0001: Money is Decimal end to end

• Status: accepted
• Date: 2026-09-30

Context

Matching compares printed totals, line amounts, and unit prices to the cent. Binary floats can't represent most decimal amounts exactly, so 0.1 + 0.2 != 0.3, and a float amount serialized to JSON loses trailing zeros (12.10 becomes 12.1). Either one produces false mismatches or misleading output.

Decision

Every amount is a decimal.Decimal: parsing in normalize.py, the Pydantic models in schema.py, the arithmetic checks, the matcher, and the JSON output, where amounts serialize as strings. Code never converts an amount to float. Tolerances in MatchPolicy are Decimal too.

Consequences

• Arithmetic checks are exact, and the tolerance settings mean what they say.
• JSON consumers get strings for amounts and must parse them as decimals.
• Property-based tests check that parsing a formatted amount gives back the same Decimal.