Using novel credit data, we show that hedge fund borrowing is significantly overcollateralized, primarily with rehypothecable securities. An idiosyncratic liquidity shock to a major prime broker significantly decreases credit to connected hedge funds. The dominant channel behind this shock transmission is credit supply reduction rather than precautionary demand reduction. Funds posting more rehypothecable collateral are less affected because their collateral alleviates prime broker liquidity constraints. Exposed funds subsequently have lower aggregate credit with worse terms, suggesting imperfect substitutability across hedge fund credit sources. Funds subject to the decrease in balance sheet leverage subsequently increase portfolio illiquidity, embedded leverage, and derivatives exposure.

We exploit the 2020 Treasury market shock to analyze how external and internal constraints impact arbitrageurs. Using regulatory filings, we find that hedge funds reduced arbitrage activities and increased cash holdings, despite stable credit and low contemporaneous redemptions. Creditors’ regulatory and liquidity constraints were not propagated to hedge funds through repo—Treasury arbitrageurs’ predominant financing source. Fund-creditor borrowing data reveal more regulated dealers provided, and more important clients received, disproportionately higher funding. Value-at-risk reported by funds suggests internal risk constraints were binding. Our results support theoretical predictions that arbitrageur risk constraints and precautionary liquidity management can amplify market instability even when contemporaneous financing remains resilient.