diff --git a/mobile/__tests__/api.test.ts b/mobile/__tests__/api.test.ts
new file mode 100644
index 0000000..3ab1f91
--- /dev/null
+++ b/mobile/__tests__/api.test.ts
@@ -0,0 +1,267 @@
+import { api } from '../utils/api';
+
+// ------------------------------------------------------------------ mock fetch
+
+const mockFetch = jest.fn() as jest.MockedFunction<typeof fetch>;
+global.fetch = mockFetch;
+
+function jsonResponse(body: unknown, status = 200): Response {
+  return {
+    ok: status >= 200 && status < 300,
+    status,
+    text: () => Promise.resolve(JSON.stringify(body)),
+  } as Response;
+}
+
+beforeEach(() => {
+  mockFetch.mockReset();
+});
+
+// ------------------------------------------------------------------ health
+
+describe('api.health', () => {
+  it('returns status and timestamp', async () => {
+    const payload = { status: 'ok', timestamp: '2026-04-05T00:00:00Z' };
+    mockFetch.mockResolvedValue(jsonResponse(payload));
+
+    const result = await api.health();
+    expect(result).toEqual(payload);
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/health',
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+});
+
+// ------------------------------------------------------------------ farms
+
+describe('api.getFarms', () => {
+  it('fetches array of farms', async () => {
+    const farms = [{ id: 'f1', name: 'Finca Alta', location: 'Ahuachapán', altitude_m: 1200, owner_name: 'Carlos', created_at: '2026-01-01' }];
+    mockFetch.mockResolvedValue(jsonResponse(farms));
+
+    const result = await api.getFarms();
+    expect(result).toEqual(farms);
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/farm',
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+});
+
+describe('api.getFarm', () => {
+  it('fetches a single farm by id', async () => {
+    const farm = { id: 'f1', name: 'Finca Alta' };
+    mockFetch.mockResolvedValue(jsonResponse(farm));
+
+    const result = await api.getFarm('f1');
+    expect(result).toEqual(farm);
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/farm/f1',
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+});
+
+describe('api.createFarm', () => {
+  it('posts farm data', async () => {
+    const body = { name: 'Finca Nueva', location: 'Sonsonate', altitude_m: 900, owner_name: 'Maria' };
+    const created = { id: 'f2', ...body, created_at: '2026-04-05' };
+    mockFetch.mockResolvedValue(jsonResponse(created));
+
+    const result = await api.createFarm(body);
+    expect(result).toEqual(created);
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/farm',
+      expect.objectContaining({
+        method: 'POST',
+        body: JSON.stringify(body),
+      })
+    );
+  });
+});
+
+// ------------------------------------------------------------------ workers
+
+describe('api.getWorkers', () => {
+  it('fetches all workers when no farmId', async () => {
+    mockFetch.mockResolvedValue(jsonResponse([]));
+    await api.getWorkers();
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/worker',
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+
+  it('filters by farm_id when provided', async () => {
+    mockFetch.mockResolvedValue(jsonResponse([]));
+    await api.getWorkers('f1');
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/worker?farm_id=f1',
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+});
+
+describe('api.createWorker', () => {
+  it('posts worker data with defaults', async () => {
+    const body = { farm_id: 'f1', name: 'Pedro' };
+    const created = { id: 'w1', ...body, role: 'worker', liquid_address: 'lq1abc' };
+    mockFetch.mockResolvedValue(jsonResponse(created));
+
+    const result = await api.createWorker(body);
+    expect(result).toEqual(created);
+  });
+});
+
+describe('api.updateWorker', () => {
+  it('sends PUT request', async () => {
+    const updated = { id: 'w1', name: 'Pedro Updated' };
+    mockFetch.mockResolvedValue(jsonResponse(updated));
+
+    await api.updateWorker('w1', { name: 'Pedro Updated' });
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/worker/w1',
+      expect.objectContaining({ method: 'PUT' })
+    );
+  });
+});
+
+describe('api.getWorkerPayments', () => {
+  it('fetches payment history', async () => {
+    const payments = { worker: { id: 'w1', name: 'Pedro' }, payments: [], total_paid_sats: 0 };
+    mockFetch.mockResolvedValue(jsonResponse(payments));
+
+    const result = await api.getWorkerPayments('w1');
+    expect(result).toEqual(payments);
+  });
+});
+
+// ------------------------------------------------------------------ shifts
+
+describe('api.createShift', () => {
+  it('posts shift data', async () => {
+    const body = { farm_id: 'f1', foreman_id: 'w2' };
+    const shift = { id: 's1', ...body, status: 'open', qr_image: 'data:image/png;base64,...' };
+    mockFetch.mockResolvedValue(jsonResponse(shift));
+
+    const result = await api.createShift(body);
+    expect(result).toEqual(shift);
+  });
+});
+
+describe('api.checkIn', () => {
+  it('posts checkin to shift', async () => {
+    const checkin = { id: 'c1', shift_id: 's1', worker_id: 'w1', checked_in_at: '2026-04-05T08:00:00Z' };
+    mockFetch.mockResolvedValue(jsonResponse({ checkin, worker: { id: 'w1', name: 'Pedro' } }));
+
+    const result = await api.checkIn('s1', { worker_id: 'w1' });
+    expect(result.checkin).toEqual(checkin);
+  });
+});
+
+describe('api.closeShift', () => {
+  it('posts close request', async () => {
+    const resp = { shift: { id: 's1', status: 'closed' }, checkin_count: 5, liquid_record: { asset_id: 'abc' }, workers: [] };
+    mockFetch.mockResolvedValue(jsonResponse(resp));
+
+    const result = await api.closeShift('s1');
+    expect(result.shift.status).toBe('closed');
+  });
+});
+
+// ------------------------------------------------------------------ lots
+
+describe('api.createLot', () => {
+  it('posts lot data', async () => {
+    const body = { shift_id: 's1', weight_kg: 45, grade: 'A' as const };
+    const resp = { lot: { id: 'l1', ...body }, asset: { asset_id: 'lq-asset' }, provenance_url: '/provenance/l1', qr_image: '' };
+    mockFetch.mockResolvedValue(jsonResponse(resp));
+
+    const result = await api.createLot(body);
+    expect(result.lot.weight_kg).toBe(45);
+  });
+});
+
+describe('api.transferLot', () => {
+  it('posts transfer data', async () => {
+    const body = { to_entity: 'Mill Co', entity_type: 'wet_mill' as const };
+    const resp = { transfer: { id: 't1', lot_id: 'l1', ...body }, liquid_tx: { tx_id: 'tx123' } };
+    mockFetch.mockResolvedValue(jsonResponse(resp));
+
+    const result = await api.transferLot('l1', body);
+    expect(result.transfer.to_entity).toBe('Mill Co');
+  });
+});
+
+// ------------------------------------------------------------------ payroll
+
+describe('api.runPayroll', () => {
+  it('posts payroll request', async () => {
+    const resp = { shift_id: 's1', shift_date: '2026-04-05', total_workers: 3, paid_count: 3, failed_count: 0, total_sats_paid: 3000, payments: [], lightning_demo_mode: true };
+    mockFetch.mockResolvedValue(jsonResponse(resp));
+
+    const result = await api.runPayroll({ shift_id: 's1' });
+    expect(result.total_sats_paid).toBe(3000);
+    expect(result.lightning_demo_mode).toBe(true);
+  });
+});
+
+// ------------------------------------------------------------------ provenance
+
+describe('api.getProvenance', () => {
+  it('fetches provenance data', async () => {
+    const resp = { lot: { id: 'l1' }, farm: { id: 'f1' }, shift: { id: 's1' }, workers: [], transfers: [], payment_summary: { worker_count: 0, total_sats: 0 } };
+    mockFetch.mockResolvedValue(jsonResponse(resp));
+
+    const result = await api.getProvenance('l1');
+    expect(result.lot.id).toBe('l1');
+    expect(mockFetch).toHaveBeenCalledWith(
+      'http://localhost:3000/provenance/l1/data',
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+});
+
+// ------------------------------------------------------------------ error handling
+
+describe('error handling', () => {
+  it('throws on network failure', async () => {
+    mockFetch.mockRejectedValue(new Error('Network request failed'));
+    await expect(api.health()).rejects.toThrow('No se pudo conectar al servidor');
+  });
+
+  it('throws on non-JSON response', async () => {
+    mockFetch.mockResolvedValue({
+      ok: true,
+      status: 200,
+      text: () => Promise.resolve('not json'),
+    } as Response);
+    await expect(api.health()).rejects.toThrow('Respuesta inválida del servidor');
+  });
+
+  it('throws server error message', async () => {
+    mockFetch.mockResolvedValue(jsonResponse({ error: 'Farm not found' }, 404));
+    await expect(api.getFarm('bad-id')).rejects.toThrow('Farm not found');
+  });
+
+  it('throws generic error when no message in response', async () => {
+    mockFetch.mockResolvedValue(jsonResponse({}, 500));
+    await expect(api.health()).rejects.toThrow('Error 500');
+  });
+});
+
+// ------------------------------------------------------------------ headers
+
+describe('request headers', () => {
+  it('always sends Content-Type: application/json', async () => {
+    mockFetch.mockResolvedValue(jsonResponse({ status: 'ok' }));
+    await api.health();
+    expect(mockFetch).toHaveBeenCalledWith(
+      expect.any(String),
+      expect.objectContaining({
+        headers: { 'Content-Type': 'application/json' },
+      })
+    );
+  });
+});
diff --git a/mobile/jest.config.js b/mobile/jest.config.js
new file mode 100644
index 0000000..3f40cbe
--- /dev/null
+++ b/mobile/jest.config.js
@@ -0,0 +1,10 @@
+/** @type {import('jest').Config} */
+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node',
+  roots: ['<rootDir>/__tests__'],
+  moduleFileExtensions: ['ts', 'tsx', 'js', 'json'],
+  transform: {
+    '^.+\\.tsx?$': ['ts-jest', { tsconfig: { jsx: 'react-jsx' } }],
+  },
+};
diff --git a/mobile/package.json b/mobile/package.json
index 3ab058e..6dfb522 100644
--- a/mobile/package.json
+++ b/mobile/package.json
@@ -6,7 +6,7 @@
     "start": "expo start",
     "android": "expo run:android",
     "ios": "expo run:ios",
-    "test": "echo \"No tests yet\" && exit 0"
+    "test": "jest --verbose"
   },
   "dependencies": {
     "@expo-google-fonts/dm-sans": "^0.4.2",
@@ -32,8 +32,11 @@
   },
   "devDependencies": {
     "@babel/core": "^7.25.2",
+    "@types/jest": "^30.0.0",
     "@types/react": "~19.1.10",
     "babel-preset-expo": "^54.0.10",
+    "jest": "^30.3.0",
+    "ts-jest": "^29.4.9",
     "typescript": "^5.8.2"
   },
   "engines": {