diff --git a/src/lib/utils/money.test.ts b/src/lib/utils/money.test.ts
new file mode 100644
index 0000000..11461ba
--- /dev/null
+++ b/src/lib/utils/money.test.ts
@@ -0,0 +1,178 @@
+import { describe, it, expect } from 'vitest';
+import { MoneyHelper } from './money';
+
+describe('MoneyHelper', () => {
+  describe('toProto()', () => {
+    it('should handle normal values', () => {
+      const money = MoneyHelper.fromAmount(123.45, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.currencyCode).toBe('USD');
+      expect(proto.units).toBe('123');
+      expect(proto.nanos).toBe(450_000_000);
+    });
+
+    it('should normalize when nanos rounds to 1_000_000_000', () => {
+      // Create a value that when rounded will produce exactly 1_000_000_000 nanos
+      // 0.9999999995 should round to 1.0 when multiplied by 1_000_000_000
+      const money = MoneyHelper.fromAmount(0.9999999995, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('1');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should normalize when nanos rounds to -1_000_000_000', () => {
+      // Create a value that when rounded will produce exactly -1_000_000_000 nanos
+      const money = MoneyHelper.fromAmount(-0.9999999995, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('-1');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should handle positive values close to rounding boundary', () => {
+      const money = MoneyHelper.fromAmount(5.9999999995, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('6');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should handle negative values close to rounding boundary', () => {
+      const money = MoneyHelper.fromAmount(-5.9999999995, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('-6');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should handle zero', () => {
+      const money = MoneyHelper.fromAmount(0, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('0');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should handle large positive values', () => {
+      const money = MoneyHelper.fromAmount(999999.9999999995, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('1000000');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should handle large negative values', () => {
+      const money = MoneyHelper.fromAmount(-999999.9999999995, 'USD');
+      const proto = money.toProto();
+      
+      expect(proto.units).toBe('-1000000');
+      expect(proto.nanos).toBe(0);
+    });
+
+    it('should maintain consistency: from -> toProto -> from should preserve value', () => {
+      const originalAmount = 123.456789012;
+      const money1 = MoneyHelper.fromAmount(originalAmount, 'USD');
+      const proto = money1.toProto();
+      
+      // Verify proto has valid nanos (this is the bug fix test)
+      expect(proto.nanos).toBeGreaterThanOrEqual(-999_999_999);
+      expect(proto.nanos).toBeLessThanOrEqual(999_999_999);
+      
+      const money2 = MoneyHelper.from(proto);
+      
+      // The round-trip should preserve the value within reasonable precision
+      expect(money2.toNumber()).toBeCloseTo(originalAmount, 9);
+    });
+
+    it('should handle arithmetic results that may round to boundary', () => {
+      // Test a division that might produce a value close to rounding boundary
+      const money = MoneyHelper.fromAmount(10, 'USD');
+      const result = money.div(3).mul(3); // 10/3*3 may have precision issues
+      const proto = result.toProto();
+      
+      // Should not throw and should have valid nanos
+      expect(proto.nanos).toBeGreaterThanOrEqual(-999_999_999);
+      expect(proto.nanos).toBeLessThanOrEqual(999_999_999);
+    });
+
+    it('should always produce valid nanos for any amount', () => {
+      // Test various random amounts to ensure nanos is always valid
+      const testAmounts = [
+        0.9999999995, -0.9999999995,
+        1.9999999995, -1.9999999995,
+        999.9999999995, -999.9999999995,
+        0.123456789, -0.123456789,
+        12345.6789, -12345.6789,
+      ];
+
+      for (const amount of testAmounts) {
+        const proto = MoneyHelper.fromAmount(amount, 'USD').toProto();
+        expect(proto.nanos).toBeGreaterThanOrEqual(-999_999_999);
+        expect(proto.nanos).toBeLessThanOrEqual(999_999_999);
+      }
+    });
+  });
+
+  describe('from()', () => {
+    it('should construct from valid proto', () => {
+      const proto = {
+        currencyCode: 'USD',
+        units: '123',
+        nanos: 450_000_000,
+      };
+      const money = MoneyHelper.from(proto);
+      
+      expect(money.toNumber()).toBeCloseTo(123.45, 9);
+      expect(money.currency).toBe('USD');
+    });
+
+    it('should handle null/undefined', () => {
+      const money1 = MoneyHelper.from(null);
+      expect(money1.toNumber()).toBe(0);
+      
+      const money2 = MoneyHelper.from(undefined);
+      expect(money2.toNumber()).toBe(0);
+    });
+  });
+
+  describe('arithmetic operations', () => {
+    it('should add correctly', () => {
+      const m1 = MoneyHelper.fromAmount(100.50, 'USD');
+      const m2 = MoneyHelper.fromAmount(50.25, 'USD');
+      const result = m1.add(m2);
+      
+      expect(result.toNumber()).toBeCloseTo(150.75, 9);
+    });
+
+    it('should subtract correctly', () => {
+      const m1 = MoneyHelper.fromAmount(100.50, 'USD');
+      const m2 = MoneyHelper.fromAmount(50.25, 'USD');
+      const result = m1.sub(m2);
+      
+      expect(result.toNumber()).toBeCloseTo(50.25, 9);
+    });
+
+    it('should multiply correctly', () => {
+      const money = MoneyHelper.fromAmount(100, 'USD');
+      const result = money.mul(1.5);
+      
+      expect(result.toNumber()).toBeCloseTo(150, 9);
+    });
+
+    it('should divide correctly', () => {
+      const money = MoneyHelper.fromAmount(100, 'USD');
+      const result = money.div(4);
+      
+      expect(result.toNumber()).toBeCloseTo(25, 9);
+    });
+
+    it('should throw on currency mismatch', () => {
+      const m1 = MoneyHelper.fromAmount(100, 'USD');
+      const m2 = MoneyHelper.fromAmount(100, 'EUR');
+      
+      expect(() => m1.add(m2)).toThrow('Currency mismatch');
+    });
+  });
+});
diff --git a/src/lib/utils/money.ts b/src/lib/utils/money.ts
index 05aa1ea..955e064 100644
--- a/src/lib/utils/money.ts
+++ b/src/lib/utils/money.ts
@@ -61,13 +61,23 @@ export class MoneyHelper {
    * 对应 Go 的 ToEntityValues
    */
   toProto(): MoneyProto {
-    const unitsDec = this.amount.trunc();
-    const nanosDec = this.amount.minus(unitsDec).times(NANOS_MOD).round();
+    let units = this.amount.trunc();
+    let nanos = this.amount.minus(units).times(NANOS_MOD).round();
+
+    // Normalize: carry overflow/underflow nanos into units
+    // Use >= and <= for defensive programming, though rounding should only produce exactly ±NANOS_MOD
+    if (nanos.gte(NANOS_MOD)) {
+      units = units.plus(1);
+      nanos = nanos.minus(NANOS_MOD);
+    } else if (nanos.lte(-NANOS_MOD)) {
+      units = units.minus(1);
+      nanos = nanos.plus(NANOS_MOD);
+    }
 
     return {
       currencyCode: this.currency,
-      units: unitsDec.toString(),
-      nanos: nanosDec.toNumber(),
+      units: units.toString(),
+      nanos: nanos.toNumber(),
     };
   }