WASHINGTON >> A deal on Capitol Hill that could cut prescription drug costs for millions of Medicare beneficiaries was cautiously cheered by older Americans and their advocates Thursday even as many worried it might never come to fruition.

The health care and climate agreement struck by Senate Majority Leader Chuck Schumer and Democratic Sen. Joe Manchin includes landmark provisions that could help senior citizens, including a cap on out-of-pocket Medicare drug costs and a requirement that the government negotiate prices on some high-cost drugs.

Some of the issues been talked about for decades and proved elusive.

Several prongs of the proposal have the potential to bring relief to millions of people:

• It would cap out-of-pocket drug costs at $2,000 a year for Medicare beneficiaries. No such limit exists today, and some older people on costly drugs can run up bills of tens of thousands of dollars.

• It would give Medicare, for the first time, the ability to directly negotiate with pharmaceutical companies over the price of drugs, though the number of medications subject to the provision would be limited.

• It would create a new “inflation rebate” requiring drug companies to give refunds to Medicare beneficiaries for increases in prescription drug prices that exceed the rate of inflation.

• It would provide coverage of vaccines for seniors.