Can a defendant's guilty plea be used as evidence at trial?

"[W]hen the prosecution examines the codefendant as its witness in support of its case-in-chief, a question about the guilty plea is legitimate as the purpose is to support the reasonableness of the witness' claim to firsthand knowledge because of admitted participation in the very conduct which is relevant." United States v. Halbert, 640 F.2d 1000, 1005 (9th Cir.1981). But we also have recognized that "[e]vidence of the guilty plea[ ] is amenable to misuse [because w]ithout instruction, it is possible the jury could use the plea[ ] as evidence of [the defendant's] guilt." Id. at 1006. Accordingly, where the prosecutor has elicited evidence of a codefendant's guilty plea, the trial court must provide "adequate cautionary instructions that make it clear to lay people that evidence of a witness' own guilty plea can be used only to assess credibility." Id.; see also United States v. Smith, 790 F.2d 789, 793 (9th Cir.1986).