Fintech Development

Tiered Fee Calculation Engine Prompt

Build a rate and fee calculation engine that correctly handles tiered pricing, promotional overrides, and precise rounding at every step.

Anselm Fowel / Public collection27 Sept 2026

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The prompt

You are a fintech engineer who has built fee calculation engines for a payments platform and has debugged the specific class of bugs where fee logic is technically correct in isolation but produces a wrong total once tiers, overrides, and rounding interact.

My platform charges [FEE STRUCTURE: A FLAT FEE PLUS PERCENTAGE / TIERED VOLUME-BASED PRICING / INTERCHANGE-PLUS PRICING] to [CUSTOMER TYPE: MERCHANTS / END USERS] based on [PRICING DIMENSION: MONTHLY VOLUME / TRANSACTION COUNT / ACCOUNT TIER]. Some customers have [PROMOTIONAL OVERRIDES: A TEMPORARY DISCOUNT / A CUSTOM NEGOTIATED RATE] that must take precedence over standard tiers.

First, design the core calculation logic for tiered pricing: whether tiers are cliff-based (the whole amount is charged at the rate of the tier it falls into) or graduated (each portion is charged at its own tier's rate, like a tax bracket), confirm which model matches [FEE STRUCTURE], and show the calculation for a concrete example amount crossing multiple tiers.

Next, design how promotional and negotiated overrides layer on top of or replace the standard tier calculation, including the precedence order when a customer qualifies for multiple potential overrides at once (e.g., a volume discount and a time-limited promo), and how the engine logs which rule actually applied so a billing dispute can be traced to a specific rule, not just a final number.

Then address rounding: where in the calculation chain rounding is applied when multiple percentage-based fees or a percentage-plus-flat-fee combination are computed, why rounding intermediate results separately versus rounding only the final total can produce different (and both defensible-looking) answers, and which approach this system should standardize on and why. Also design how the engine handles a rate change or promo expiring mid-billing-period, and how charges are prorated across the rate boundary without double-charging or under-charging for that period.

Finally, output the calculation engine's core function in [LANGUAGE/FRAMEWORK] with comments explaining tier logic, override precedence, and rounding points, followed by a table of 6 test cases (crossing a tier boundary, overlapping promos, mid-period rate change, and a rounding edge case) with expected output for each.

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