Overview

What PredArena is, what it is not, and how it compares to venue demo environments.

PredArena is a trading and trade-analytics API for prediction markets. You place orders through a REST API; PredArena fills them by walking the LIVE production orderbooks of Kalshi and Polymarket, applies each venue's real fee schedule, and keeps full portfolio accounting — orders, fills, positions, a cash ledger, realized/unrealized P&L, and execution-quality analytics (per-order slippage vs. mid, Sharpe/Sortino, attribution).

It exists for one job: letting bot builders and traders measure whether a strategy actually works — with realistic fills and fees — before risking capital.

What you can do

  • Place market and limit orders (GTC / IOC / FOK) against live Kalshi and Polymarket books — limit remainders rest and fill as makers when the book crosses them.
  • Attach stop, take_profit, or bracket (OCO) protection to positions.
  • Cancel resting orders; replay-protect everything with client_order_id idempotency.
  • Tag orders with a strategy label and get P&L attribution per strategy, series, and venue.
  • Audit every fill: each order records the pre-trade mid price and signed slippage.
  • Export orders, fills, trades, and the cash ledger as CSV.

What PredArena is not

Read 'Simulation vs. Reality' before trusting any backtest-like number.

PredArena fills are not the same as real-money venue execution. PredArena does not model market impact, queue position, or mid-flight book changes — and we document every known divergence, with the data we record so you can measure it yourself. See the Simulation vs. Reality page.

PredArena also does not (yet) offer historical replay/backtesting against recorded books, or order routing to real venues. It runs forward, against live data.

When to use Kalshi's official demo instead

Kalshi runs a free demo environment (docs) with its own API root and separate credentials. If you only need to verify that your code can talk to Kalshi's real API surface — auth signing, order payloads, WebSockets — use the demo. It is the venue's own API and will always be the highest-fidelity integration test.

PredArena is built for a different question: execution and strategy quality. The demo is a separate sandbox environment, not production markets; PredArena fills against the production orderbooks your strategy would actually trade, models venue fees explicitly (with sources cited on every trade), supports Polymarket alongside Kalshi in one account, and layers on the analytics (slippage, attribution, risk ratios) that neither venue's sandbox provides.

Base URL and versioning

https://predarena.org/api/v2

The v2 portfolio API is the current stable surface. Breaking changes will ship under a new version prefix; additive fields may appear on existing responses at any time — parse leniently.