The Supreme Court on Monday unanimously ruled that the First Step Act, the bipartisan 2018 law that overhauled aspects of the criminal justice system, did not require new sentences for some low-level drug offenders.

Though all of the justices agreed on the bottom line, the decision nonetheless featured a sharp disagreement about the background of a 1986 law that had subjected drug dealers selling crack cocaine to the same sentences as ones selling 100 times as much powder cocaine.

Justice Sonia Sotomayor issued a concurring opinion accusing the majority of including “an unnecessary, incomplete and sanitized history” of the law, one she said had imposed disproportionately harsh sentences on Black offenders.

Justice Clarence Thomas, writing for eight members of the court, said the 1986 law had enjoyed overwhelming bipartisan support, adding that “a majority of the Congressional Black Caucus co-sponsored and voted for the bill.”

“Many black leaders in that era professed two concerns,” Justice Thomas wrote. “First, crack was fueling crime against residents in inner cities, who were predominantly black,” he wrote, noting that the president of an N.A.A.C.P. chapter in the Washington region had called crack “the worst thing to hit us since slavery.”

“Second,” Justice Thomas wrote, “there were concerns that prosecutors were not taking these kinds of crimes seriously enough because the victims were disproportionately black.”

Justice Sotomayor responded that “the full history is far less benign.”

“Most egregiously,” she wrote, “the court barely references the ratio’s real-world impact” of the sentencing disparities between offenses involving crack and powder cocaine.