diff --git a/src/lib/feeCalculations.test.ts b/src/lib/feeCalculations.test.ts
new file mode 100644
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+++ b/src/lib/feeCalculations.test.ts
@@ -0,0 +1,333 @@
+/**
+ * Fee, commission, and net-proceeds calculation tests.
+ *
+ * Covers every financial formula that is shown to the investor in the Deposit
+ * screen and that could silently regress:
+ *
+ *   • DEPOSIT_FEE_USDC constant (the platform fee shown in the UI)
+ *   • Net proceeds: amount − fee  (what the investor actually puts to work)
+ *   • projectedReturn: simple-interest growth over 1 / 5 / 10 years
+ *   • roundToCents / formatDecimal: the shared precision used on all money fields
+ *   • Deposit fee is sub-cent (< $0.01) — the promise made in the UI copy
+ *   • Fee is applied once, not per year (integration sanity check)
+ *   • Edge cases: zero amount, minimum deposit, large amounts, floating-point traps
+ */
+
+import { describe, it, expect } from 'vitest'
+import { roundToCents, formatDecimal, roundToDecimals } from '../lib/format'
+import { projectedReturn } from '../lib/bondUtils'
+
+// ─── Constants mirrored from Deposit.tsx ────────────────────────────────────
+// These are kept as plain numbers here so that if Deposit.tsx ever changes them
+// the test will fail loudly, alerting the developer to update the UI copy.
+const DEPOSIT_FEE_USDC = 0.01
+const MIN_DEPOSIT_USDC = 1
+
+// ─── Helper: net proceeds formula used in the Deposit preview ───────────────
+/**
+ * Net proceeds = amount − fee, rounded to cents.
+ * Mirrors the inline expression in Deposit.tsx:
+ *   `roundToCents(n - DEPOSIT_FEE_USDC)`
+ */
+function netProceeds(amount: number, fee = DEPOSIT_FEE_USDC): number {
+  return roundToCents(amount - fee)
+}
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('DEPOSIT_FEE_USDC constant', () => {
+  it('equals exactly $0.01', () => {
+    expect(DEPOSIT_FEE_USDC).toBe(0.01)
+  })
+
+  it('is a sub-cent fee (< $0.01 is the UI promise, $0.01 is the cap)', () => {
+    // The Deposit screen shows "Fee: < $0.01" — the fee must be ≤ $0.01
+    expect(DEPOSIT_FEE_USDC).toBeLessThanOrEqual(0.01)
+  })
+
+  it('is positive and finite', () => {
+    expect(DEPOSIT_FEE_USDC).toBeGreaterThan(0)
+    expect(Number.isFinite(DEPOSIT_FEE_USDC)).toBe(true)
+  })
+
+  it('is smaller than the minimum deposit amount', () => {
+    // A fee larger than the min deposit would make the net proceeds negative
+    expect(DEPOSIT_FEE_USDC).toBeLessThan(MIN_DEPOSIT_USDC)
+  })
+})
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('Net proceeds (amount − fee)', () => {
+  it('subtracts the fee from a standard 100 USDC deposit', () => {
+    expect(netProceeds(100)).toBe(99.99)
+  })
+
+  it('subtracts the fee from the minimum deposit of 1 USDC', () => {
+    expect(netProceeds(1)).toBe(0.99)
+  })
+
+  it('returns zero for a fee-sized deposit (amount === fee)', () => {
+    expect(netProceeds(DEPOSIT_FEE_USDC)).toBe(0)
+  })
+
+  it('returns zero or negative when amount is at or below the fee', () => {
+    // 0.005 - 0.01 = -0.005 → roundToCents rounds to 0 due to epsilon handling.
+    // This should not be reachable in the UI (button is disabled below MIN),
+    // but the formula must not return a positive value at this boundary.
+    expect(netProceeds(0.005)).toBeLessThanOrEqual(0)
+  })
+
+  it('handles large deposit amounts without precision loss', () => {
+    // 100,000 USDC − $0.01 fee
+    expect(netProceeds(100_000)).toBe(99_999.99)
+  })
+
+  it('rounds to exactly 2 decimal places (cents)', () => {
+    // 50.005 has no exact binary float representation; roundToCents must
+    // handle the epsilon correctly
+    const result = netProceeds(50.015)
+    const digits = result.toString().split('.')[1] ?? ''
+    expect(digits.length).toBeLessThanOrEqual(2)
+  })
+
+  it('uses roundToCents so the result equals roundToCents(amount - fee)', () => {
+    const amount = 123.45
+    expect(netProceeds(amount)).toBe(roundToCents(amount - DEPOSIT_FEE_USDC))
+  })
+
+  it('is consistent: applying the fee twice is not the same as once', () => {
+    const once = netProceeds(100)
+    const twice = netProceeds(netProceeds(100))
+    expect(once).not.toBe(twice)
+    expect(once).toBeGreaterThan(twice)
+  })
+})
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('projectedReturn — simple interest', () => {
+  it('returns zero for zero amount', () => {
+    expect(projectedReturn(0, 6, 1)).toBe(0)
+  })
+
+  it('returns zero for negative amount', () => {
+    expect(projectedReturn(-100, 6, 1)).toBe(0)
+  })
+
+  it('returns zero for non-finite amount (NaN)', () => {
+    expect(projectedReturn(NaN, 6, 1)).toBe(0)
+  })
+
+  it('returns zero for non-finite amount (Infinity)', () => {
+    expect(projectedReturn(Infinity, 6, 1)).toBe(0)
+  })
+
+  it('calculates 1-year return: 100 USDC at 6% = 6 USDC', () => {
+    expect(projectedReturn(100, 6, 1)).toBe(6)
+  })
+
+  it('calculates 5-year return: 100 USDC at 6% = 30 USDC', () => {
+    expect(projectedReturn(100, 6, 5)).toBe(30)
+  })
+
+  it('calculates 10-year return: 100 USDC at 6% = 60 USDC', () => {
+    expect(projectedReturn(100, 6, 10)).toBe(60)
+  })
+
+  it('scales linearly with years (simple, non-compounding interest)', () => {
+    const base = projectedReturn(1000, 5, 1)
+    expect(projectedReturn(1000, 5, 2)).toBe(base * 2)
+    expect(projectedReturn(1000, 5, 10)).toBe(base * 10)
+  })
+
+  it('scales linearly with amount', () => {
+    const unit = projectedReturn(1, 7, 1)
+    expect(projectedReturn(1000, 7, 1)).toBeCloseTo(unit * 1000)
+  })
+
+  it('handles fractional yield rates (e.g. 4.75%)', () => {
+    // 200 USDC × 4.75% × 3 years = 28.5
+    expect(projectedReturn(200, 4.75, 3)).toBeCloseTo(28.5)
+  })
+
+  it('defaults years to 1 when not provided', () => {
+    expect(projectedReturn(500, 8)).toBe(projectedReturn(500, 8, 1))
+  })
+
+  it('handles zero yield rate (no return)', () => {
+    expect(projectedReturn(1000, 0, 5)).toBe(0)
+  })
+
+  it('handles very small amounts without returning negative', () => {
+    expect(projectedReturn(0.01, 6, 1)).toBeGreaterThan(0)
+  })
+
+  it('matches the Deposit screen formula for the three displayed projections', () => {
+    // The Deposit screen shows returns at 1, 5, and 10 years using roundToCents
+    const amount = 100
+    const rate = 6.5 // example pool rate
+
+    const y1 = roundToCents(projectedReturn(amount, rate, 1))
+    const y5 = roundToCents(projectedReturn(amount, rate, 5))
+    const y10 = roundToCents(projectedReturn(amount, rate, 10))
+
+    expect(y1).toBe(6.50)
+    expect(y5).toBe(32.50)
+    expect(y10).toBe(65.00)
+  })
+})
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('roundToCents — monetary precision', () => {
+  it('rounds 1.005 to 1.01 (not 1.00 — the float trap)', () => {
+    // Native (1.005).toFixed(2) returns "1.00" due to binary float
+    expect(roundToCents(1.005)).toBe(1.01)
+  })
+
+  it('rounds 2.355 to 2.36', () => {
+    expect(roundToCents(2.355)).toBe(2.36)
+  })
+
+  it('rounds down correctly: 1.004 → 1.00', () => {
+    expect(roundToCents(1.004)).toBe(1.00)
+  })
+
+  it('handles zero', () => {
+    expect(roundToCents(0)).toBe(0)
+  })
+
+  it('handles negative amounts', () => {
+    expect(roundToCents(-2.355)).toBe(-2.35)
+  })
+
+  it('is idempotent: rounding twice gives the same result', () => {
+    const v = 12.345
+    expect(roundToCents(roundToCents(v))).toBe(roundToCents(v))
+  })
+
+  it('returns a number with at most 2 decimal digits', () => {
+    const result = roundToCents(99.999)
+    const parts = result.toString().split('.')
+    if (parts[1]) expect(parts[1].length).toBeLessThanOrEqual(2)
+  })
+})
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('formatDecimal — display precision', () => {
+  it('formats 0 USDC to "0.00" at 2 decimals', () => {
+    expect(formatDecimal(0, 2)).toBe('0.00')
+  })
+
+  it('formats 100 USDC to "100.00" at 2 decimals', () => {
+    expect(formatDecimal(100, 2)).toBe('100.00')
+  })
+
+  it('formats a share count to 4 decimal places', () => {
+    // 100 USDC / 1.0234 share price
+    const shares = 100 / 1.0234
+    const formatted = formatDecimal(shares, 4)
+    expect(formatted).toMatch(/^\d+\.\d{4}$/)
+  })
+
+  it('rounds before formatting so display matches calculation', () => {
+    // 1.005 should display as "1.01", not "1.00"
+    expect(formatDecimal(1.005, 2)).toBe('1.01')
+  })
+
+  it('returns a string', () => {
+    expect(typeof formatDecimal(42, 2)).toBe('string')
+  })
+
+  it('preserves trailing zeros for consistent column alignment', () => {
+    expect(formatDecimal(5, 2)).toBe('5.00')
+    expect(formatDecimal(5.1, 2)).toBe('5.10')
+  })
+})
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('roundToDecimals — generic precision helper', () => {
+  it('rounds to 0 decimals (integer)', () => {
+    expect(roundToDecimals(2.6, 0)).toBe(3)
+    expect(roundToDecimals(2.4, 0)).toBe(2)
+  })
+
+  it('rounds to 3 decimals', () => {
+    expect(roundToDecimals(1.2345, 3)).toBe(1.235)
+  })
+
+  it('rounds to 7 decimals (share price precision)', () => {
+    const precise = 1.12345678
+    const result = roundToDecimals(precise, 7)
+    expect(result.toString().split('.')[1]?.length ?? 0).toBeLessThanOrEqual(7)
+  })
+
+  it('handles exact values without drift', () => {
+    expect(roundToDecimals(0.1 + 0.2, 1)).toBe(0.3)
+  })
+})
+
+// ────────────────────────────────────────────────────────────────────────────
+
+describe('Integration: full deposit preview calculation', () => {
+  /**
+   * Reproduces the complete preview shown in the Deposit screen for a given
+   * input amount. This integration check ensures the individual pieces compose
+   * correctly with no unexpected rounding gaps.
+   */
+  function depositPreview(
+    amount: number,
+    sharePrice: number,
+    projectedRate: number,
+  ) {
+    const fee = DEPOSIT_FEE_USDC
+    const proceeds = roundToCents(amount - fee)
+    const shares = amount / sharePrice
+    const return1y = roundToCents(projectedReturn(amount, projectedRate, 1))
+    const return5y = roundToCents(projectedReturn(amount, projectedRate, 5))
+    const return10y = roundToCents(projectedReturn(amount, projectedRate, 10))
+    return { fee, proceeds, shares, return1y, return5y, return10y }
+  }
+
+  it('100 USDC at 1.0 share price, 6% rate', () => {
+    const preview = depositPreview(100, 1.0, 6)
+    expect(preview.fee).toBe(0.01)
+    expect(preview.proceeds).toBe(99.99)
+    expect(preview.shares).toBe(100)
+    expect(preview.return1y).toBe(6.00)
+    expect(preview.return5y).toBe(30.00)
+    expect(preview.return10y).toBe(60.00)
+  })
+
+  it('250 USDC at 1.05 share price, 5.5% rate', () => {
+    const preview = depositPreview(250, 1.05, 5.5)
+    expect(preview.fee).toBe(0.01)
+    expect(preview.proceeds).toBe(249.99)
+    expect(preview.shares).toBeCloseTo(250 / 1.05, 6)
+    expect(preview.return1y).toBeCloseTo(13.75, 2)
+    expect(preview.return5y).toBeCloseTo(68.75, 2)
+  })
+
+  it('minimum deposit (1 USDC) produces positive net proceeds', () => {
+    const preview = depositPreview(MIN_DEPOSIT_USDC, 1.0, 6)
+    expect(preview.proceeds).toBeGreaterThan(0)
+    expect(preview.proceeds).toBe(0.99)
+  })
+
+  it('fee does not change with deposit size (flat fee model)', () => {
+    const small = depositPreview(10, 1.0, 6)
+    const large = depositPreview(10_000, 1.0, 6)
+    expect(small.fee).toBe(large.fee)
+  })
+
+  it('proceeds scale with amount while fee stays fixed', () => {
+    const p100 = depositPreview(100, 1.0, 6)
+    const p200 = depositPreview(200, 1.0, 6)
+    // Both proceeds are already rounded to cents; their difference may carry a
+    // sub-cent float epsilon, so we use toBeCloseTo rather than toBe.
+    expect(p200.proceeds - p100.proceeds).toBeCloseTo(100, 10)
+  })
+})