diff --git a/tests/bigint.test.ts b/tests/bigint.test.ts
new file mode 100644
index 0000000..f9b30b9
--- /dev/null
+++ b/tests/bigint.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest';
+import { toBigintOrThrow } from '../src/utils/bigint';
+import { SdkError } from '../src/errors';
+
+const ctx = { code: 'CONFIG' as const, name: 'amount', detail: {} };
+
+describe('toBigintOrThrow', () => {
+  it('passes through bigint values', () => {
+    expect(toBigintOrThrow(42n, ctx)).toBe(42n);
+    expect(toBigintOrThrow(0n, ctx)).toBe(0n);
+  });
+
+  it('converts numeric strings', () => {
+    expect(toBigintOrThrow('123', ctx)).toBe(123n);
+    expect(toBigintOrThrow('0', ctx)).toBe(0n);
+  });
+
+  it('converts finite numbers', () => {
+    expect(toBigintOrThrow(100, ctx)).toBe(100n);
+    expect(toBigintOrThrow(0, ctx)).toBe(0n);
+  });
+
+  it('throws SdkError for invalid input', () => {
+    expect(() => toBigintOrThrow('not-a-number', ctx)).toThrow(SdkError);
+    expect(() => toBigintOrThrow(null, ctx)).toThrow(SdkError);
+    expect(() => toBigintOrThrow(undefined, ctx)).toThrow(SdkError);
+    expect(() => toBigintOrThrow({}, ctx)).toThrow(SdkError);
+  });
+
+  it('includes field name in error message', () => {
+    try {
+      toBigintOrThrow('bad', ctx);
+    } catch (err) {
+      expect(err).toBeInstanceOf(SdkError);
+      expect((err as SdkError).message).toContain('amount');
+      expect((err as SdkError).code).toBe('CONFIG');
+    }
+  });
+});
diff --git a/tests/hex.test.ts b/tests/hex.test.ts
new file mode 100644
index 0000000..2f07e93
--- /dev/null
+++ b/tests/hex.test.ts
@@ -0,0 +1,42 @@
+import { describe, expect, it } from 'vitest';
+import { isHexStrict } from '../src/utils/hex';
+
+describe('isHexStrict', () => {
+  it('accepts valid hex strings', () => {
+    expect(isHexStrict('0xab')).toBe(true);
+    expect(isHexStrict('0xABCDEF0123456789')).toBe(true);
+    expect(isHexStrict('0x00')).toBe(true);
+  });
+
+  it('rejects non-string input', () => {
+    expect(isHexStrict(42)).toBe(false);
+    expect(isHexStrict(null)).toBe(false);
+    expect(isHexStrict(undefined)).toBe(false);
+    expect(isHexStrict({})).toBe(false);
+  });
+
+  it('rejects strings without 0x prefix', () => {
+    expect(isHexStrict('abcd')).toBe(false);
+  });
+
+  it('rejects empty payload (bare 0x)', () => {
+    expect(isHexStrict('0x')).toBe(false);
+  });
+
+  it('rejects odd-length payload', () => {
+    expect(isHexStrict('0xabc')).toBe(false);
+    expect(isHexStrict('0x1')).toBe(false);
+  });
+
+  it('rejects non-hex characters', () => {
+    expect(isHexStrict('0xgh')).toBe(false);
+    expect(isHexStrict('0x00zz')).toBe(false);
+  });
+
+  it('enforces minBytes option', () => {
+    expect(isHexStrict('0xab', { minBytes: 1 })).toBe(true);
+    expect(isHexStrict('0xab', { minBytes: 2 })).toBe(false);
+    expect(isHexStrict('0xaabbccdd', { minBytes: 4 })).toBe(true);
+    expect(isHexStrict('0xaabb', { minBytes: 4 })).toBe(false);
+  });
+});
diff --git a/tests/json.test.ts b/tests/json.test.ts
new file mode 100644
index 0000000..9aaf2f2
--- /dev/null
+++ b/tests/json.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it } from 'vitest';
+import { bigintReplacer, serializeBigInt, stableStringify } from '../src/utils/json';
+
+describe('bigintReplacer', () => {
+  it('converts bigint to string', () => {
+    expect(bigintReplacer('x', 42n)).toBe('42');
+  });
+
+  it('passes non-bigint values through', () => {
+    expect(bigintReplacer('x', 'hello')).toBe('hello');
+    expect(bigintReplacer('x', 99)).toBe(99);
+    expect(bigintReplacer('x', null)).toBe(null);
+  });
+});
+
+describe('serializeBigInt', () => {
+  it('serializes objects with bigint fields', () => {
+    const result = serializeBigInt({ a: 1n, b: 'text' });
+    expect(JSON.parse(result)).toEqual({ a: '1', b: 'text' });
+  });
+});
+
+describe('stableStringify', () => {
+  it('sorts object keys for deterministic output', () => {
+    const a = stableStringify({ z: 1, a: 2 });
+    const b = stableStringify({ a: 2, z: 1 });
+    expect(a).toBe(b);
+    expect(JSON.parse(a)).toEqual({ a: 2, z: 1 });
+  });
+
+  it('handles nested objects with sorted keys', () => {
+    const result = stableStringify({ b: { d: 1, c: 2 }, a: 3 });
+    const keys = Object.keys(JSON.parse(result));
+    expect(keys).toEqual(['a', 'b']);
+  });
+
+  it('handles arrays without reordering', () => {
+    const result = stableStringify([3, 1, 2]);
+    expect(JSON.parse(result)).toEqual([3, 1, 2]);
+  });
+
+  it('converts bigint values', () => {
+    const result = stableStringify({ amount: 100n });
+    expect(JSON.parse(result)).toEqual({ amount: '100' });
+  });
+
+  it('strips undefined values', () => {
+    const result = stableStringify({ a: 1, b: undefined });
+    expect(JSON.parse(result)).toEqual({ a: 1 });
+  });
+
+  it('handles null and primitives', () => {
+    expect(stableStringify(null)).toBe('null');
+    expect(stableStringify(42)).toBe('42');
+    expect(stableStringify('hi')).toBe('"hi"');
+  });
+});
diff --git a/tests/signal.test.ts b/tests/signal.test.ts
new file mode 100644
index 0000000..509a87d
--- /dev/null
+++ b/tests/signal.test.ts
@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { signalTimeout, signalAny } from '../src/utils/signal';
+
+describe('signalTimeout', () => {
+  it('returns an AbortSignal', () => {
+    const signal = signalTimeout(5_000);
+    expect(signal).toBeInstanceOf(AbortSignal);
+    expect(signal.aborted).toBe(false);
+  });
+
+  it('aborts after the specified timeout', async () => {
+    const signal = signalTimeout(50);
+    expect(signal.aborted).toBe(false);
+    await new Promise((r) => setTimeout(r, 100));
+    expect(signal.aborted).toBe(true);
+  });
+});
+
+describe('signalAny', () => {
+  it('returns undefined for empty input', () => {
+    expect(signalAny([])).toBeUndefined();
+  });
+
+  it('returns undefined when all entries are undefined', () => {
+    expect(signalAny([undefined, undefined])).toBeUndefined();
+  });
+
+  it('aborts when any input signal aborts', () => {
+    const c1 = new AbortController();
+    const c2 = new AbortController();
+    const combined = signalAny([c1.signal, c2.signal]);
+    expect(combined).toBeDefined();
+    expect(combined!.aborted).toBe(false);
+
+    c1.abort(new Error('first'));
+    expect(combined!.aborted).toBe(true);
+  });
+
+  it('is already aborted if an input signal is pre-aborted', () => {
+    const c = new AbortController();
+    c.abort(new Error('already'));
+    const combined = signalAny([c.signal]);
+    expect(combined!.aborted).toBe(true);
+  });
+});