Coach definition, not a live verdict. These pages show how the X-ray Coach defines this concept and where its thresholds sit, sourced from the coach's own code. Unlike the tactic pages, nothing here runs a live detector — example boards illustrate the rule, they don't execute it.
What it means
- The foundation metric the whole coach is built on: how often, and how badly, your moves lose ground.
- Severity is split into tiers — turning a winning position into equal, versus turning an equal position into losing.
The exact rule the X-ray Coach applies
- A move is a blunder when win probability drops by ≥ 30% (matching Lichess). When win-probability is unavailable, the fallback is an eval drop of ≥ 300 cp.
- Severity tiers: winning→equal blunders and equal→losing blunders are counted separately, so a thrown win is not lumped with a thrown draw.
Why it matters
Win-probability drop, not raw centipawns, is the unit — a 300 cp swing matters far more near equality than deep in a won or lost game.