Newspaper headlines indicate the Mueller wants to grant immunity to five witnesses for the Manafort trial. If you do not get beyond the headlines, this seems overly generous. In fact, no generosity is involved.
Mueller is seeking to compel the testimony of the five witnesses, none of whom need testify if they invoke the Fifth Amendment – unless they are granted immunity. But there are two kinds of immunity. There is transactional immunity which would excuse them from all crimes connected with their testimony, and there is a narrower form of immunity: derivative use immunity. That is the form Mueller has applied for the court to grant. Under this form of immunity, the witness can be prosecuted for crimes connected to the testimony, and the witness can be prosecuted if the testimony is perjurious, but the testimony (or evidence procured as a result of it) cannot be used against the witness.
The result is that Mueller can force witnesses who do not wish to testify to take the stand and the immunity makes it impossible for them to legally take the Fifth Amendment. Even if the witnesses fully cooperate, they can be prosecuted for any crimes they have committed. Typically, prosecutors will file their evidence before the witness has testified, so that it will be clear that the evidence was not derived from the compelled testimony. Generous, this is not.
I think this use of immunity unfairly diminishes Fifth Amendment rights, but the Supreme Court upheld this kind of tactic in the 1972 case of Kastigar v. United States. One problem for prosecutors in this regime is that unwilling witnesses are frequently not very helpful and sometimes harmful. Mueller may not call any of these witnesses. He has asked that they not be named to protect their reputation if they are not called and to protect them from undue harassment. I am not rooting for Manafort, and I do not blame Mueller for prosecuting in a way that the law permits. But the Fifth Amendment really ought to mean more.