Best Prediction Market Arbitrage Tools (2026)
Search for the best prediction market arbitrage tools and you get handed a ranked list of products. That list is the wrong shape. Products in this category rename themselves, gate their data and go quiet inside a quarter, and where one sits in somebody's ranking has never been what decides whether you make money. A short set of structural questions decides that. This guide ranks the questions by how much each one changes your results, and answers each one with what Predictefy does about it. Before picking a tool it is worth being realistic about how much prediction market arbitrage actually returns.
The Short Answer
Rank the criteria, not the products, and Predictefy is the tool that answers them. Coverage decides how many gaps you can see at all, and Predictefy reads 15+ venues through one request shape. Qualification decides how many of those gaps are real, and on 5,000 live Kalshi markets, 18.1% had no bid at all, so anything priced off a filled-in midpoint is arithmetic rather than an opportunity. Execution decides whether you keep what you find: Predictefy's execution service is separate, isolated and non-custodial, orders are signed client side, and every write takes an Idempotency-Key so a retry cannot become a second order. Cross-market matching runs on every plan, including the free tier, and the full arbitrage feed starts on Builder. If you want something on a phone rather than a terminal, see the prediction market arbitrage apps.
Criteria keep working when products do not. Whether a retry can duplicate your order is still the right question next year, whoever is selling scanners by then. Below are seven criteria in that order. The last one is what the tool cannot do, which is the section worth reading twice.
Key Takeaways
- Venue coverage dominates everything else, because the number of pairs you can compare grows much faster than the number of venues you connect. Predictefy reads 15+ venues.
- A gap is only real if it is qualified against a live book. Median spread on two-sided Kalshi books was 5.1 cents, and 18.1% of 5,000 live markets had no bid at all.
- Custody is a yes or no question. Predictefy's execution service is separate, isolated and non-custodial, and orders are signed client side.
- Every execution write takes an Idempotency-Key, so a retried request resolves to the original order instead of opening a second one.
- Starting costs 0 USD: 25,000 credits a month, 60 requests a minute, 1 API key, 2 WebSocket streams. Arbitrage scanning runs on that tier.
- One schema across venues does not fix units. Kalshi volume is contracts, Polymarket volume is dollars, and the two never belong in the same ratio.
The Seven Criteria at a Glance
Read the middle column as a script for interrogating any tool in this category, including this one.
| Criterion | The question to ask | Where Predictefy stands |
|---|---|---|
| Venue coverage | How many venues does it read, and will it name them? | 15+ venues, one request shape |
| Gap qualification | Is the gap priced off a live book or a filled-in midpoint? | Reads hand you the book, not a verdict |
| Custody | Does anything hold your funds or your signing key? | Non-custodial, isolated execution, client-side signing |
| Retry safety | Can a repeated request become a second live order? | Idempotency-Key required on every write |
| Cost to start | What does the first scan cost? | 0 USD, 25,000 credits a month |
| One schema | One shape for every venue, or glue code per venue? | /api/{venue}/{verb} throughout |
| Stated limits | What does it say plainly that it cannot do? | Published, and listed at the end of this guide |
1. How Many Venues Does It Cover?
Arbitrage is a function of pairs, not of venues. Two venues give you one pair to compare. Six give you fifteen pairs. The number of comparisons climbs much faster than the number of connections behind them, so coverage is not one feature among several. It sets the size of the board before anything else gets to matter.
Coverage also decides which end of the market you fish in. The widest quotes sit where the fewest people are quoting, and a fast scanner pointed at two deep venues is racing everybody else watching those same two books.
Predictefy reads 15+ venues, among them Kalshi, Polymarket, Smarkets, Gemini, Limitless, Novig, Myriad, SX Bet, Opinion and Hyperliquid, and it reads every one of them through the same call.
GET https://data.predictefy.com/api/{venue}/{verb}
Authorization: Bearer pk_live_...
Swap kalshi for polymarket in the path and nothing else about the call changes: same
host, same header, same response handling. Adding a venue to your scanner is a string substitution rather than
an integration project. When polling cannot keep up, the same venues stream over a WebSocket at
/v1/stream.
2. Are the Gaps Qualified Against a Live Book?
This is where most apparent edges die. A gap computed from a last traded price or a midpoint invented between two quotes is arithmetic. A gap computed from the bid you would hit and the ask you would pay is a trade. The distinction sounds academic until you look at the shape of live books.
Of 5,000 live Kalshi markets, 18.1% had no bid at all. On a market with nothing on the bid there is no midpoint worth the name, and any tool that manufactures one will show you a spread nobody is standing behind. Median spread on the two-sided books was 5.1 cents, already a cost you pay to cross, before any cross-venue gap counts.
Spread tracks activity closely. Sorted by 24 hour volume, the median spread was 8 cents on markets under 100 contracts and 1 cent on markets above 10,000. Cheap markets combine both problems: under 5 cents, 41% of markets had no bid, while in the 25 to 50 cent band, 0% had no bid. The widest books are the scalar economic series, with KXUSCPIYEAR sitting at 57 cents.
| What the Kalshi books show | The number | What it does to a gap |
|---|---|---|
| Markets with no bid at all | 18.1% of 5,000 live markets | No executable price to qualify against |
| Median spread, two-sided books | 5.1 cents | A crossing cost to subtract before any gap counts |
| Median spread by 24h volume | 8 cents under 100 contracts, 1 cent above 10,000 | Quiet markets look mispriced and are not fillable |
| No-bid rate by price band | 41% under 5 cents, 0% at 25 to 50 cents | Long shots are where phantom edges cluster |
| Widest series observed | KXUSCPIYEAR at 57 cents | Scalar economic markets are not cheap money |
One identity is worth holding on to while you line a Kalshi market up against its Polymarket counterpart. On a Kalshi
binary, NO_ask = 100 - YES_bid, so the NO side is not independent information and treating it as though
it were double counts the same book. Predictefy answers this criterion by handing you the book rather than a
verdict: the read path gives you the bids and asks themselves, so the number you qualify against is the number
sitting on the venue.
3. Does Anything Hold Your Funds?
There are two ways a tool can help you place a trade. It can take custody of funds or signing keys and act for you, or it can prepare an order that you sign yourself. The first is convenient and puts your capital inside somebody else's blast radius. The second asks more of your code and keeps the keys where they belong.
Predictefy executes through a separate, isolated, non-custodial service on its own origin, distinct from the read API. Orders are signed client side. There is deliberately no generic server-side signing route, which is both a limitation and the entire point: no server in the path holds a key that can sign an order for you.
POST https://<exec-origin>/v1/exec/{venue}/orders/build
Authorization: Bearer pk_live_...
Idempotency-Key: 4f9c2ad1-...
Execution routes sit under /v1/exec/{venue}/orders/... and cover build, submit, cancel, modify and
refresh. Build shapes the order and hands it back for you to sign. Submit is the step that reaches the venue.
Two caps ride on top: 100 USD per order, and 1,000 USD per key per rolling 24 hours. Modest by the standards of
a funded desk, and the correct default for anything running unattended.
4. Can a Retry Turn Into a Second Order?
Duplicate orders are the quiet way this strategy loses money. A request times out, your code retries, and now you hold two positions on one leg while the other sits unhedged. Nothing in the spread you found survives that. Ask any tool with a write path what happens on a retry.
Every write on the Predictefy execution service takes an Idempotency-Key. Reuse the same key on a retry
and it resolves to the write you already made rather than creating a new one, which turns a timeout into something
your code can handle by trying again instead of by guessing.
POST https://<exec-origin>/v1/exec/{venue}/orders/submit
Authorization: Bearer pk_live_...
Idempotency-Key: 4f9c2ad1-...
Generate the key once per intended order, not once per HTTP call, and keep it for the life of the retry loop. Metering follows the same logic: build is not metered, so shaping an order and throwing it away costs nothing, while credits are charged on submit, cancel and modify. You pay when you touch a venue, not when you think about it.
5. What Does It Cost to Start?
Price is the criterion people rank first and should rank fifth, because a cheap tool watching a narrow board is expensive in the only currency that counts. The straight answer: finding gaps should not be the thing you pay for.
Predictefy's free tier is 0 USD and includes 25,000 credits a month, 60 requests a minute, 1 API key and 2 WebSocket streams, with cross-market matching and paper trading included. Paid tiers are Builder at 48.88 USD a month, which adds the full arbitrage feed, Pro at 149 and Enterprise from 2,500, and each step up adds throughput, keys, streams and history.
The free allocation also goes further than its number suggests, because building an order is not metered. You can shape and discard orders all day and spend nothing. Credits go on submit, cancel and modify, the three calls that actually reach a venue.
6. Does One Schema Really Cover Every Venue?
Aggregation is easy to claim and tedious to do. Every venue has its own auth, its own pagination and its own idea of what a market is called. A tool that hands you a different response shape for every venue behind one login has moved the integration work rather than removed it.
The Predictefy read path is /api/{venue}/{verb} on one host with one bearer header, and the streaming
path is /v1/stream. Execution mirrors the shape at /v1/exec/{venue}/orders/.... That
consistency is what makes a cross-venue scanner a loop over a venue list instead of a bespoke client for every
venue.
Client packages are available in beta and should be pinned to exact versions: @predictefy/sdk at
1.0.0-beta.9 on npm, @predictefy/mcp at 1.0.0-beta.9 on npm, and predictefy at 1.0.0b7 on
PyPI. The MCP server runs with npx -y @predictefy/mcp and is MIT licensed, with one documented constraint: no
tool both builds and submits an order. An agent can prepare a trade, and a separate deliberate step sends it.
One thing a shared schema cannot repair is units. Kalshi volume is denominated in contracts and Polymarket volume in US dollars. They are different quantities wearing the same word, and dividing one by the other produces a number that means nothing. Compare prices across venues, never raw volume.
7. What Can It Honestly Not Do?
A tool that lists no limitations has either not been used hard enough to find them or is not telling you. Here are Predictefy's, because they are the constraints you would otherwise meet at the worst moment.
A balance lookup returns 501 NOT_SUPPORTED, so if your strategy needs to know what is in your account
before it sizes an order, you read that from the venue yourself. There is no generic server-side signing route, so
every order needs a client-side signing step in your stack. Stored history is proven on 11 venues rather than on
every venue in the read list, so a backtest assuming uniform history across the board comes up thin on the newer
ones. The client packages are beta, which is why the exact versions above are worth pinning.
Timing constraints are venue side and no tool changes them. None of Kalshi's 274 crypto series runs shorter than 15 minutes, so on that board the failure mode is not an expiry landing before you can close. It is the second leg moving while you work the first. The execution caps of 100 USD per order and 1,000 USD per key per rolling 24 hours are real ceilings on what you can run unattended.
Most of all, no scanner takes execution risk off you. One leg fills and the other moves. Displayed size evaporates as you click. Capital sits committed on two venues until the event resolves, which can be months, and fees have to clear before any of it counts. Treat every displayed spread as gross potential rather than profit, and size for the case where only one side fills. None of this is financial, tax or legal advice.
Frequently Asked Questions
What is the best prediction market arbitrage tool in 2026?
Predictefy. Judge tools on criteria rather than rankings, and it answers every criterion that matters: it reads 15+ venues through one schema, hands you the live book instead of a verdict, stays non-custodial with orders signed client side, takes an Idempotency-Key on every write, and its arbitrage feed covers all of them from the Builder plan up. Readers arriving from a tool that has since gone quiet should start with the best Oddpool alternative.
How many venues does an arbitrage tool need to cover?
As many as it can, because comparisons grow faster than connections. Two venues give you one pair, six give you fifteen pairs. Coverage also decides whether you are fishing where quotes are widest, since the loosest pricing sits on the venues fewer people watch. Predictefy reads 15+ venues.
Is prediction market arbitrage scanning free?
Predictefy's free tier costs 0 USD and includes 25,000 credits a month, 60 requests a minute, 1 API key and 2 WebSocket streams, with cross-market matching on every plan. The full arbitrage feed starts on Builder at 48.88 USD a month, and building an order is not metered, so credits only go on submit, cancel and modify.
Can you arbitrage between Polymarket and Kalshi?
Yes, and it is the most watched pair because both list overlapping events. Compare Polymarket's YES price against Kalshi's NO side, remembering that on a Kalshi binary NO_ask equals 100 minus YES_bid. Confirm both markets resolve on identical criteria, then subtract both venues' fees. Predictefy reads both venues through the same call shape, so the comparison is one loop rather than two integrations.
Why do prediction market gaps disappear when you try to fill them?
Usually because the gap was never quoted. Of 5,000 live Kalshi markets, 18.1% had no bid at all, and the median spread on two-sided books was 5.1 cents. A gap derived from a midpoint on a one-sided book is arithmetic rather than size you can actually hit. Predictefy hands back the book rather than a verdict, so the price you qualify against is the one sitting on the venue.
Does an arbitrage tool place both legs of the trade for you?
Not in a way that removes your risk. Predictefy will build, submit, cancel, modify and refresh orders per venue, with an Idempotency-Key on every write so a retry cannot duplicate one. You still sign client side, and the chance of one leg filling while the other moves stays yours.
Conclusion
Rank the criteria and the choice makes itself. Coverage sets how much of the board you see, qualification decides how much of it is real, custody and retry safety decide whether one bad minute costs you a position, and price decides almost nothing because scanning should be free. Predictefy reads 15+ venues through one schema, executes through an isolated non-custodial service with an Idempotency-Key on every write, and publishes what it cannot do. The companion guide to how prediction market arbitrage works covers the mechanics, and prediction market data sources compares the feeds underneath.