My app charges (or is about to charge) customers in more than one currency. Audit and fix how I handle currency everywhere. 1. Storage: every amount in my database must store its currency code alongside it (ISO 4217, like USD, EUR, JPY). An amount without a currency is a bug waiting to happen. Add the column and backfill it. 2. Minor units: amounts must be stored in the currency's smallest unit (cents for USD/EUR, but zero-decimal currencies like JPY have no cents, and a few currencies use three decimals). Do not assume two decimal places anywhere. Use my payment provider's currency metadata for the exponent. 3. Formatting: all display of money goes through Intl.NumberFormat with the correct currency, never string concatenation like a currency symbol glued to amount. Verify JPY and a zero-decimal currency render without fake decimals. 4. Presentment vs settlement: if my provider lets customers pay in their local currency while I settle in mine, document which currency each record is in, and store the exchange rate or converted amount the provider reports so my records reconcile. 5. Consistency: make sure a customer's subscription stays in the currency it started in; plan changes and upgrades must not silently switch currency. 6. Fees and refunds: refunds go out in the original currency at the current conversion, handled by the provider; my records should reference the original transaction rather than recompute conversion myself. Do not convert currencies with hardcoded or scraped exchange rates in my own math; the provider's conversion is the source of truth. Give me: the schema changes, every code path that touches currency with what changed, and test cases for USD, EUR, and JPY covering display, charging, and refund records.