Polymarket CLOB API

What it is

The trading half of Polymarket, and the only place its prices actually come from. The CLOB API at clob.polymarket.com serves the order book for an outcome token, the best bid and ask, the midpoint, the spread and the last traded price, in single and batch form — and then, behind authentication, takes orders, cancels them and reports your open ones.

The split from the Gamma API is not cosmetic. Reading a book here takes nothing at all; placing an order takes a wallet, a signed message, derived credentials, a per-request HMAC signature and collateral on Polygon. Those are two different products with two different sets of preconditions, which is why they are two cards.

Real-time comes as two WebSocket channels: a public market channel carrying book snapshots, price changes, last trade prices and tick-size changes, and an authenticated user channel carrying your own orders and fills.

Availability

Trading is blocked in the United States on this platform. Polymarket's international site says so in its own words and offers a US visitor view-only mode, directing them to Polymarket US — a separate entity operating as a CFTC-designated contract market, with its own app and its own API. The footer states plainly that the international platform is not regulated by the CFTC.

Everywhere else, availability is a wallet question rather than a signup question. There is no KYC step in the documented flow: control of a Polygon address, a signature, and collateral are the whole of it. That also means there is no account to be refused, and no support desk that reviewed your case — the geoblock sits in front of the web app, not in front of the endpoint.

For a developer this is the sharp edge worth knowing before you build: the read endpoints will answer you from anywhere, so a successful call proves nothing about whether you may trade what you just read.

Pricing

The API is free. The trades are not.

Polymarket charges takers only, and the fee is a curve rather than a rate: fee = shares × feeRate × price × (1 − price). The rate depends on the market's category — 0.07 for crypto, 0.05 for sports, economics, culture, weather and the general bucket, 0.04 for finance, politics, mentions and tech, and zero on geopolitical and world-event markets. Because of the price × (1 − price) term, the fee in dollars peaks at 50 cents and falls symmetrically toward both ends: on a 100-share crypto trade Polymarket's own table shows $1.75 at 50 cents and 33 cents at 5 cents or 95 cents. Makers pay nothing and are rebated a share of taker fees daily.

This card's figure is that general 0.05 rate expressed at its worst point — 1.25% of a dollar of payout on a fill at 50 cents. Read the category table before budgeting, because the spread between crypto and a geopolitical market is the whole fee.

Deposits and withdrawals carry no Polymarket fee, though the docs note that intermediaries may charge their own.

Markets & resolution

Resolution is the UMA optimistic oracle, not Polymarket's judgement. Anyone may propose an outcome against a bond of roughly 750 pUSD; there is a two-hour challenge window; a dispute triggers a second proposal round, and a second dispute escalates to a UMA token-holder vote after a debate period of 24 to 48 hours. Polymarket's published timeline is about two hours undisputed and four to six days disputed.

Three consequences for code:

• A market that stops trading has not paid. Redemption happens after resolution, through the collateral adapter, and returns pUSD.
• A UMA vote can return "Unknown", in which case the market resolves 50/50 and both tokens redeem at 50 cents.
• Rules can be clarified after trading starts, through an on-chain bulletin board entry that voters are expected to weigh. The clarification cannot change the intent of the question, but it can change which real-world result counts.

Integrations

REST plus two WebSocket channels, with unified TypeScript and Python SDKs published by Polymarket and an OpenAPI description linked from the docs. Authentication is two layers: an EIP-712 ClobAuth message signed by the wallet (L1) to create or derive API credentials, then HMAC-SHA256 request signing with a key, secret and passphrase (L2). The signed payload is timestamp plus method plus path plus the exact body bytes, which is the usual place an implementation goes wrong.

Price history is not here. It sits on Polymarket's Data API, keyed by token ID, with bucket widths from 60 seconds upward — and the docs are explicit that only coarse buckets cover the full history, while finer ones return the last 30 days.

Limitations

• No US trading on this platform, whatever the endpoint will answer.
• No published rate limits. The builder tiers say Standard and Highest and leave it there.
• No test environment. There is no sandbox or testnet for the production CLOB; rehearsal means small live orders.
• Fees are a formula with a per-category coefficient, so a single number on any comparison page — including this one — is an approximation.
• The matching engine restarts. Polymarket documents maintenance windows and a post-restart post-only mode, which a naive bot will read as rejected orders.
• No historical order book. You can reconstruct prices from the history endpoint, but the depth that produced them is not retained.

Alternatives

For discovery and market metadata against the same venue, use the Polymarket Gamma API — it is the other half of the same integration and needs none of the signing above. If the trading side has to be a US-regulated exchange with identity verification and a published rate-limit table, that is the Kalshi API. For an on-chain book with maker rebates and a much shorter market lifespan, see the Limitless API.