The dispute centers on M & K Employee Solutions, LLC, and related petitioners, who withdrew from the IAM National Pension Fund and were assessed roughly $10 million in withdrawal liability. The petitioners argued that under the correct actuarial assumptions — specifically, the discount rate that had been in place before an ad hoc January 2018 meeting at which the fund's actuary presented trustees with two options (7.5 percent or 7 percent) but the trustees ultimately settled on a rate of 6.5 percent, a figure that had not been modeled in the presentation — the liability would have been closer to one-third of that amount. The discount rate matters enormously, counsel for petitioners explained, because of the compounding effect over the decades-long horizon of projected pension payments.

The core statutory question is what the phrase as of the end of the plan year in Section 1391 of ERISA requires. Counsel for petitioners, Michael E. Kenneally, Jr., argued that the as of language freezes all inputs into the unfunded-vested-benefits calculation — including actuarial assumptions — as of December 31 of the year preceding the employer's withdrawal. Any assumption selected after that date, he contended, impermissibly changes the amount of unfunded vested benefits after the valuation date. He warned that the opposing rule would allow a plan to hire a new actuary after employers have already withdrawn, potentially quadrupling liability, as allegedly occurred in the Second Circuit's Metz decision.

Counsel for the respondent fund, John E. Roberts, countered that as of is a reference point for retrospective work, not a deadline. He argued that actuaries have selected assumptions after the measurement date for roughly 40 years, that the phrase contemplates work being performed later to determine the plan's financial condition at an earlier date, and that the Second Circuit's Metz decision was completely atextual — it did not rely on the as of language and instead drew on legislative history of a provision that does not even deal with assumptions. He characterized Metz as a bad-facts case — the plan had replaced its actuary after employers withdrew, and the new actuary's interest rate was dramatically lower — that should have been resolved through arbitration challenges to the reasonableness of the assumptions rather than a categorical timing rule.

Kevin J. Barber, arguing for the United States as amicus curiae supporting the fund, told the Court that the Second Circuit in Metz became the first court to prohibit post-measurement-date assumption selection, that various ERISA provisions make clear Section 1391's as of language contemplates a retrospective determination, and that petitioners' rule would force actuaries to use stale assumptions keyed to the prior plan year's measurement date — here, the end of 2016 — in tension with Section 1393's requirement that assumptions represent the actuary's best estimate.

Several justices pressed both sides on administrability and manipulation risk. Justice Sotomayor noted that nothing in the fund's proposed rule would prevent post-withdrawal changes to assumptions, and asked what protects against gamesmanship; Roberts acknowledged that post-withdrawal changes would be permissible under his reading but pointed to arbitration challenges and the statutory requirement that assumptions be reasonable and represent the actuary's best estimate. Justice Kavanaugh raised the concern that an employer's decision to withdraw might itself be based on an estimate of withdrawal liability, making a post-withdrawal assumption change that multiplies the bill by several times particularly disruptive. Justice Jackson pressed petitioners on whether the as of language applies only to hard data and not to the actuary's forward-looking judgment, while Justice Kagan asked whether petitioners' timing rule would equally bar post-measurement-date updates to actuarial methods. Justice Alito asked whether the D.C. Circuit's information available as of test — the secondary question of what data an actuary may consider when selecting assumptions after the measurement date — creates administrability problems; Roberts argued that question is not presented and that there is no circuit split on it.

In rebuttal, Kenneally argued that the as soon as practicable language in Section 1399(c) is not much of a guardrail at all, pointing to cases where two and a half years passed between withdrawal and assessment, and that a remand would require deposing the actuary to reconstruct what happened at the January 2018 meeting — exactly the kind of fact-bound, expensive-to-litigate controversy the MPPAA was not designed to create. He also noted that a pending PBGC proposed rule, if enacted, would eliminate the reasonableness and best-estimate standards that the fund cited as protections against manipulation, leaving the timing rule as the only check on surprise post-withdrawal liability bills.