Session: fix-rounding-in-tax-calculation — 2026-10-06

Project: ledger-api · Branch: fix/tax-rounding · Mode: default · Model: claude-opus-4-7

Summary

Tracked down a one-cent mismatch between invoice totals and the PDF: tax was rounded per line instead of per invoice. Switched to Decimal with banker's rounding at the invoice level and added a property-based test with Hypothesis.

Conversation

Turn 1 — User

Customers report invoices off by one cent vs the PDF.

Turn 1 — Assistant

Root cause: tax is rounded on every line item, then summed. Rounding once per invoice with ROUND_HALF_EVEN fixes it. Adding a Hypothesis test that the sum of lines always equals the invoice total.

Tools used in this turn:

• Bash: pytest -k tax -q — 1 failed (reproduces)
• Edit: ledger/tax.py
• Write: tests/test_tax_properties.py
• Bash: pytest -q — 41 passed

Connections

• [[ledger-api]]
• [[Decimal]]
• [[Hypothesis]]
• [[BankersRounding]]