Prediction Market Data APIs

What is in this category

APIs that hand you the state of somebody else's prediction market: what is listed, what it costs right now, what the book looks like behind that price, and what happened after the question closed. Most of them are published by a venue about its own markets, which means the card you are reading is about a product with a different audience from the venue it belongs to — the venue is bought by someone taking a position, the API by someone writing code against prices.

No venue API below needs a funded account to read, so most of what is claimed here can be checked from a terminal. The aggregators and archive vendors are where that stops. FinFeedAPI, Predictefy and Predexon each answer an unauthenticated request with 401, and FinFeedAPI sells no free plan at all; Adjacent's keyless tier is delayed and, in practice, one venue wide. DepthFeed is the exception among them — it keeps one keyless demo endpoint that returns a real order book.

"Real money" is a question about you, not about the API

Every venue in this catalogue trades contracts that pay 1 dollar to the winning side. Whether you personally can hold one is decided somewhere else entirely: by where you live, by which corporate entity operates the platform you connected to, and by which regulator, if any, stands behind it.

The same brand can be two answers at once. Polymarket's international platform tells a US visitor that trading is blocked and offers view-only mode, pointing at Polymarket US — a separate entity operating as a CFTC-designated contract market. Limitless's terms, last updated 15 September 2026, bar trading by users in the United States, the Republic of China, Ontario and Alberta — while its public API endpoints, which those terms say nothing about, answered an unauthenticated request all the same. Kalshi is a CFTC-regulated exchange whose market data is public and whose account is not.

So "real money" tells you nothing on its own. The questions that do: which entity's terms am I accepting, does it serve my jurisdiction, and what does it do when it decides I am not eligible — refuse the signup, decline the deposit, or let me read the book and nothing else.

"Resolution" means four different things

A card that says a market "resolves against official data" has answered one quarter of the question. Four separate facts hide in that word, and each of them changes what a price means and how long your capital is locked up.

Who decides. Polymarket resolution runs through the UMA optimistic oracle — anyone may propose an outcome against a bond, and a contested proposal ends at a vote of UMA token holders, not at the company. Limitless resolves the majority of its markets automatically from Pyth Network, short crypto markets from a Chainlink time-weighted average, and custom event markets by hand. Kalshi determines the outcome itself, as the exchange, under the rules of the series.

Against which source. Never the headline, always the document. The source lives in the individual market's rules, and the rules also define the edge cases; the title is only the question.

On what timetable. An undisputed Polymarket resolution is roughly two hours after the proposal, a disputed one four to six days. Limitless pays out automatically at the deadline where an oracle decides, and in 24 to 72 hours where a person does. Timetables matter to anyone writing code, because a market that has stopped trading is not yet a market that has paid.

What happens to an ambiguous question. This is the part nobody documents on a comparison page. A UMA vote can return "Unknown", in which case every token redeems at 50 cents each. Limitless treats an unapplicable rule as a misresolution and refunds the amount put in — not the dollar a winning share would have paid. These are different outcomes for the identical situation, and neither is visible in an order book.

Reading data and placing an order are two different permissions

The gap between the two is the most common surprise in this category, and it is wider than "get a key".

On Polymarket, the read endpoints take no credentials at all. Placing an order takes a Polygon wallet, a signature chain of an EIP-712 message and then HMAC-signed requests, and a balance in pUSD — the USDC-backed collateral token every Polymarket trade now settles in. No identity check, and no trading at all from the US on the international platform.

On Kalshi the shape is inverted. Market data is an open REST call, but an account is a regulated brokerage-style relationship: identity verification first, then an RSA key pair generated in the account settings, then every request signed. The exchange's own WebSocket will not even complete a handshake without those headers.

On Limitless, a scoped API token is issued from the web UI to an account that is a connected wallet, and orders are signed with EIP-712 against the market's venue contract on Base. There is no test environment — the docs say so plainly, and suggest rehearsing with minimum-size live orders.

What is actually free, and where the free part ends

The venue APIs below are free to read and none of them sells a data plan; the aggregators and archive vendors do sell one, and one of them has no free tier at all. On the venue side, what varies is what "free" runs out against.

Rate limits are the real ceiling, and how legible that ceiling is depends entirely on the venue. Kalshi documents a token bucket per account tier — most endpoints cost 10 tokens, the entry tier refills 200 read tokens and 100 write tokens per second — so you can compute your sustained request rate before writing a line. Polymarket US states 20 requests a second, per API key on the authenticated half and per IP on the public one. Manifold states 500 a minute per IP, and Futuur 2,000 a minute and a million a day per user. Polymarket's builder tiers name rate limits as "Standard" and "Highest" without saying what either is. Limitless states that limits exist, returns 429 with a Retry-After header when present, and invites you to write in for a quota.

The second ceiling is history. Live endpoints do not go back forever, and each venue draws the line differently: an archival cutoff that keeps moving on Kalshi, a bucket-width-dependent window on Polymarket, and on Limitless a series that cannot outlive the market it belongs to.

What to check before you build on one

• Does the read endpoint answer with no key? If yes, confirm it yourself in a terminal before believing a table about it.
• Is the WebSocket public? Public prices over REST and an authenticated-only stream is a common combination, and it decides whether you can watch the book without an account.
• Where is the history, and how far back? Ask for the oldest bucket you actually need, and check what comes back rather than what the parameter list implies.
• What does the venue charge the order this feed will produce? Every fee here is a curve, not a rate: Polymarket's taker fee is a formula over the price, Kalshi's is the same shape, and Limitless's buy fee falls from 3.00% to 0.40% as the price rises. Makers pay nothing on Polymarket and Limitless; on Kalshi they pay nothing under the default fee type, but a minority of series carry a maker-fee variant instead — 160 of the 11,078 the series endpoint returned across its categories on 19 September 2026.
• Whose terms apply to the account, not to the data? That is the answer that decides whether the code you wrote can ever place the order it computed.