The Supreme Court unanimously decided Monday that a 2018 criminal sentencing law does not allow a chance for those convicted for the smallest amounts of crack cocaine under an old tough-on-crime law to reduce their prison sentences.

In doing so, the justices ruled against what a bipartisan group of senators who championed the 2018 law known as the First Step Act, including the Senate Judiciary Committee’s top members, Chair Richard J. Durbin of Illinois and ranking Republican Charles E. Grassley of Iowa, said they intended with the law.

The senators argued in a brief in the case that they meant to give a chance at resentencing for everyone in prison under the 1986 law that treated crack 100 times more severely than powder cocaine.

“Unfortunately, the text will not bear that reading,” Justice Sonia Sotomayor wrote in a concurrence. “Fortunately, Congress has numerous tools to right this injustice.”

The bill’s authors could pass new standalone legislation, or include a provision in other criminal justice measures the Senate is considering. Justice Brett M. Kavanaugh, during oral argument on the case in May, said Congress could have said “something simple” in the law such as, everyone sentenced for crack offenses under that law is eligible for resentencing.

Justice Clarence Thomas, in the main opinion, wrote that the 2018 law was written in a way that means it applies to defendants who had larger amounts of crack cocaine but doesn’t apply to those who had the smallest amounts.

For that conclusion, the justices had to backtrack through decades of changes in sentencing for powder and crack cocaine that sought to address how Black and Latino defendants disproportionately received harsher sentences.

The prisoner bringing the challenge, Tarahrick Terry, pleaded guilty to possessing 3.9 grams of crack in Florida in 2008 and was sentenced to more than 15 years in prison under the Anti-Drug Abuse Act of 1986, which had three levels of penalties based on how much crack cocaine was possessed. The amount meant Terry was in the lowest level.

In 2010, a law known as the Fair Sentencing Act changed the threshold for how much crack cocaine defendants had to have possessed before they qualified for a higher level and mandatory stiffer sentences.

Thomas wrote that the 2018 law says that those in prison are eligible for a sentence reduction only if they received a sentence for “a covered offense.” And the law defined a covered offense as one changed by the 2010 law, which raised the threshold amount for the higher two categories but not for the lowest level of penalties such as Terry’s.

“The question here is whether crack offenders who did not trigger a mandatory minimum qualify,” Thomas wrote. “They do not.”

Sotomayor wrote separately to “clarify the consequences of today’s decision.”

“While the Fair Sentencing Act of 2010 and First Step Act of 2018 brought us a long way toward eradicating the vestiges of the 100-to-1 crack-to-powder disparity, some people have been left behind,” Sotomayor wrote.

Among those are Terry, whose sentencing was complicated by his designation as a “career offender,” in part because his possession offense now is treated “as a far less serious crime,” Sotomayor wrote.