Trial court prejudicially violated petitioner’s constitutional right to presence at a critical stage where it held a Penal Code section 1172.6(d)(3) hearing in petitioner’s absence and without a knowing and intelligent waiver of his right to be present. After issuing an order to show cause on Quan’s petition for relief under section 1170.95 (now 1172.6), the court held a (d)(3) hearing without ensuring either that Quan was present or that he had waived his right to be present. The court denied the petition and Quan appealed. Held: Reversed. A petitioner has a due process right to be personally present at a section 1172.6(d)(3) hearing because the petitioner may offer information or testimony at the fact-intensive inquiry into his own mental state. A waiver of this right requires more than defense counsel’s blanket assertion that her client did not need to be present; it requires a record of a knowing and intelligent waiver. The error was prejudicial because the Attorney General could not demonstrate beyond a reasonable doubt that Quan would not have offered testimony or assisted his counsel in rebutting the prosecution’s claims about his mental state.