Read the SPD for two sentences, not one. “Roth 401(k)” is the elective cap. You need “after-tax / voluntary after-tax employee contributions” and either in-plan Roth conversion of that source or an in-service withdrawal that can go to a Roth IRA. Highlight both. If either is missing, stop — mega is a different employer’s feature.
Separate mega from the IRA backdoor on paper. IRA backdoor: $7,500, Form 8606, pro-rata on every traditional IRA. Mega: 401(k) after-tax, 1099-R, no IRA pro-rata unless you roll the after-tax into a traditional IRA (never). Keep the two checklists on two pages. The regular backdoor playbook is the other one.
Fill elective and catch-up first. $24,500 elective (pre-tax or Roth). Catch-up $8,000 / $11,250 at 60–63 sits outside 415(c). If prior-year SS wages from this employer exceeded $150,000, 2026 catch-up must be Roth — that is not mega, it is a SECURE 2.0 wage test. Do not skip the match to “make room.”
Get this year’s employer dollar, not last year’s estimate. Match true-up, profit sharing, and nonelective all eat 415(c). Ask payroll / the recordkeeper for year-to-date annual additions before you set an after-tax percent that will bounce in December.
Size after-tax off leftover 415(c), then off cash you will not miss. Plug numbers into the calculator. Cut the after-tax so the emergency fund and high-APR debt plan still exist. A $38,000 mega funded with a 22% card is a round-trip.
Ask for the payroll source code in writing. After-tax is a different box from Roth elective. If they “just raise Roth %,” you filled the $24,500 cap and created no mega. Confirm the first paycheck actually split: elective, catch-up, after-tax, as three lines.
Convert or roll the same week as the hit. Standing in-plan conversion if the recordkeeper has a toggle; otherwise a weekly in-service form. Earnings in the after-tax source are ordinary income. Same-week keeps box 2a in pennies. Year-end conversion in a bull market is how people swear mega is “taxable.”
Watch ACP / refunds. If last year the plan refunded after-tax to HCEs, this year will too unless the testing changed. Safe-harbor plus a designed after-tax source is the pattern that actually works. A refund in April is the plan telling you mega is not for your pay band.
File the 1099-R even when taxable is $12. Box 1 gross, box 2a earnings, box 5 basis. In-plan conversions still issue the form. Staple it to the return. If basis and earnings got rolled to the wrong IRA type, call the recordkeeper the same week — do not “fix it next year.”
Solo 401(k) is a document problem, not a vibe. The adoption agreement must allow after-tax and in-plan Roth or in-service distribution. Many $100/year solo plans do not. Owner profit-sharing still counts toward $72,000. If you are 1099, read first-year 1099 before you open a second plan to chase mega.
Hard-stop list: rolling after-tax to a traditional IRA, calling Roth elective “mega,” emptying the HYSA to hit 415(c), skipping the conversion because “I’ll do a lump in December,” and assuming the next employer copied this SPD. Re-read features on day one of a new job. If 415(c) is already full of employer money, mega is $0 — do not invent room.
One-page decision
Both features in the SPD + leftover 415(c) + cash after match, debt, and reserves: run after-tax and convert weekly. Missing either feature: you have a regular backdoor problem or a different 401(k), not a mega. Dirty traditional IRAs: clean them for the IRA backdoor; they do not block mega and mega does not clean them.