ProbBrain — Free Live Polymarket & Kalshi Arbitrage Scanner with a Public Track Record

ProbBrain is a free, public dashboard for prediction-market enthusiasts. We do two things: (1) scan Polymarket and Kalshi for live arbitrage opportunities, and (2) publish individual market signals with a transparent track record. Both run automatically, and both are visible at probbrain.com.

This post walks through what we just shipped, the numbers behind it, and why we ship our scoreboard publicly even when it makes us look bad.

The Live Arbitrage Scanner

The scanner runs every 15 minutes against both Polymarket and Kalshi. It surfaces opportunities in three tiers, sorted by net edge after fees:

• Tier 1 — Actionable. True risk-free arbs after estimated fees. These are rare on purpose and get arbed away in seconds when they appear.
• Tier 2 — Inconsistencies. Within-Polymarket sum violations, within-Kalshi monotonicity breaks, or cross-platform mismatches that aren't quite risk-free but still represent real mispricings worth watching.
• Tier 3 — Signals. Lower-conviction watch list — useful for tracking how spreads evolve.

Each card shows the legs (which markets to take which side of), gross and net edge after our conservative fee model (Polymarket taker + Kalshi 0.07·P·(1−P) + 1.5% spread + 0.5% gas + 8% capital cost annualized), executable size estimates, and a one-line "weakest link" summary that names what could go wrong.

You can filter the entire list by underlying (BTC, ETH, SOL, FED rate, CPI, NFP) and by platform (Within-Poly, Within-Kalshi, Cross-platform). The platform filter applies globally across all tiers, and an inline counter on each tier header shows how many opportunities match what you've selected.

Curated Cross-Platform Pairs

Most automated cross-platform matching breaks because Polymarket and Kalshi sell different products on the same underlying — Polymarket ladders by year-end while Kalshi ladders by daily, weekly, or by settlement window. We hand-validate pairs where the two venues are genuinely asking the same question, and the scanner uses those curated entries to find arbs that strict canonical-key matching would miss.

Currently live: BTC year-end 2026, ETH year-end 2026, and SOL year-end 2026 — each pairs the Polymarket year-end price ladder against the matching Kalshi MAXY series. The full list of curated pairs grows as we manually verify each candidate (no fuzzy matching allowed in this layer — that's a hard architectural rule).

The Signals Track Record

Alongside the scanner, we publish individual market signals — calls where we think a Polymarket market is materially mispriced. Every signal is logged the moment it goes out, and every resolution updates the public scoreboard.

Current numbers as of today:

• 35 signals resolved
• 15 correct (42.9% accuracy)
• Brier score: 0.37
• 6/6 correct in the last 24 hours (Iran-overpriced cluster all settled NO as April closed; Bitcoin held above $55k)

That overall accuracy number is honest — we don't cherry-pick winning streaks. Some categories perform much better than others; geopolitics has been our weakest bucket and the dashboard reflects that explicitly.

Why Honesty Beats Hype

Most signal services bury their losers. We do the opposite for one specific reason: a public scoreboard is the only credibility primitive that can't be faked.

Two concrete commitments:

• Calibration is applied everywhere. When our model says "high confidence" but our historical accuracy in that category is poor, the displayed confidence on the dashboard is shrunk toward the market price. Both the Telegram alert and the public dashboard derive their confidence label from the same calibrated gap, so the two surfaces always agree.
• Tier 1 is allowed to be empty. If there are no risk-free arbs right now, the dashboard says so plainly. We'd rather show zero Tier 1 cards than promote a borderline opportunity to make the page look busy.

Same logic applies to the curated arbitrage pairs: every entry includes a "weakest link" summary explaining what could break the assumption. If we can't articulate one, the pair doesn't ship.

Where to Start

Open probbrain.com/arbitrage for the live scanner, or probbrain.com for the signals dashboard and current accuracy. The methodology page (probbrain.com/methodology) documents every detection pass, the fee model, and how tier assignment works — useful if you want to verify our claims rather than take them on faith.

Follow @ProbBrain on X for new signal calls and resolution recaps as they happen.