Comment by skissane

The mental health evidence is introduced by the defence, not the prosecution. In most cases, the prosecution has no interest in raising defendant’s alleged mental heath issues (since it may well result in a lighter sentence, in more extreme cases even be grounds for an acquittal), while the interests of the defence is obvious.

And if the defence introduces mental health evidence into the sentencing, the judge is legally obliged to rule on it - explaining whether it was accepted or not, and if it was, how big an impact it had on the sentencing decision-if the judge didn’t do that, they risk either party successfully appealing the sentence, in extreme cases even being disciplined.

And even if it is a “plea deal” - the sentencing procedure is fundamentally the same as if there isn’t one, it is just the prosecution commits not to ask for a harsher sentence than agreed, the defence still has to introduce mitigating evidence and the judge has to rule on it; if the defence doesn’t, there’s a risk the judge may decide the agreed sentence is too lenient and overrule the agreement.