What is the test for a criminal defendant to be declared mentally unfit to stand trial?

A criminal defendant may not be tried or convicted " ' "if he . . . lacks ' "sufficient present ability to consult with his lawyer with a reasonable degree of rational understanding -- [or lacks] . . . a rational as well as a factual understanding of the proceedings against him." ' " ' " (People v. Sattiewhite (2014) 59 Cal.4th 446, 464.)

We agree that a defendant must exhibit more than bizarre behavior or a preexisting condition (People v. Ramos, supra, 34 Cal.4th at p. 508), but " '[i]f a qualified mental health expert who has examined the defendant " 'states under oath with particularity that in his professional opinion the accused is, because of mental illness, incapable of understanding the purpose or nature of the criminal proceedings being taken against him or is incapable of assisting in his defense or cooperating with counsel,' " that is substantial evidence of incompetence' " (People v. Sattiewhite, supra, 59 Cal.4th at p. 465).