Tail Correlation API

Measures the thing that destroys portfolios: correlations that look comfortably low in calm markets but spike toward 1 exactly when the market crashes, so the diversifiers you were counting on all fall together — computed live from Yahoo Finance daily closes, no key, nothing stored. A normal full-sample correlation hides this by averaging the calm days with the crisis days; this API instead conditions on the benchmark's extremes. For each asset it returns the ordinary correlation to the benchmark, the crash correlation (measured only on the benchmark's worst days — its lower tail), the rally correlation (on its best days), and the breakdown: how much the correlation rises in a crash versus normal. A bond, gold or commodity position with a low normal correlation but a high crash correlation is a false diversifier; one whose correlation stays low or falls in the tail is a genuine hedge. The asset endpoint returns one instrument's full tail-correlation profile; the screener endpoint ranks the cross-asset universe by crash correlation, surfacing which holdings actually fail when you need them. This is the conditional / tail-correlation cut — distinct from the unconditional cross-asset, sector and FX correlation matrices (which average all days together), the up/down capture API (magnitudes, not co-movement) and the price APIs. It is correlation when it matters: in the crash.