5500 for first year of a spin off plan

We had a plan with 4 sponsors - a controlled group.

In 2025 2 of the sponsor's participants were spun off to a new plan in anticipation of those 2 sponsor being sold in 2026 and no longer being a controlled group.

We were told to make sure the new plan for the spin off had the same effective date at the original plan so that it was clear it was a continuation and not a new plan that would be subject to the auto enrollment rules.

For the first 5500 for 2025, for the plan's effective date, do I use the original plan's effective date from a prior year with a full plan year - or the date of the spin off and show it as a short 1st year?

I think that yes is generally the better answer if, as you state, the spin-off is intended to be a continuation of the portion of the existing plan attributable to those entities, rather than the establishment of a brand-new plan.

So, for the first year Form 5500:

In Part I  For … fiscal plan year beginning “spin off date (whatever that is)” and ending “end of plan year (whatever that is) 

Part IB mark “first return/report”, 

Part II 1b likely “001” 1c “original effective date (whatever that is).

Nothing in line 4

That coincides with your facts:
•    401(k) plan has existed for years. 
•    The two entities participated in that plan for years. 
•    Assets and liabilities attributable to those subsidiaries are being spun off into a separate plan under IRC §414(l). 
•    The intent is for the new plan to be a continuation of the benefits previously maintained for those employees, not a newly established retirement program. 

I mean the Form 5500 instructions ask for the plan's effective date, not the date the plan first filed a Form 5500 or the date of the spin-off.

Legally, speaking, a §414(l) spin-off is generally treated as a continuation of the transferred portion of the original plan. Participants' accrued benefits, vesting service, distribution restrictions, and other plan rights continue uninterrupted. The spin-off itself is a transfer of assets and liabilities, not the creation of new retirement benefits.  The legal substance (a continuation through a §414(l) spin-off) should drive the reporting, rather than simply using the date on which the separate trust or separate Form 5500 first comes into existence.

That said, make sure the plan documents are drafted consistently.  E.g., the AA says:  Effective Date: January 1, 2026 while the Form 5500 reports: Original Effective Date: January 1, 2012 (or whatever the date is), that inconsistency could create unnecessary questions.  So, make sure ALL plan documents and communications are consistent (AA or individual plan doc, first 5500, new SPD, resos, etc.

If an IRS agent or DOL investigator were to ask: "Your Form 5500 says the plan's original effective date was ___ Why is this the first Form 5500 filed on ____?" the answer is straightforward:  "Because, this is the first annual return filed for the XYZ 401(k) Plan as a separately maintained plan following a spin-off from the ABC Corporation 401(k) Plan effective January 1, 2026. Prior to the spin-off, the transferred participants and assets were reported as part of the ABC Plan on Forms 5500 filed for plan years 20__ through 2025.  The XYZ Plan is just a continuation of the prior ABC Plan"… or something like that.  That's a perfectly logical explanation.  Perhaps memorialize this statement to put in the administrative file (for those folks who work on this 5 years from now when everyone has forgotten the transaction).

No authority for anything said above except maybe the Form 5500 instructions.... so caveat emptor