Dodd-Frank does allow the Fed to set up emergency loan facilities that many firms can tap in a crisis. And the legislation required that the Fed devise measures that would prevent the loans from going to insolvent firms.

Members of Congress suggested some of the measures that the Fed included in its final rule. To make sure the loans are broadly available, the rule says that at least five firms have to be eligible to borrow from an emergency loan program, preventing the loans from being tailored to suit just one. That number of eligible firms was contained in a bill introduced into the Senate in May by Elizabeth Warren, Democrat of Massachusetts, and David Vitter, Republican of Louisiana.

The Fed’s rule also contains specific measures to make sure insolvent firms are disqualified from the emergency loans. The Fed’s rule says an entity cannot borrow for the purpose of lending to another entity that is insolvent. Representative Jeb Hensarling, Republican of Texas and chairman of the House Financial Services Committee, called for such a measure in a letter that he sent to the Fed last year.

Consumer advocates said they were pleased with some of the measures that the Fed included. “This rule contains a lot of commitments to be tougher,” said Marcus Stanley, policy director at Americans for Financial Reform, a group that has lobbied for tougher financial regulation.

But they said the Fed had taken steps to protect its latitude to act. “The rule still has very extensive Federal Reserve discretion in it,” Mr. Stanley said.

And consumer advocates say the rule could yet allow seriously stressed firms to borrow from the Fed. While the Fed may offer a loan to many firms, one or two banks that are in desperate need may benefit the most from the Fed’s assistance, effectively obtaining a bailout on the sly. In 2008, for instance, a handful of potentially insolvent banks might have collapsed had they not borrowed many billions of dollars from the Fed’s emergency facilities.

One challenge for the rule is that solvency can be a hard-to-determine concept, particularly in a crisis.