Elective deferrals *to* your traditional 401(k) from your paycheck are not included in your AGI and therefore do not appear in your MAGI. They are excluded from the amount reported in box 1 of your W-2. They are not reported anywhere on your tax return as a deduction.
Elective deferrals to a Designated Roth Account in your 401(k) (a Roth 401(k)) do appear in your AGI and MAGI since they are not excluded from the amount in box 1 of your W-2.
