Form 8959 — Additional Medicare Tax

This skill produces an audit-grade draft of Form 8959 from the user's wages, self-employment income, RRTA compensation, filing status, and any Additional Medicare Tax already withheld by an employer. It walks through Parts I–V line by line, applies the IRC §1401(b)(2) and §3101(b)(2) surtax rules, validates the result, and emits a deliverable the user can transcribe to a paper or e-file form.

The 0.9% Additional Medicare Tax is mechanically simple — a flat surtax above a status-based threshold — but it is easy to get wrong because the threshold does not phase, does not adjust for inflation, and is split unevenly across income types. Employer withholding kicks in only at $200,000 of wages from a single employer regardless of filing status, so MFJ couples with two earners under $200K each will under-withhold and owe at filing. The agent's job is to catch that gap and produce the reconciliation cleanly.

When to invoke

Engage this skill when any of the following is true:

• The user explicitly mentions Form 8959, "Additional Medicare Tax", or the 0.9% Medicare surtax
• The user has W-2 wages above the threshold for their filing status (see Prerequisites) — single/HoH/QSS at $200,000; MFJ at $250,000; MFS at $125,000
• The user has self-employment income and wages combined above their threshold
• The user has Box 6 (Medicare tax withheld) on a W-2 that exceeds 1.45% of Box 5 (Medicare wages) — that excess is Additional Medicare Tax already withheld by the employer and must reconcile on Form 8959 Part V
• The user has RRTA compensation (railroad retirement) with similar high-income exposure
• A return is being assembled and Schedule 2 Line 11 needs a number