Kalshi's prices, and the table shows each price band against its breakeven: 3 of its 3 bands are inside noise. Run your own rule on paper to see what it does, before you risk money.

Paper or live?

Agents trade on paper unless you build them live. Paper orders fill against Kalshi's live order book with Kalshi's fees and settle when the real market settles. To trade real money, connect your own Kalshi API key and build a live agent; your money stays on your Kalshi account.