Solo 401k employee contribution; IRS reporting

[TeeNixx]

As an owner/employee I did a lumpsum employee contribution to my solo 401k plan late in the tax year to drive down my AGI. Since it's an employee contribution it won't show on the W-2.
Vanguard will not send any contribution notice to me or IRS. How and where should I do the deduction of the contribution?
 

[seattlecyclone]

That goes on Schedule 1.

[TeeNixx]

I understand it would go on sch 1, @seattlecyclone
The problem is that Vanguard won't produce any document to prove my deposit. How to verify the contribution for the IRS?

[dandarc]

Employee 401k deferrals show up in Box 12, code D on your W-2. So it does show up on your W-2. And then your W-2 Box 1 reflects the deduction - absent any other things, that box would be lower than the Medicare wages in Box 5 by the amount of the employee 401k contribution. That's assuming you're treating yourself like an employee and actually issuing a W-2 to yourself. This would be typical if you're being taxed as an S-Corp.

If you're taxed as a sole-proprietor, and not issuing a W-2 to yourself, then the entire SoloK contribution for the year - both employee and employer sides combined - is shown / deducted on Schedule 1.

[bacchi]

It's not a tax document but the 401k 12/31 statement lists your contributions. For any prior year contributions (e.g., contributions made in 2022 for 2021 taxes), you'll need to wait until the 6/30 statement.

[seattlecyclone]

I understand it would go on sch 1, @seattlecyclone
The problem is that Vanguard won't produce any document to prove my deposit. How to verify the contribution for the IRS?

If you're a sole proprietor you just put the contribution amount on Schedule 1 and you can show account statements to the IRS if they ask about it. That's it!

[TeeNixx]

Thanks for the input. I'll make sure that I have the contribution statement on file.