Do I have to notify beneficiaries when a trust becomes irrevocable?

Yes, and the notice is what starts the clock on any contest. Serving it early is what closes that window early.

Yes. California requires the trustee to serve a formal notification on beneficiaries and on the deceased settlor's heirs when a revocable trust becomes irrevocable on death. It is not optional, it is not discretionary, and the deadline is measured in days.

Why trustees skip it, and why they should not

The usual reason is delicacy. Sending a legal notice to your own brother weeks after your mother's funeral feels like an act of aggression, so it gets put off until things settle down.

The notice works in exactly the opposite direction. Serving it starts the period within which someone can bring a contest. Until it goes out, that period has not begun, and the trust remains open to challenge long after everyone assumed the matter was closed. The delicate choice is the one that leaves the family exposed for years.

What the notice has to contain

It identifies the settlor and the trust, names the trustee and says how to reach them, states the beneficiary's right to request a complete copy of the trust, and gives the warning about the time limit for bringing a contest. Getting the contents wrong is as good as not serving it, so keep proof of what was sent and when.

A separate notice goes to the Department of Health Care Services where the deceased received Medi-Cal, which is a step families almost never know about.

The other document: a certification of trust

This is the one banks and title companies ask for, and it is a different thing entirely. A certification is a short document proving your authority as trustee without handing over the whole trust instrument. It confirms the trust exists, who the trustee is, and what powers they hold, while keeping the distribution terms private.

You are entitled to give a certification instead of the full document, and you generally should. There is no reason for a branch manager to know how your mother divided her estate.