The agreement that is now embraced by lawmakers of both parties would apply only to the federal prison system, which houses less than 10 percent of the nation’s 2.2 million inmates. It includes one provision to address an issue that has been contentious for years, since the era when a mandatory minimum sentence would apply to a far smaller amount of crack cocaine than powder cocaine.

The disparity, now widely viewed as racially discriminatory, was significantly reduced in 2010, but the change was not made retroactive, so between 2,500 and 3,000 people remain in prison on sentences that would have been much lower today.

The bill would change the system going forward, with more than 2,000 people a year potentially able to avoid mandatory minimum sentences, out of some 60,000 admitted to federal prison each year.

Some provisions, such as one that clarifies that prisoners are allowed to earn 54 days of credit off their sentence for good behavior each year instead of 47 days, would provide a small benefit to a large number of people.

Others would give a large benefit to a very small number of people. The bill would end the practice of counting gun offenses for which the person has not yet been convicted as priors that could add 25 years to a sentence.

Some advocates said measures that would not significantly reduce the prison population were counterproductive. “Creating confusion over what’s significant reform and what isn’t, allowing them to portray minor improvements as major victories, enables them to preserve all of the key pieces of an infrastructure of mass human caging while making the public think they’re dismantling it,” said Alec Karakatsanis, the executive director of Civil Rights Corps, a public interest law firm in Washington.

Support for overhauling the criminal justice system has grown in recent years because the nation has incarcerated far more people than any other country in the world, while crime has been on the decline for more than two decades. Evidence has mounted showing that the country’s sprawling system of punishment was counterproductive, resulting in high recidivism rates. Studies showed that even brief stays in jail disrupt people’s lives and make them more likely to commit crime. And many states realized that without substantive change they would be spending an ever-greater portion of their budgets on prisons.