One hurdle to a possible fix for recent volatility in the short-term cash markets: hedge funds.

Federal Reserve officials are considering a new tool to ease stresses in the market for Treasury repurchase agreements, or repos. Through the repo market, banks and hedge funds borrow cash overnight, while pledging safe securities such as government bonds as collateral. In September, an unexpected shortage of available cash to lend sparked a surge in the cost of repo-market borrowing, prompting the Fed to intervene for the first...