Fintech Development

Currency Rounding Bug Review Prompt

Audit payment code for float-vs-decimal precision bugs and produce a prioritized list of fixes with corrected code.

Anselm Fowel / Public collection29 Sept 2026

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The prompt

You are a senior fintech code reviewer who has traced production discrepancies of fractions of a cent back to floating-point arithmetic, and you treat any float used for money as a bug regardless of how small the observed error currently is.

Here is a section of my codebase that handles money calculations: [PASTE CODE]. It's written in [LANGUAGE/FRAMEWORK], stores amounts as [DATA TYPE CURRENTLY USED], and handles [OPERATION: SPLITTING A CHARGE / APPLYING A DISCOUNT / CALCULATING TAX / CONVERTING CURRENCY] in [CURRENCY(IES)].

First, scan the code and list every operation where a rounding or precision bug could occur, including floating-point arithmetic on money values, division that isn't followed by explicit rounding, and repeated additions of rounded intermediate values that drift from the true total. For each, state the exact line or expression and why it's unsafe.

Next, explain which representation this codebase should standardize on (integer minor units, i.e., cents, versus an arbitrary-precision decimal type) given its currencies, and note any currencies in [CURRENCY(IES)] that don't use two decimal places (e.g., JPY, BHD) and how that changes the minor-unit conversion.

Then rewrite the flagged code using the correct representation, specifying the exact rounding mode (banker's rounding vs. round-half-up) at each point money is divided or a percentage is applied, and explain why that rounding mode was chosen for this specific operation.

Finally, write 5 unit test cases specifically designed to catch the rounding bugs you found, using amounts and percentages known to expose floating-point error (like 0.1 + 0.2 or splitting $10 three ways). Output everything as: a numbered list of bugs found, the corrected code with inline comments, and the test cases in [LANGUAGE/FRAMEWORK]'s testing framework.

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