diff --git a/apps/worker/src/whatsapp/session-pool.test.ts b/apps/worker/src/whatsapp/session-pool.test.ts
new file mode 100644
index 0000000..3daeb4d
--- /dev/null
+++ b/apps/worker/src/whatsapp/session-pool.test.ts
@@ -0,0 +1,76 @@
+import { WhatsAppSessionPool } from './session-pool';
+
+// Mock createWhatsAppSession so we don't need a real WhatsApp connection
+jest.mock('./session', () => ({
+  createWhatsAppSession: jest.fn().mockResolvedValue({
+    sendMessage: jest.fn().mockResolvedValue({}),
+    logout: jest.fn().mockResolvedValue({}),
+  }),
+}));
+
+describe('WhatsAppSessionPool', () => {
+  let pool: WhatsAppSessionPool;
+
+  beforeEach(() => {
+    pool = new WhatsAppSessionPool();
+    jest.clearAllMocks();
+  });
+
+  it('add() stores a session by accountId', async () => {
+    await pool.add('acc_1', './sessions/1', jest.fn(), jest.fn(), jest.fn());
+    expect(pool.get('acc_1')).toBeDefined();
+  });
+
+  it('get() returns undefined for unknown accountId', () => {
+    expect(pool.get('acc_unknown')).toBeUndefined();
+  });
+
+  it('getAll() returns the sessions map', async () => {
+    await pool.add('acc_1', './sessions/1', jest.fn(), jest.fn(), jest.fn());
+    expect(pool.getAll().size).toBe(1);
+    expect(pool.getAll().has('acc_1')).toBe(true);
+  });
+
+  it('sendMessage() throws with informative error when no session exists', async () => {
+    await expect(pool.sendMessage('acc_missing', '1234@g.us', 'hello')).rejects.toThrow(
+      'No active session for account acc_missing',
+    );
+  });
+
+  it('sendMessage() includes available accounts in error when others exist', async () => {
+    await pool.add('acc_1', './sessions/1', jest.fn(), jest.fn(), jest.fn());
+    await expect(pool.sendMessage('acc_missing', '1234@g.us', 'hello')).rejects.toThrow(
+      'Active accounts: [acc_1]',
+    );
+  });
+
+  it('remove() removes session from pool', async () => {
+    await pool.add('acc_1', './sessions/1', jest.fn(), jest.fn(), jest.fn());
+    expect(pool.get('acc_1')).toBeDefined();
+    await pool.remove('acc_1');
+    expect(pool.get('acc_1')).toBeUndefined();
+  });
+
+  it('remove() is a no-op for unknown accountId', async () => {
+    await expect(pool.remove('acc_unknown')).resolves.not.toThrow();
+  });
+
+  it('add() injects accountId into onMessage callback', async () => {
+    const onMessage = jest.fn();
+    const { createWhatsAppSession } = require('./session');
+
+    let capturedOnMessage: any;
+    (createWhatsAppSession as jest.Mock).mockImplementationOnce(
+      (_accountId: string, _path: string, cb: any) => {
+        capturedOnMessage = cb;
+        return Promise.resolve({ sendMessage: jest.fn(), logout: jest.fn() });
+      },
+    );
+
+    await pool.add('acc_1', './sessions/1', onMessage, jest.fn(), jest.fn());
+
+    const fakeMsg = { platformMsgId: 'abc', content: 'hi', sourceGroupJid: '1@g.us', senderJid: '2@s.whatsapp.net', accountId: 'acc_1' };
+    await capturedOnMessage(fakeMsg);
+    expect(onMessage).toHaveBeenCalledWith(fakeMsg, 'acc_1');
+  });
+});
diff --git a/apps/worker/src/whatsapp/session-pool.ts b/apps/worker/src/whatsapp/session-pool.ts
index 816991b..0c8c24f 100644
--- a/apps/worker/src/whatsapp/session-pool.ts
+++ b/apps/worker/src/whatsapp/session-pool.ts
@@ -42,7 +42,10 @@ export class WhatsAppSessionPool {
 
   async sendMessage(accountId: string, groupJid: string, text: string): Promise<void> {
     const sock = this.sessions.get(accountId);
-    if (!sock) throw new Error(`No active session for account ${accountId}`);
+    if (!sock) {
+      const available = Array.from(this.sessions.keys()).join(', ') || 'none';
+      throw new Error(`No active session for account ${accountId}. Active accounts: [${available}]`);
+    }
     await sock.sendMessage(groupJid, { text });
   }