---

@Frustrated-in-IL wrote:

I suppose a copy of my allocation worksheet, the last 8606 of the deceased, and something showing FMV on date of death.

---

I think that's probably as good as it's going to get.  But I'm not sure the 8606 "proves" anything, maybe consistency (so about as much as a medical record "proves" that your dependent lived with you--oh look, they told the doctor's office the same thing they told the IRS).

Out in the real world though, how many IRA beneficiaries are going to get that much information?  I think that's going to be a giant sinkhole where a majority of IRA basis will get lost in the future.  I would provide the info to heirs if I were involved in the process (and kudos to you for doing it!)  But I'd guess we're in the minority.  I'm sure once this becomes a widespread problem, Congress will release a patch in SECURE 9.0 when passed in 2047.