Tax engine

The tax engine lives in @fiscode/tax as pure functions: no React, no DOM, no DB. It takes a plain bundle and returns plain estimates.

Strategy seam

interface TaxStrategy {
  compute(input: TaxInput, year: YearConfig): TaxEstimate;
  quarterly(input: TaxInput, year: YearConfig, method: "even" | "annualized"): QuarterlyPlan;
}

engine.computeForYear(year) picks a strategy based on the active entity record for that year, not a global. Today there are two strategies:

• SoleProprietorStrategy — full implementation. SE tax (with wage-base + additional Medicare), QBI, federal brackets per filing status, state, quarterly + annualized + safe-harbor.
• SCorpStrategy — NotImplemented stub. The interface exists so adding it later is data + an implementation, not a refactor. Excluded from the UI by the isSCorp flag.

Year config

packages/tax/src/config/<year>.ts is the canonical example. Constants that change annually (brackets, SS wage base, mileage rate, standard deduction, QBI thresholds, etc.) are marked // todo: verify against IRS/SSA for <year> in the source.

getYearConfig(year) falls back to the nearest defined year and surfaces a console warning + UI banner — stale data is loud, not silent.

What fiscode does not do

• No IRS form generation. No filing.
• No actual-expense vehicle method (standard mileage only).
• No retirement / Roth UI in v1 (schema + CSV + flags only).