diff --git a/packages/player/package.json b/packages/player/package.json
index 8abafdc..2d5b547 100644
--- a/packages/player/package.json
+++ b/packages/player/package.json
@@ -18,7 +18,7 @@
     "build": "tsup src/index.ts --format esm,cjs --dts --clean --sourcemap",
     "dev": "tsup src/index.ts --format esm,cjs --dts --watch",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run"
+    "test": "vitest run --passWithNoTests"
   },
   "dependencies": {
     "@scormflow/sdk": "workspace:*"
diff --git a/packages/react/package.json b/packages/react/package.json
index e9f4ad3..528bf53 100644
--- a/packages/react/package.json
+++ b/packages/react/package.json
@@ -18,7 +18,7 @@
     "build": "tsup src/index.ts --format esm,cjs --dts --clean --sourcemap --external react",
     "dev": "tsup src/index.ts --format esm,cjs --dts --watch --external react",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run"
+    "test": "vitest run --passWithNoTests"
   },
   "peerDependencies": {
     "react": ">=18",
diff --git a/packages/sdk/src/errors.test.ts b/packages/sdk/src/errors.test.ts
new file mode 100644
index 0000000..afa96fd
--- /dev/null
+++ b/packages/sdk/src/errors.test.ts
@@ -0,0 +1,105 @@
+import { describe, expect, it } from 'vitest';
+import {
+  ScormError,
+  ScormHttpError,
+  ScormNetworkError,
+  ScormTimeoutError,
+  ScormAbortError,
+  isRetryableStatus,
+} from './errors.js';
+
+describe('ScormError', () => {
+  it('is an Error subclass with the correct name', () => {
+    const e = new ScormError('boom');
+    expect(e).toBeInstanceOf(Error);
+    expect(e.name).toBe('ScormError');
+    expect(e.message).toBe('boom');
+  });
+
+  it('attaches the cause when provided', () => {
+    const cause = new Error('underlying');
+    const e = new ScormError('boom', { cause });
+    expect((e as Error & { cause?: unknown }).cause).toBe(cause);
+  });
+});
+
+describe('ScormHttpError', () => {
+  it('lifts code, requestId, and details from the response body', () => {
+    const e = new ScormHttpError(
+      404,
+      { code: 'course_not_found', message: 'no such course', requestId: 'req-1', details: { id: 'x' } },
+      'fallback',
+      { code: 'course_not_found' },
+    );
+    expect(e.status).toBe(404);
+    expect(e.code).toBe('course_not_found');
+    expect(e.message).toBe('no such course');
+    expect(e.requestId).toBe('req-1');
+    expect(e.details).toEqual({ id: 'x' });
+    expect(e.name).toBe('ScormHttpError');
+  });
+
+  it('falls back to fallbackMessage and `http_<status>` code when body is missing fields', () => {
+    const e = new ScormHttpError(500, undefined, 'POST /x → 500');
+    expect(e.code).toBe('http_500');
+    expect(e.message).toBe('POST /x → 500');
+    expect(e.requestId).toBeUndefined();
+    expect(e.details).toBeUndefined();
+  });
+
+  it('preserves the raw body for inspection', () => {
+    const raw = { code: 'oops', extra: 'field' };
+    const e = new ScormHttpError(500, raw, 'fallback', raw);
+    expect(e.body).toBe(raw);
+  });
+});
+
+describe('ScormNetworkError', () => {
+  it('captures the original cause', () => {
+    const cause = new TypeError('fetch failed');
+    const e = new ScormNetworkError('network down', cause);
+    expect(e.name).toBe('ScormNetworkError');
+    expect((e as Error & { cause?: unknown }).cause).toBe(cause);
+  });
+
+  it('works without a cause', () => {
+    const e = new ScormNetworkError('network down');
+    expect((e as Error & { cause?: unknown }).cause).toBeUndefined();
+  });
+});
+
+describe('ScormTimeoutError', () => {
+  it('reports the timeout in the message and exposes timeoutMs', () => {
+    const e = new ScormTimeoutError(5000);
+    expect(e.name).toBe('ScormTimeoutError');
+    expect(e.timeoutMs).toBe(5000);
+    expect(e.message).toMatch(/5000ms/);
+  });
+});
+
+describe('ScormAbortError', () => {
+  it('has a fixed message and name', () => {
+    const e = new ScormAbortError();
+    expect(e.name).toBe('ScormAbortError');
+    expect(e.message).toBe('Request was aborted');
+  });
+});
+
+describe('isRetryableStatus', () => {
+  it('marks 408, 429, and 5xx as retryable', () => {
+    expect(isRetryableStatus(408)).toBe(true);
+    expect(isRetryableStatus(429)).toBe(true);
+    expect(isRetryableStatus(500)).toBe(true);
+    expect(isRetryableStatus(503)).toBe(true);
+    expect(isRetryableStatus(599)).toBe(true);
+  });
+
+  it('marks 2xx, 3xx, and other 4xx as non-retryable', () => {
+    expect(isRetryableStatus(200)).toBe(false);
+    expect(isRetryableStatus(301)).toBe(false);
+    expect(isRetryableStatus(400)).toBe(false);
+    expect(isRetryableStatus(401)).toBe(false);
+    expect(isRetryableStatus(404)).toBe(false);
+    expect(isRetryableStatus(422)).toBe(false);
+  });
+});
diff --git a/packages/sdk/src/http/retry.test.ts b/packages/sdk/src/http/retry.test.ts
new file mode 100644
index 0000000..b8fdf12
--- /dev/null
+++ b/packages/sdk/src/http/retry.test.ts
@@ -0,0 +1,97 @@
+import { describe, expect, it } from 'vitest';
+import { computeBackoff, parseRetryAfter, sleep } from './retry.js';
+
+describe('computeBackoff', () => {
+  it('grows exponentially up to maxMs', () => {
+    expect(computeBackoff({ baseMs: 100, maxMs: 10_000, attempt: 1, jitter: false })).toBe(100);
+    expect(computeBackoff({ baseMs: 100, maxMs: 10_000, attempt: 2, jitter: false })).toBe(200);
+    expect(computeBackoff({ baseMs: 100, maxMs: 10_000, attempt: 3, jitter: false })).toBe(400);
+    expect(computeBackoff({ baseMs: 100, maxMs: 10_000, attempt: 4, jitter: false })).toBe(800);
+  });
+
+  it('caps growth at maxMs', () => {
+    expect(computeBackoff({ baseMs: 1000, maxMs: 2000, attempt: 5, jitter: false })).toBe(2000);
+  });
+
+  it('uses the injected random fn when jitter is on', () => {
+    const value = computeBackoff({
+      baseMs: 100,
+      maxMs: 10_000,
+      attempt: 4,
+      jitter: true,
+      random: () => 0.5,
+    });
+    expect(value).toBe(400);
+  });
+
+  it('floors jittered values', () => {
+    const value = computeBackoff({
+      baseMs: 100,
+      maxMs: 10_000,
+      attempt: 1,
+      jitter: true,
+      random: () => 0.999,
+    });
+    expect(Number.isInteger(value)).toBe(true);
+  });
+
+  it('treats attempt <= 0 as the first attempt', () => {
+    expect(computeBackoff({ baseMs: 100, maxMs: 10_000, attempt: 0, jitter: false })).toBe(100);
+    expect(computeBackoff({ baseMs: 100, maxMs: 10_000, attempt: -3, jitter: false })).toBe(100);
+  });
+});
+
+describe('parseRetryAfter', () => {
+  it('returns null for missing header', () => {
+    expect(parseRetryAfter(null)).toBeNull();
+  });
+
+  it('parses seconds as a number', () => {
+    expect(parseRetryAfter('5')).toBe(5000);
+    expect(parseRetryAfter('0')).toBe(0);
+  });
+
+  it('parses HTTP-date and returns milliseconds from now', () => {
+    const future = new Date('2030-01-01T00:00:30Z').toUTCString();
+    const result = parseRetryAfter(future, () => Date.parse('2030-01-01T00:00:00Z'));
+    expect(result).toBe(30_000);
+  });
+
+  it('clamps past dates to 0', () => {
+    const past = new Date('2000-01-01T00:00:00Z').toUTCString();
+    const result = parseRetryAfter(past, () => Date.parse('2030-01-01T00:00:00Z'));
+    expect(result).toBe(0);
+  });
+
+  it('returns null for garbage', () => {
+    expect(parseRetryAfter('not-a-date-or-number')).toBeNull();
+  });
+});
+
+describe('sleep', () => {
+  it('resolves after the given delay', async () => {
+    const start = Date.now();
+    await sleep(20);
+    expect(Date.now() - start).toBeGreaterThanOrEqual(15);
+  });
+
+  it('resolves immediately for non-positive delays', async () => {
+    const start = Date.now();
+    await sleep(0);
+    await sleep(-100);
+    expect(Date.now() - start).toBeLessThan(10);
+  });
+
+  it('rejects when the signal is aborted mid-sleep', async () => {
+    const controller = new AbortController();
+    const p = sleep(1000, controller.signal);
+    controller.abort();
+    await expect(p).rejects.toBeDefined();
+  });
+
+  it('rejects immediately when the signal is already aborted', async () => {
+    const controller = new AbortController();
+    controller.abort();
+    await expect(sleep(1000, controller.signal)).rejects.toBeDefined();
+  });
+});