How a Single Rupee Can Trigger ₹62,400 in Extra Tax Due to a Coding Bug

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A software developer building an India tax-regime comparison tool discovered a severe cliff-edge flaw in naive progressive tax implementations. Under India's new tax regime, incomes up to ₹12,00,000 attract zero liability through a rebate, but a poorly coded function drops the rebate entirely at ₹12,00,001, instantly imposing roughly ₹62,400 in tax on just one extra rupee of income. The law addresses this through marginal relief, which caps the tax on income just above the threshold to the amount by which earnings actually exceed it. The developer identified three such discontinuities — rebate cliff, surcharge entry points, and marginal relief calculations — and resolved them using a binary search approach in TypeScript. The article serves as a technical guide for developers building accurate tax calculators, highlighting how statutory order of operations and edge-case testing are critical to correctness.

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