diff --git a/tests/fixed-decimal.test.ts b/tests/fixed-decimal.test.ts
index d4e88a4..c50128e 100644
--- a/tests/fixed-decimal.test.ts
+++ b/tests/fixed-decimal.test.ts
@@ -1,4 +1,5 @@
-import assert from "node:assert/strict";
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
 import {
   addFixed,
   subFixed,
@@ -6,44 +7,139 @@ import {
   divFixed,
   roundFixed,
   formatFixed,
-} from "../src/lib/data/fixed-decimal";
+} from '../src/lib/data/fixed-decimal';
 
+// Baseline inline checks
 assert.equal(addFixed(0.1, 0.2, 2), 0.3);
-assert.equal(addFixed("12.345", "0.005", 2), 12.35);
-assert.equal(subFixed("1.0", "0.9", 2), 0.1);
-assert.equal(subFixed("-1.25", "-2.50", 2), 1.25);
-
-assert.equal(mulFixed("19.99", 3, 2), 59.97);
-assert.equal(mulFixed("1.25", "2.5", 3), 3.125);
+assert.equal(addFixed('12.345', '0.005', 2), 12.35);
+assert.equal(subFixed('1.0', '0.9', 2), 0.1);
+assert.equal(subFixed('-1.25', '-2.50', 2), 1.25);
+assert.equal(mulFixed('19.99', 3, 2), 59.97);
+assert.equal(mulFixed('1.25', '2.5', 3), 3.125);
 assert.equal(divFixed(1, 3, 2), 0.33);
 assert.equal(divFixed(10, 4, 3), 2.5);
 assert.throws(() => divFixed(1, 0), RangeError);
+assert.equal(roundFixed('2.5', 0, 'half-even'), 2);
+assert.equal(roundFixed('3.5', 0, 'half-even'), 4);
+assert.equal(roundFixed('-2.5', 0, 'half-even'), -2);
+assert.equal(roundFixed('2.5', 0, 'half-up'), 3);
+assert.equal(roundFixed('-2.5', 0, 'half-up'), -3);
+assert.equal(roundFixed('2.99', 0, 'truncate'), 2);
+assert.equal(roundFixed('-2.99', 0, 'truncate'), -2);
+assert.equal(formatFixed('1250.5', { currencyPrefix: '$' }), '$1,250.50');
+assert.equal(formatFixed('12', { precision: 4 }), '12.0000');
+assert.equal(formatFixed('-1234.567'), '-1,234.57');
+assert.equal(formatFixed('1234.5', { commaSeparated: false }), '1234.50');
+assert.equal(formatFixed('42', { precision: 0 }), '42');
+
+describe('Fixed-Point Decimal Arithmetic (fixed-decimal.ts)', () => {
+  describe('addFixed', () => {
+    it('corrects classic floating point addition drift (0.1 + 0.2 === 0.3)', () => {
+      assert.strictEqual(addFixed(0.1, 0.2), 0.3);
+      assert.strictEqual(addFixed('0.1', '0.2'), 0.3);
+      assert.strictEqual(addFixed(0.07, 0.01), 0.08);
+    });
+
+    it('handles negative operands and zeroes', () => {
+      assert.strictEqual(addFixed(-10.5, 5.25), -5.25);
+      assert.strictEqual(addFixed(0, 0), 0);
+      assert.strictEqual(addFixed(100.5, -100.5), 0);
+    });
+
+    it('supports custom precision', () => {
+      assert.strictEqual(addFixed(0.0001, 0.0002, 4), 0.0003);
+    });
+
+    it('rejects invalid inputs', () => {
+      assert.throws(() => addFixed('invalid', 5), TypeError);
+      assert.throws(() => addFixed(5, 5, -1), RangeError);
+    });
+  });
+
+  describe('subFixed', () => {
+    it('corrects floating point subtraction drift (1.0 - 0.9 === 0.1)', () => {
+      assert.strictEqual(subFixed(1.0, 0.9), 0.1);
+      assert.strictEqual(subFixed('100.05', '0.05'), 100);
+    });
+
+    it('handles negative results', () => {
+      assert.strictEqual(subFixed(50, 100), -50);
+    });
+  });
+
+  describe('mulFixed', () => {
+    it('corrects floating point multiplication drift (19.99 * 3 === 59.97)', () => {
+      assert.strictEqual(mulFixed(19.99, 3), 59.97);
+      assert.strictEqual(mulFixed('19.99', '3'), 59.97);
+    });
+
+    it('applies default banker rounding on fractional results', () => {
+      assert.strictEqual(mulFixed(10.55, 0.1, 2), 1.06);
+    });
+  });
+
+  describe('divFixed', () => {
+    it('computes division and rounds properly', () => {
+      assert.strictEqual(divFixed(100, 3, 2), 33.33);
+      assert.strictEqual(divFixed(10, 2), 5);
+    });
+
+    it('throws RangeError on division by zero', () => {
+      assert.throws(() => divFixed(100, 0), RangeError);
+      assert.throws(() => divFixed(100, '0'), RangeError);
+    });
+  });
+
+  describe('roundFixed (Banker\'s Rounding / half-even)', () => {
+    it('implements half-even rounding correctly', () => {
+      assert.strictEqual(roundFixed(2.5, 0, 'half-even'), 2);
+      assert.strictEqual(roundFixed(3.5, 0, 'half-even'), 4);
+      assert.strictEqual(roundFixed(-2.5, 0, 'half-even'), -2);
+      assert.strictEqual(roundFixed(-3.5, 0, 'half-even'), -4);
+    });
+
+    it('supports half-up mode', () => {
+      assert.strictEqual(roundFixed(2.5, 0, 'half-up'), 3);
+      assert.strictEqual(roundFixed(3.5, 0, 'half-up'), 4);
+    });
+
+    it('supports truncate mode', () => {
+      assert.strictEqual(roundFixed(2.99, 0, 'truncate'), 2);
+      assert.strictEqual(roundFixed(-2.99, 0, 'truncate'), -2);
+    });
+  });
+
+  describe('formatFixed', () => {
+    it('formats with thousands commas and currency prefix', () => {
+      assert.strictEqual(
+        formatFixed(1250.5, { currencyPrefix: '$' }),
+        '$1,250.50'
+      );
+      assert.strictEqual(
+        formatFixed('1250000.75', { currencyPrefix: '€' }),
+        '€1,250,000.75'
+      );
+    });
+
+    it('supports disabling commas', () => {
+      assert.strictEqual(
+        formatFixed(1250.5, { commaSeparated: false }),
+        '1250.50'
+      );
+    });
+
+    it('formats negative balances correctly', () => {
+      assert.strictEqual(
+        formatFixed(-45.2, { currencyPrefix: '$' }),
+        '-$45.20'
+      );
+    });
 
-assert.equal(roundFixed("2.5", 0, "half-even"), 2);
-assert.equal(roundFixed("3.5", 0, "half-even"), 4);
-assert.equal(roundFixed("-2.5", 0, "half-even"), -2);
-assert.equal(roundFixed("2.5", 0, "half-up"), 3);
-assert.equal(roundFixed("-2.5", 0, "half-up"), -3);
-assert.equal(roundFixed("2.99", 0, "truncate"), 2);
-assert.equal(roundFixed("-2.99", 0, "truncate"), -2);
-
-assert.equal(formatFixed("1250.5", { currencyPrefix: "$" }), "$1,250.50");
-assert.equal(formatFixed("12", { precision: 4 }), "12.0000");
-assert.equal(formatFixed("-1234.567"), "-1,234.57");
-assert.equal(
-  formatFixed("1234.5", { commaSeparated: false }),
-  "1234.50",
-);
-assert.equal(formatFixed("42", { precision: 0 }), "42");
-
-assert.equal(addFixed("1e-2", "2e-2", 2), 0.03);
-assert.equal(mulFixed("-0.1", "0.2", 3), -0.02);
-assert.equal(roundFixed("0.000123", 4), 0.0001);
-
-assert.throws(() => addFixed("abc", 1), TypeError);
-assert.throws(() => roundFixed(Number.NaN), TypeError);
-assert.throws(() => roundFixed(1, -1), RangeError);
-assert.throws(() => roundFixed(1, 101), RangeError);
-assert.throws(() => roundFixed(1, 2, "invalid" as never), RangeError);
-
-console.log("fixed-decimal tests passed");
+    it('supports custom precision formatting', () => {
+      assert.strictEqual(
+        formatFixed(0.123456, { precision: 4 }),
+        '0.1235'
+      );
+    });
+  });
+});