In California a joint bank account, or “multiple party account,” is presumed to belong to the surviving account holder. See California probate code 5302(a) which reads, in part, “…(a) Sums remaining on deposit at the death of a party to a joint account belong to the surviving party or parties as against the estate of the decedent unless there is clear and convincing evidence of a different intent.…” I added the emphasis so then the question is what is clear and convincing evidence of a different intent?
I would say that arguably a written statement, or letter, by the decedent saying they wanted the joint account person added to “help pay my bills” or something like that would be good evidence. Or a will specifically mentioning the account would be good. The key is th