What is the legal test for a defendant's competence to stand trial?

Once a defendant has been found competent to stand trial, a second competency hearing is required only if the evidence discloses a substantial change of circumstances or new evidence is presented casting serious doubt on the validity of the prior finding of the defendant's competence. (See People v. Kelly (1992) 1 Cal.4th 495, 542-543, 3 Cal.Rptr.2d 677, 822 P.2d 385 [no change in circumstance to justify second hearing]; People v. Jones (1991) 53 Cal.3d 1115, 1153-1154, 282 Cal.Rptr. 465, 811 P.2d 757 [general assertion of defendant's worsening condition and inability to cooperate with counsel inadequate to justify second hearing].)