Paper trade Kalshi against the live order book

Kalshi's demo vs paper trading the real markets

Kalshi runs a separate demo site, demo.kalshi.co, where you can practice trading with mock funds. A demo account has its own login, apart from your real Kalshi account, and no real money is involved.

The demo is an environment apart from the real exchange, and Kalshi's API documentation says this about its markets:

“The price and behavior of markets in the demo environment may not be reflective of those in real markets.”

Windmill paper trades the real markets: the real order books, Kalshi's real fee schedule and the real settlements. The only difference is that no order reaches Kalshi and no money moves.

How it works

1. Describe a strategy in plain English. For example: “Buy NO on longshots under 10¢ in the NFL series, at most $20 a market, check every hour.” The builder asks about anything it still needs.
2. It becomes an agent. The agent wakes on the schedule you set, reads the live markets and places paper orders.
3. You see every run. Each run shows the agent's reasoning, the tools it called and the orders it placed. Each order shows its fills, their prices and the fees.
4. Go live when you choose. Connect your own Kalshi API key when you want an agent trading for real. A live agent trades on your Kalshi account, never past the limits you gave it, and your money stays on Kalshi.

Why the fills matter

A paper account that fills every order at the midpoint, with no fee, makes most strategies look better than they are. Windmill's paper fills follow what is really in the book.

• An order that crosses the book fills at the prices resting there. A buy takes the asks from the lowest up, level by level, as far as its limit price and only as much as each level shows. Whatever is left rests in the book.
• A resting order fills only when the price moves past it. A resting buy fills when the best ask drops below its limit, and a resting sell when the best bid rises above it, at the order's own price. Touching the limit is not enough, because a paper order has no place in the queue. These fills pay the maker fee.
• Fees follow Kalshi's fee schedule, including its rounding. Each fill pays the taker or maker fee of its market's series.
• Positions settle when Kalshi settles the market, at Kalshi's settlement value.

A control agent that picked sides with a coin flip lost about 0.4¢ a contract over more than 250 bets, which is about what the fees cost.

What we learned running it ourselves

From August 27 to September 6, 2026, we ran about 90 strategies on paper: 4,200 settled bets in all.

• No strategy showed an edge that held up as its sample grew. The early leaders drifted back toward zero.
• Buying 89–95¢ “sure things” in thin entertainment markets lost 36¢ a contract.
• Favorites in the last hour before a game were priced at 87% and won 80% of the time, over 139 bets.

That is the case for testing a strategy on paper first. The full write-up has the numbers and the caveats.

Questions

Is it free?

Yes. Paper trading is free, and signing up needs no card. The pricing page lists how many agent runs a day the free plan includes and what Pro adds.

Do I need to know how to code?

No. You describe the strategy in plain English, and the builder turns it into an agent.

Do I need a Kalshi account to paper trade?

No. Paper trading needs only a Windmill account. To trade live, you connect your own Kalshi API key.

Does Kalshi have a demo account?

Yes. Kalshi runs a demo site, demo.kalshi.co, with mock funds and logins of its own. Kalshi says the price and behavior of its demo markets may not be reflective of those in real markets. Windmill paper trades the real markets.

Can I use it on a phone?

Yes. You can build an agent and follow its runs from a phone.

How often can an agent check the markets?

As often as every 10 minutes. Each check is one agent run, and runs count against your account's run budget over a rolling 24 hours.