What is the legal test for overturning a judgment of conviction where a defendant is found incompetent to stand trial?

We next turn to the appropriate remedy. A judgment of conviction must be reversed where a trial court fails to hold a competency hearing in the face of substantial evidence of the defendant's incompetency. (People v. Young (2005) 34 Cal.4th 1149, 1216-1217.) "Under section 1368 of the Penal Code the trial court has no power to proceed with the trial once a doubt arises as to the sanity of the defendant. In trying defendant without first determining at a hearing his competence to stand trial, the court both denie[s] to defendant a substantial right [citations] and pronounce[s] judgment on

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him without jurisdiction to do so. In such cases the error is per se prejudicial." (People v. Pennington (1967) 66 Cal.2d 508, 521.) The state is free to retry the defendant, "assuming, of course, that at the time of such trial he is competent to be tried." (Drope v. Missouri, supra, 420 U.S. at p. 183.)