Polymarket Kalshi Arbitrage
Cross-venue prediction markets for AI agents: Polymarket + Kalshi, costed and attributed
- Transport
- Not stated
- Package
- —
- Registry id
- app.oddsrail/polymarket-kalshi-arbitrage
No install snippet on purpose. A working MCP config is a command, its arguments and an environment block — the last two are where API keys live, so this catalogue never stores them and cannot publish them. Follow the link above for the authors' own instructions.
The rail AI agents use to trade prediction markets.
An MCP server that gives any agent (Claude Code, Claude Desktop, or anything MCP-compatible) prediction-market access across Polymarket and Kalshi: market search, orderbooks, price history, positions, and order routing, with on-chain builder-code attribution on Polymarket, plus two premium signal tools (in-play overshoot/fade detection, resolution dispute-risk).
Free to use, and free of fees. oddsrail ships with a project builder code
registered at 0 bps, so orders routed through it are attributed without
adding a single basis point to anyone's trade. The project's income is a share
of Polymarket's weekly builder reward pool, paid by Polymarket's own program,
not by you. Running your own builder profile instead is one environment
variable (ODDSRAIL_BUILDER_CODE), and server_info always tells you which
code is in use. No fee tiers, no paywalled tools, no account required.
How oddsrail compares
Verified against each alternative directly (their repos, live endpoints, and registry entries, September 2026), not from their marketing:
| oddsrail | raw venue APIs | pmxt | Simmer | Polymarket agent-skills | |
|---|---|---|---|---|---|
| What it is | self-hosted MCP server | the venues themselves | unified API + SDK + MCP, "CCXT for prediction markets" | agent trading platform + SDK + MCP | markdown skill docs for agents |
| Venues you can trade | Polymarket, Kalshi | one each | Polymarket, Opinion, Limitless (hosted writes); a dozen more for data | Polymarket, Kalshi, plus its own $SIM sandbox markets | Polymarket only |
| Custody | non-custodial; keys never leave your machine | yours | hosted mode: "PMXT handles custody, signing infrastructure"; self-hosted mode: your keys | self-custody, local signing | yours (documentation only) |
| Attribution you control | yes: ODDSRAIL_BUILDER_CODE overrides the 0 bps default |
n/a | not documented | not documented | documents builder headers for your own code |
| Cost to the trader | 0 bps, free tools | free | hosted pricing not in the README | not documented | free |
| Open source | MIT, full source | n/a | MIT, ~2.1k stars | not stated | docs; license not stated |
| Operator guardrails | notional caps, open-order cap, allowed markets; enforced pre-request, in dry-run too | none | not documented | per-trade limits, daily caps, stop-loss/take-profit, kill switch | none |
| Paper trading | dry-run fills against the live book, P&L | none | not documented | virtual $SIM sandbox, then graduate to real money | none |
| Book-walked cost, settlement audit, jurisdiction-classified failures, dated venue-quirk notes | yes, all four | no | not documented | not documented | quirks partly documented |
| Realtime | watch_book, bounded |
websocket, yours to wire | not documented in the README | not documented | websocket documented |
Verified 2026-09-02 from each project's own README or docs (pmxt: github.com/pmxt-dev/pmxt; Simmer: docs.simmer.markets; agent-skills: github.com/Polymarket/agent-skills). "Not documented" means exactly that, not "absent". Re-check before quoting; these projects move.
The wedge, in one line: pmxt is the reference for trading everywhere; Simmer is the reference for an agent economy with a sandbox and a reputation layer; oddsrail is the reference for trading correctly, non-custodially, with attribution you own.
Where the others are honestly ahead: pmxt trades three venues to our two and covers a dozen more for data, with hosted convenience and a community many times ours. Simmer has a virtual-balance sandbox, stop-loss and take-profit rails we do not have, a public reasoning/reputation layer, and a strategy-skills marketplace. Polymarket's agent-skills is the venue's own documentation and covers bridging and deposits, which oddsrail does not.
From the project's README.