Is there a prejudicial error if a witness is a cooperating witness?

^2. We need not decide whether United States v. Richter, 826 F.2d 206 (2d Cir. 1987), applies to cases, such as this one, involving cooperating witnesses. In Richter we held that prosecutorial statements asking the jury to contrast the defendant's credibility with that of the government's witness who was a law enforcement agent was prejudicial error because the jury may perceive law enforcement agents to be truthful. Id. at 208-10. Mathieu argues the same logic applies to government cooperators. But here, the cooperators were participants in the charged criminal activity, and it is unclear why the jury would be likely to presume their truthfulness. Indeed, the district court instructed the jury that it could consider the "fact that a witness is an accomplice ... as bearing upon her credibility" but cautioned against assuming "that simply because a person has admitted participating in one or more crimes, she is incapable of telling the truth about what happened." App'x 157. This instruction would have been unnecessary if the jury were inclined to regard the government cooperators as highly credible.