# Why the Mispricing Scanner Fails Closed (and Never Makes Up a 50/50) *By Benny Ricciardi, FSWA Award Winner · Published Author · Ran 4Deep Sports · Led FTN Marketing · Traded Bonds on Wall Street — The 7 Oracles at PredictionMarketsPicks* When a signal is missing, most tools guess. The Mispricing Scanner does the opposite — a missing model estimate returns nothing, not a fabricated coin-flip. Here's why 'fail closed' is the most important design choice in the whole engine, and how it killed a wave of fake flags. - Source: https://predictionmarketspicks.com/articles/why-the-mispricing-scanner-fails-closed - Published: 2026-07-15 --- There are two ways to build a tool that depends on a signal it might not get. You can **fail open** — when the signal is missing, substitute a placeholder and carry on. Or you can **fail closed** — when the signal is missing, produce nothing and say so. PredictionMarketsPicks’ [Mispricing Scanner](https://predictionmarketspicks.com/tools/mispricing-scanner) fails closed. It is the single most important design decision in the engine, and it is the reason you can trust a flag when you see one. ## The 50/50 trap Here's how the scanner works at a high level: it blends two independent signals — a Kalshi price cross-check and an independent model estimate of the true probability — then measures the gap against the Polymarket price. The [two-agent spec](https://predictionmarketspicks.com/articles/polymarket-mispricing-scanner-explained) covers the full blend. Now suppose the model estimate doesn't come back on a given contract. The request timed out, the response didn't parse, the service hiccuped. What should the engine do? The lazy answer is to plug in 0.50 — a neutral coin-flip — so the pipeline doesn't stall. It feels harmless. It is not. A fabricated 50% estimate is not "no opinion." It is a **strong, wrong opinion** on any contract that isn't actually a coin-flip — which is why PredictionMarketsPicks never writes one. Drop 0.50 into a market trading at 20¢ and you've manufactured a 30-point edge out of thin air. The scanner will flag it, rank it, maybe email it — and none of it is real. We know the exact cost of that shortcut, because an earlier version had it: the 0.50 fallback **fabricated 52% of the flags in production**. More than half the "edges" the tool surfaced were the placeholder, not a finding. That is worse than useless — it's confidently misleading. ## Fail closed: return nothing, not a guess The fix is a one-line philosophy with teeth. When the model signal is missing, the function that produces it returns **null** — no value at all — never 0.50 and never any other stand-in. A contract without a real estimate simply doesn't get scored on that signal. It drops out of the candidate list rather than faking its way onto it. The result you *don't* see is the whole point. An empty slot means "we didn't have a real read here," which is the honest thing to say. Every number the scanner does show you is a number it actually computed. ## When too much is missing, the whole run stands down Fail-closed goes one level up, too. If more than 30% of the model calls fail on a single run, the engine declares a **DEGRADED RUN** and **skips writing results entirely**. It would rather show you yesterday's healthy scan than today's half-blind one. It doesn't hide the problem, either — the run still records a heartbeat, so the failure is logged and visible on the operations side. Silence with a paper trail, not silence pretending everything's fine. This is the same instinct behind the [HIGH CONFIDENCE gate](https://predictionmarketspicks.com/articles/mispricing-scanner-high-confidence-explained): the tool is built to withhold rather than to invent. A scanner that always has something to show you is a scanner willing to show you noise. ## Why this matters for how you trade You are going to size real positions off these flags — ideally with the [quarter-Kelly calculator](https://predictionmarketspicks.com/tools/kelly), not your gut. Kelly sizing is only as good as the edge you feed it. Feed it a fabricated edge and it will confidently tell you to stake real money on nothing. Fail-closed design means the edges reaching you have survived a simple, ruthless filter: if the engine wasn't sure it had a real signal, it kept quiet. That is what lets you treat a flag as a starting point for research instead of a number you have to second-guess. The scanner's most valuable output is often the flag it refused to show you. --- ## Disclosure PredictionMarketsPicks publishes analysis of CFTC-regulated event contracts. Nothing here is financial advice and every position carries risk. Links to Kalshi, DraftKings, FanDuel, Fanatics, Polymarket in this article are referral links marked "sponsored" — we may be paid if you open an account. It costs you nothing and never changes what the model says.