wanderbooks/docs/requirements.md

Requirements — accounting semantics (Phase 1)

Numbered requirements for the engine. R-N is current scope; R-FUTURE-N is explicitly out of scope for now but designed for. Agents: if code and a requirement conflict, the requirement wins.

Core double-entry

• R-1 — Balanced entries. Every journal entry has at least two lines and total debits exactly equal total credits (Decimal equality, no epsilon). Unbalanced entries are rejected at post time with the offending totals.
• R-2 — Exact arithmetic. All money is Decimal. No binary floats in any money path. No silent rounding: amounts are exact end to end. (Presentation rounding, when the UI arrives, is half-up to the currency's minor unit and is a display concern only.)
• R-3 — Immutability. Posted entries cannot be edited or deleted. The only correction mechanism is a reversing entry (same lines, sides flipped) linked via reversal_of. Draft → Posted is one-way.
• R-4 — Chart of accounts. Five closed types (Asset, Liability, Equity, Revenue, Expense) with fixed normal sides (debit-normal: Asset, Expense; credit-normal: Liability, Equity, Revenue). Codes are unique and non-empty. Accounts may have parents (hierarchy). Placeholder (header) accounts organize the chart and reject postings.
• R-5 — Single currency per ledger (day one). Every account and every journal line uses the ledger's base currency. Anything else is rejected, not converted. Multi-currency with translation is R-FUTURE-1.
• R-6 — Line hygiene. Zero-amount lines are rejected, not ignored. Unknown accounts are rejected. Single-line entries are rejected.

Reports

• R-7 — Trial balance. Every non-zero, non-placeholder account appears with its debit or credit balance, sorted by code. Total debits must equal total credits (the balanced() check).
• R-8 — Balance sheet. Assets, liabilities, equity grouped by account type. The accounting equation holds exactly: assets = liabilities + equity. Because period close (R-FUTURE-2) doesn't exist yet, current earnings (net income to date) are folded into equity so the equation holds on a live book.
• R-9 — Profit and loss. Revenue and expenses for the life of the ledger, with net income. Period filtering arrives with R-FUTURE-2.
• R-10 — Canonical snapshots. reports::snapshot() renders the three core reports as deterministic JSON (decimal strings, sorted lines, fixed keys). Two engines fed the same books must produce identical output. This is the oracle interchange format.

Auditability

• R-11 — Append-only log. Posted entries are kept in posting order. The log plus the chart fully determine every report.
• R-12 — Command surface. All mutations go through Ledger methods. There is no second way to change the books.

Receipt policy (MCP phase)

• R-13a — Receipt corrections. Posted entries are immutable (R-3); their evidence is not frozen but is append-only. A receipt is replaced by attaching the new one and unlinking the old with a reason; unwanted receipts and documents are withdrawn with a reason. Unlinking may never leave an entry the policy (R-13) requires a receipt for with none. Every step is a journal event; no blob is deleted.
• R-13 — Receipt enforcement. The MCP layer (not the engine — the engine stays policy-free) enforces a configurable receipt policy: postings at or above a threshold (require_receipts_above, default "25.00" in base currency) must have at least one attached receipt, with per-account overrides. post_entry with confirmed: true refuses with a typed receipt_required error otherwise. The draft preview always includes the receipt checklist (attached: [...], required_but_missing: bool), so the human sees it before confirming. Full spec: docs/mcp-interface.md.

Explicitly future work

• R-FUTURE-6 — Budgets and scheduled transactions. (Budgets explicitly declined as bloat; scheduled transactions remain future.)

Promoted from future (done)

• R-? — Cash flow statement (indirect method). Was R-FUTURE-3. ledger-core::reports::cash_flow + books.cash_flow.
• R-? — QuickBooks-compatible import/export. Was R-FUTURE-4. ledger-import crate: CSV bank/chart import, IIF and XLSX export.
• R-? — Multi-currency (foreign-currency lines). Was R-FUTURE-1. JournalLine carries optional foreign currency/amount; amount is the base-currency equivalent. Ledger::add_line_fx, Ledger::set_fx_rate. Reports work in base currency. (Full per-account currencies and translation remain future.)
• R-? — Periods and close. Was R-FUTURE-2. ledger-core::periods: create_period, close_period (posts closing entry to 3999 Retained Earnings), closed periods reject new entries.
• R-? — A/R and A/P workflows. Was R-FUTURE-5. ledger-core::arap: customers, vendors, invoices, bills, payments, AR/AP aging. MCP tools for the full workflow.
• R-? — Bank reconciliation. Cleared flags on posted lines, reconciliation_report with uncleared-line detail.
• R-? — Payroll (accounting only). ledger-core::payroll: run_payroll posts one balanced entry (Dr 6100 gross, Dr 6110 employer FICA, Cr 2110–2150 payables, Cr cash net); remit_payroll_taxes clears the payables. No tax tables — the caller supplies every number.
• R-? — Tracking classes (divisional P&L). ledger-core::classes: JournalLine.class_id, add_class, set_line_class (drafts only), profit_and_loss_filtered, profit_and_loss_by_class. Shared costs are split explicitly across lines at entry time.
• R-? — CPA handoff package. books.export_cpa_package: zip with trial-balance XLSX, general-ledger XLSX (every posted line), P&L and balance-sheet PDFs (printpdf, MIT).

Non-goals (all phases of the engine)

The engine never does I/O, networking, wall-clock time sourcing, or UI. It is a pure domain library: data in, validated books out.