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Ultra-Sharp Agent Skills — a research-first skill-authoring system + 72 production-ready skills for AI agents.

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# name: filing-us-business-taxes description: Guides US business tax filings by entity type — Schedule C, Form 1065, 1120-S, 1120 — plus estimated taxes, payroll forms, and 1099s, with deadlines and penalty rules. Use when the user asks how to file business taxes in the US, which form their LLC, S-corp, C-corp, partnership, or sole proprietorship files, about quarterly estimated taxes, self-employment tax, K-1s, 941/940 payroll filings, W-2s, 1099-NEC, or business tax extensions. Do not use for personal 1040-only returns (preparing-us-personal-tax-returns), Canadian filings (filing-canadian-business-taxes), or tax planning strategy (optimizing-business-taxes).

# Filing US Business Taxes

# When to use / when NOT to use

  • Use for: which federal return an entity files, filing/payment deadlines, estimated taxes, payroll and contractor reporting, extensions and penalties.
  • Do NOT use for: personal returns without a business (→ preparing-us-personal-tax-returns), Canadian entities (→ filing-canadian-business-taxes), or choosing/optimizing structure (→ optimizing-business-taxes).

# Important limits

  • Educational help, not professional tax advice — multi-state, international, or high-stakes filings go to a CPA/EA.
  • Figures and deadlines are tax-year-stamped and MUST be verified against irs.gov before use.
  • Never assist with evasion, unreported income, or falsified records; refuse and explain.

# Core rules

  1. The entity determines the return:
    Entity Return Owner receives Federal due date
    Sole prop / single-member LLC Schedule C on Form 1040 (+ Schedule SE) — April 15
    Partnership / multi-member LLC Form 1065 Schedule K-1 March 15
    S-corporation Form 1120-S Schedule K-1 (+ W-2 salary) March 15
    C-corporation Form 1120 (21% flat rate) W-2 and/or dividends April 15
  2. Pass-through returns are due a month early (March 15) so K-1s reach owners before their 1040s. Missing a 1065/1120-S deadline triggers a per-partner, per-month penalty even with no tax due.
  3. Self-employment tax rides with Schedule C: 15.3% (Social Security + Medicare) on net self-employment earnings via Schedule SE, on top of income tax; half is deductible.
  4. S-corp owners who work in the business must take a reasonable W-2 salary through payroll before distributions.
    • ✅ $70k salary + $50k distributions with payroll filings
    • ❌ $0 salary, all distributions — a classic audit trigger
  5. Estimated taxes are quarterly: April 15, June 15, September 15, January 15. Safe harbor: pay 100% of last year's tax (110% if AGI >$150k) or 90% of the current year to avoid penalties.
  6. Employees mean payroll filings: Form 941 quarterly (withholding + FICA), Form 940 annually (FUTA), W-2s to employees and the SSA by January 31, plus state equivalents.
  7. Contractors paid ≥$600 get a 1099-NEC by January 31. Collect a W-9 before first payment, not at year-end.
  8. Extensions extend filing, never payment. Form 7004 (businesses) / 4868 (individuals) gives ~6 months to file; tax owed is still due at the original deadline. Late-FILE penalties (5%/month, max 25%) dwarf late-PAY (0.5%/month) — always file on time even if you cannot pay.

# Workflow

  1. Confirm the entity type and any elections (e.g., LLC taxed as S-corp via Form 2553).
  2. Close the books for the tax year; reconcile income to bank deposits and books.
  3. Select the return from the table (rule 1) and its state counterpart(s).
  4. Compute and schedule estimated payments for the coming year (rule 5).
  5. Verify information returns: W-2s/941s consistent with wage expense; 1099-NECs issued.
  6. Validate before filing: deadline calendar cross-checked against irs.gov; book income reconciled to return income (Schedule M-1 for corporations); all K-1s issued.

# Current figures (tax year 2026 — verify before use)

Item Figure Source
C-corp federal rate 21% flat irs.gov
SE tax rate 15.3% on net SE earnings irs.gov/schedule-se
1099-NEC threshold $600 irs.gov/form-1099-nec
Estimated-tax safe harbor 100%/110% prior year or 90% current irs.gov/form-1040-es

# Edge cases & failure modes

  • LLC ≠ a tax status. An LLC files as sole prop, partnership, S-corp, or C-corp depending on members/elections — always ask which.
  • First year, no prior-year tax → safe harbor based on prior year is $0; still project and pay 90% current-year to be safe.
  • Missed 1099s → file late anyway; penalties scale with lateness.
  • State taxes vary widely (franchise taxes, gross-receipts taxes) — flag the state and direct to its authority; this skill covers federal.

# References

Entity/form/deadline tables, penalty table, and gotchas: see references/reference.md.