The Federal Reserve took the final step to ensure it can’t repeat the extraordinary steps taken to rescue American International Group Inc. and Bear Stearns Cos. in 2008, adopting formal restrictions on its ability to help failing financial firms.

Under the revised authority approved in a unanimous vote Monday, the Fed would only be able to save firms in a broad-based scenario including at least five entities at the same time. The changes are designed to reflect Congress’ intention in the 2010 Dodd-Frank Act to prevent the central bank from bailing out individual companies.

“The ability to engage in emergency lending through broad-based facilities to ensure liquidity in the financial system is a critical tool for responding to broad and unusual market stress,” Fed Chair Janet Yellen said before the vote at a meeting in Washington.

Lawmakers from both parties including Democratic Senator Elizabeth Warren complained that an initial proposal released almost two years ago didn’t go far enough and left regulators too much wiggle room to orchestrate backdoor bailouts. Yellen said the final rule included “significant changes” reflecting public comments on the earlier proposal.

The revised version defines a “broad-based” lending program as “requiring that at least five persons be eligible to participate,” according to a Fed memo. It also says solvent firms can’t borrow to pass the proceeds of emergency loans to insolvent firms.

In addition, emergency loans can only be made at a “penalty rate” that is higher than the market rate, and the Fed will be required to review every six months whether a loan program should be ended. Under the final rule, emergency credit can initially be made and renewed for a maximum of one year.