What is the test for admissibility of evidence concerning a co-defendant's guilty plea?

The admissibility of evidence concerning a co-defendant's guilty plea "turns on the purpose for which it is offered." United States v. Halbert, 640 F.2d 1000, 1004 (9th Cir.1981) (per curiam). While evidence of a co-defendant's guilty plea "may not be offered by the government and received over objection as substantive evidence of the guilt of those on trial," id., "[t]his and other circuits have consistently recognized that, under proper instruction, evidence of a guilty plea may be elicited by the prosecutor on direct examination so that the jury may assess the credibility of the witnesses the government asks them to believe." Id. "[E]vidence of the plea is relevant to credibility regardless whether government or defendant initiates inquiry about it." Id.