diff --git a/src/client.test.ts b/src/client.test.ts
new file mode 100644
index 0000000..98efec7
--- /dev/null
+++ b/src/client.test.ts
@@ -0,0 +1,41 @@
+import { describe, it, expect } from 'vitest';
+import { AgentGram } from './client.js';
+
+describe('AgentGram', () => {
+  it('should create an instance with required options', () => {
+    const client = new AgentGram({ apiKey: 'ag_test123' });
+    expect(client).toBeDefined();
+    expect(client.agents).toBeDefined();
+    expect(client.posts).toBeDefined();
+    expect(client.stories).toBeDefined();
+    expect(client.hashtags).toBeDefined();
+    expect(client.notifications).toBeDefined();
+    expect(client.ax).toBeDefined();
+  });
+
+  it('should expose all resource properties', () => {
+    const client = new AgentGram({ apiKey: 'ag_test123' });
+    const resources = ['agents', 'ax', 'posts', 'stories', 'hashtags', 'notifications'] as const;
+
+    for (const resource of resources) {
+      expect(client[resource]).toBeDefined();
+      expect(typeof client[resource]).toBe('object');
+    }
+  });
+
+  it('should accept custom base URL', () => {
+    const client = new AgentGram({
+      apiKey: 'ag_test123',
+      baseUrl: 'https://custom.example.com/api/v1',
+    });
+    expect(client).toBeDefined();
+  });
+
+  it('should accept custom timeout', () => {
+    const client = new AgentGram({
+      apiKey: 'ag_test123',
+      timeout: 60000,
+    });
+    expect(client).toBeDefined();
+  });
+});
diff --git a/src/errors.test.ts b/src/errors.test.ts
new file mode 100644
index 0000000..8603a99
--- /dev/null
+++ b/src/errors.test.ts
@@ -0,0 +1,82 @@
+import { describe, it, expect } from 'vitest';
+import {
+  AgentGramError,
+  AuthenticationError,
+  RateLimitError,
+  NotFoundError,
+  ValidationError,
+  ServerError,
+} from './errors.js';
+
+describe('AgentGramError', () => {
+  it('should create a base error with message', () => {
+    const error = new AgentGramError('test error');
+    expect(error.message).toBe('test error');
+    expect(error.name).toBe('AgentGramError');
+    expect(error.statusCode).toBeUndefined();
+    expect(error.code).toBeUndefined();
+    expect(error).toBeInstanceOf(Error);
+  });
+
+  it('should create a base error with status code and code', () => {
+    const error = new AgentGramError('test', 418, 'TEAPOT');
+    expect(error.statusCode).toBe(418);
+    expect(error.code).toBe('TEAPOT');
+  });
+});
+
+describe('AuthenticationError', () => {
+  it('should have correct defaults', () => {
+    const error = new AuthenticationError();
+    expect(error.message).toBe('Authentication failed');
+    expect(error.statusCode).toBe(401);
+    expect(error.code).toBe('AUTHENTICATION_ERROR');
+    expect(error.name).toBe('AuthenticationError');
+    expect(error).toBeInstanceOf(AgentGramError);
+  });
+
+  it('should accept custom message', () => {
+    const error = new AuthenticationError('Invalid API key');
+    expect(error.message).toBe('Invalid API key');
+  });
+});
+
+describe('RateLimitError', () => {
+  it('should have correct defaults', () => {
+    const error = new RateLimitError();
+    expect(error.statusCode).toBe(429);
+    expect(error.code).toBe('RATE_LIMIT_ERROR');
+    expect(error.name).toBe('RateLimitError');
+    expect(error).toBeInstanceOf(AgentGramError);
+  });
+});
+
+describe('NotFoundError', () => {
+  it('should have correct defaults', () => {
+    const error = new NotFoundError();
+    expect(error.statusCode).toBe(404);
+    expect(error.code).toBe('NOT_FOUND_ERROR');
+    expect(error.name).toBe('NotFoundError');
+    expect(error).toBeInstanceOf(AgentGramError);
+  });
+});
+
+describe('ValidationError', () => {
+  it('should have correct defaults', () => {
+    const error = new ValidationError();
+    expect(error.statusCode).toBe(400);
+    expect(error.code).toBe('VALIDATION_ERROR');
+    expect(error.name).toBe('ValidationError');
+    expect(error).toBeInstanceOf(AgentGramError);
+  });
+});
+
+describe('ServerError', () => {
+  it('should have correct defaults', () => {
+    const error = new ServerError();
+    expect(error.statusCode).toBe(500);
+    expect(error.code).toBe('SERVER_ERROR');
+    expect(error.name).toBe('ServerError');
+    expect(error).toBeInstanceOf(AgentGramError);
+  });
+});
diff --git a/src/http.test.ts b/src/http.test.ts
new file mode 100644
index 0000000..49fbc09
--- /dev/null
+++ b/src/http.test.ts
@@ -0,0 +1,243 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { HttpClient } from './http.js';
+import {
+  AuthenticationError,
+  NotFoundError,
+  RateLimitError,
+  ServerError,
+  ValidationError,
+  AgentGramError,
+} from './errors.js';
+
+function mockFetch(response: {
+  ok?: boolean;
+  status?: number;
+  json?: () => Promise<unknown>;
+}) {
+  return vi.fn().mockResolvedValue({
+    ok: response.ok ?? true,
+    status: response.status ?? 200,
+    json: response.json ?? (() => Promise.resolve({ success: true, data: {} })),
+  });
+}
+
+describe('HttpClient', () => {
+  let originalFetch: typeof globalThis.fetch;
+
+  beforeEach(() => {
+    originalFetch = globalThis.fetch;
+  });
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+  });
+
+  it('should make GET requests with correct headers', async () => {
+    const fetchMock = mockFetch({
+      json: () => Promise.resolve({ success: true, data: { id: '1' } }),
+    });
+    globalThis.fetch = fetchMock;
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await client.get('/agents/me');
+
+    expect(fetchMock).toHaveBeenCalledOnce();
+    const [url, opts] = fetchMock.mock.calls[0];
+    expect(url).toBe('https://api.test.com/agents/me');
+    expect(opts.method).toBe('GET');
+    expect(opts.headers.Authorization).toBe('Bearer ag_testkey');
+    expect(opts.headers['Content-Type']).toBe('application/json');
+  });
+
+  it('should make POST requests with body', async () => {
+    const fetchMock = mockFetch({
+      json: () => Promise.resolve({ success: true, data: { id: '2' } }),
+    });
+    globalThis.fetch = fetchMock;
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await client.post('/posts', { title: 'Hello', content: 'World' });
+
+    const [, opts] = fetchMock.mock.calls[0];
+    expect(opts.method).toBe('POST');
+    expect(opts.body).toBe(JSON.stringify({ title: 'Hello', content: 'World' }));
+  });
+
+  it('should include query parameters in GET requests', async () => {
+    const fetchMock = mockFetch({
+      json: () => Promise.resolve({ success: true, data: [] }),
+    });
+    globalThis.fetch = fetchMock;
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await client.get('/posts', { sort: 'hot', limit: 10 });
+
+    const [url] = fetchMock.mock.calls[0];
+    expect(url).toContain('sort=hot');
+    expect(url).toContain('limit=10');
+  });
+
+  it('should omit undefined query parameters', async () => {
+    const fetchMock = mockFetch({
+      json: () => Promise.resolve({ success: true, data: [] }),
+    });
+    globalThis.fetch = fetchMock;
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await client.get('/posts', { sort: 'hot', community: undefined });
+
+    const [url] = fetchMock.mock.calls[0];
+    expect(url).toContain('sort=hot');
+    expect(url).not.toContain('community');
+  });
+
+  it('should throw AuthenticationError on 401', async () => {
+    globalThis.fetch = mockFetch({
+      ok: false,
+      status: 401,
+      json: () =>
+        Promise.resolve({
+          success: false,
+          error: { code: 'UNAUTHORIZED', message: 'Invalid API key' },
+        }),
+    });
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'bad_key',
+      timeout: 5000,
+    });
+
+    await expect(client.get('/agents/me')).rejects.toThrow(AuthenticationError);
+  });
+
+  it('should throw ValidationError on 400', async () => {
+    globalThis.fetch = mockFetch({
+      ok: false,
+      status: 400,
+      json: () =>
+        Promise.resolve({
+          success: false,
+          error: { code: 'INVALID_INPUT', message: 'Name required' },
+        }),
+    });
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await expect(client.post('/agents/register', {})).rejects.toThrow(ValidationError);
+  });
+
+  it('should throw NotFoundError on 404', async () => {
+    globalThis.fetch = mockFetch({
+      ok: false,
+      status: 404,
+      json: () =>
+        Promise.resolve({
+          success: false,
+          error: { code: 'NOT_FOUND', message: 'Post not found' },
+        }),
+    });
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await expect(client.get('/posts/nonexistent')).rejects.toThrow(NotFoundError);
+  });
+
+  it('should throw RateLimitError on 429', async () => {
+    globalThis.fetch = mockFetch({
+      ok: false,
+      status: 429,
+      json: () =>
+        Promise.resolve({
+          success: false,
+          error: { code: 'RATE_LIMIT', message: 'Too many requests' },
+        }),
+    });
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await expect(client.post('/posts', {})).rejects.toThrow(RateLimitError);
+  });
+
+  it('should throw ServerError on 500', async () => {
+    globalThis.fetch = mockFetch({
+      ok: false,
+      status: 500,
+      json: () =>
+        Promise.resolve({
+          success: false,
+          error: { code: 'INTERNAL', message: 'Server error' },
+        }),
+    });
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await expect(client.get('/health')).rejects.toThrow(ServerError);
+  });
+
+  it('should strip trailing slash from base URL', async () => {
+    const fetchMock = mockFetch({
+      json: () => Promise.resolve({ success: true, data: {} }),
+    });
+    globalThis.fetch = fetchMock;
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com/',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await client.get('/health');
+
+    const [url] = fetchMock.mock.calls[0];
+    expect(url).toBe('https://api.test.com/health');
+  });
+
+  it('should throw on network error', async () => {
+    globalThis.fetch = vi.fn().mockRejectedValue(new Error('Network failure'));
+
+    const client = new HttpClient({
+      baseUrl: 'https://api.test.com',
+      apiKey: 'ag_testkey',
+      timeout: 5000,
+    });
+
+    await expect(client.get('/health')).rejects.toThrow(AgentGramError);
+  });
+});