See In re Richards, 213 F.3d 773, 786 (3rd Cir. 2000) (internal citations removed):
though Rule 48(a) does not confer unfettered discretion upon courts to resist motions to dismiss, it does have a procedural component that allows inquiry into the circumstances surrounding a dismissal in a manner that does not sufficiently intrude upon the prosecutorial domain to be viewed as a substantive rule.
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... we will vacate the order of the District Court with directions to remand the matter to the Territorial Court. We do not do so, however, to give that court carte blanche over prosecutorial decisions. The Territorial Court may conduct a hearing on the circumstances surrounding the Government's requested dismissal before it surrenders jurisdiction, but it lacks the power to compel the Government to take further steps to prosecute Richards.
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... Rule 48(a) is a strange animal. It reads like a procedural rule, but by interposing a discretionary act by a court between a prosecutor's decision to drop a case and ultimate dismissal, it is adorned with unmistakable substantive trappings. ...
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... there is likely little a court can do to compel action even if it denies a Rule 48(a) motion. Even if we were to uphold the Territorial Court's failure to grant the motion to dismiss, therefore, that court would seem to have few options if the day of trial came, and the prosecution refused to call witnesses or otherwise go forward with its case. The substantive bite of the rule, therefore, appears to be hampered by the dullness of its teeth. ...
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... the substantive reach of the rule appears to be effectively curtailed by the fact that even if the judge denies the motion to dismiss, there seems to be no way to compel the prosecutor to proceed.
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Even though a judge's discretion under Rule 48(a) is severely cabined, the rule may serve an important interest as an information and accountability-producing vehicle. A judge who hears a Rule 48(a) motion has independent responsibilities that may bear on his or her decision on the requested dismissal. In other words, there are independent rights, interests, and duties that a court may protect, through using Rule 48(a) as a "sunshine" provision that exposes the reasons for prosecutorial decisions.
The "dull teeth" of Rule 48(a) do have some practical utility. In the government's recent motion to dismiss the charges against Mayor Eric Adams, US District Judge Dale Ho expressed skepticism about this government's reasons for dismissal and used the discretion available under Rule 48(a) to dismiss the charges against Adams with prejudice, instead of without prejudice as requested by the government.
He also confirmed the above understanding of Rule 48(a) from In re Richards:
a court, if it were so inclined, would have no way to compel the government to prosecute a case in circumstances like those presented here. If an individual prosecutor seeks to dismiss a case for improper reasons, a court can deny the motion and send the matter back to the government, which can then reassign the case to another prosecutor. But where, as here, a court has substantive concerns about the reasons for dismissal offered by the Justice Department itself, the court does not have the same option. A court cannot force the Department of Justice to prosecute a defendant. That is by design. In our constitutional system of separation of powers, a court’s role in a criminal case is to preside over the matter — not to decide whether the defendant should be prosecuted. The Court is not aware of any authority that would empower it to appoint an independent prosecutor outside of the limited context of criminal contempt. ...
... while a court could, in theory, deny a Rule 48(a) motion even when made at the direction of DOJ leadership, such a denial would almost certainly be an exercise in futility. The court cannot appoint its own prosecutor; absent a sudden change of heart at DOJ, such a denial would produce only a staring contest, followed by an inevitable dismissal on speedy trial grounds after seventy days.
The prosecution of Steven Donzinger mentioned in another answer is not a counter-example to the above position. The 2nd Circuit described that the appointed prosecutors were "subject to supervision by the Attorney General... This authority includes supervising—and if necessary, removing—the special prosecutors." See United States v. Donzinger, 21-2486 (2nd Cir. 2022).
It is merely the initiation of contempt proceedings, to punish disobedience to court orders, that is part of the judicial function. See Young v. United States ex rel. Vuitton et Fils, 481 U.S. 787 (1987). Initiation of the contempt proceedings and appointment of the prosecutors is as far as that power goes. Once appointed, and under the supervision of the Attorney General, the special prosecutors could be directed by the DOJ to move to dismiss the charges.
