API Testing Patterns

<default_to_action> When testing APIs or designing API test strategy:

1. IDENTIFY testing level: contract, integration, or component
2. TEST the contract, not implementation (consumer perspective)
3. VALIDATE auth, input, errors, idempotency, concurrency
4. AUTOMATE in CI/CD with schema validation
5. MONITOR production APIs for contract drift

Quick Pattern Selection:

• Microservices → Consumer-driven contracts (Pact)
• REST APIs → CRUD + pagination + filtering tests
• GraphQL → Query validation + complexity limits
• External deps → Mock with component testing
• Performance → Load test critical endpoints

Critical Success Factors:

• APIs are contracts - test from consumer perspective
• Always test error scenarios, not just happy paths
• Version your API tests to prevent breaking changes </default_to_action>

Quick Reference Card

When to Use

• Testing REST or GraphQL APIs
• Validating microservice contracts
• Designing API test strategies
• Preventing breaking API changes

Testing Levels

Level	Purpose	Dependencies	Speed
Contract	Provider-consumer agreement	None	Fast
Component	API in isolation	Mocked	Fast
Integration	Real dependencies	Database, services	Slower

Critical Test Scenarios

Scenario	Must Test	Example
Auth	401/403 handling	Expired token, wrong user
Input	400 validation	Missing fields, wrong types
Errors	500 graceful handling	DB down, timeout
Idempotency	Duplicate prevention	Same idempotency key
Concurrency	Race conditions	Parallel checkout

Tools

• Contract: Pact, Spring Cloud Contract
• REST: Supertest, REST-assured, Playwright
• Load: k6, Artillery, JMeter

Agent Coordination

• qe-api-contract-validator: Validate contracts, detect breaking changes
• qe-test-generator: Generate tests from OpenAPI spec
• qe-performance-tester: Load test endpoints
• qe-security-scanner: API security testing

---

Contract Testing

Pattern: Consumer-Driven Contracts

// Consumer defines expectations
const contract = {
  request: { method: 'POST', path: '/orders', body: { productId: 'abc', quantity: 2 } },
  response: { status: 201, body: { orderId: 'string', total: 'number' } }
};

// Provider must fulfill
test('order API meets contract', async () => {
  const response = await api.post('/orders', { productId: 'abc', quantity: 2 });

  expect(response.status).toBe(201);
  expect(response.body).toMatchSchema({
    orderId: expect.any(String),
    total: expect.any(Number)
  });
});

When: Microservices, distributed systems, third-party integrations

---

Critical Test Patterns

Authentication & Authorization

describe('Auth', () => {
  it('rejects without token', async () => {
    expect((await api.get('/orders')).status).toBe(401);
  });

  it('rejects expired token', async () => {
    const expired = generateExpiredToken();
    expect((await api.get('/orders', { headers: { Authorization: `Bearer ${expired}` } })).status).toBe(401);
  });

  it('blocks cross-user access', async () => {
    const userAToken = generateToken({ userId: 'A' });
    expect((await api.get('/orders/user-B-order', { headers: { Authorization: `Bearer ${userAToken}` } })).status).toBe(403);
  });
});

Input Validation

describe('Validation', () => {
  it('validates required fields', async () => {
    const response = await api.post('/orders', { quantity: 2 }); // Missing productId
    expect(response.status).toBe(400);
    expect(response.body.errors).toContain('productId is required');
  });

  it('validates types', async () => {
    expect((await api.post('/orders', { productId: 'abc', quantity: 'two' })).status).toBe(400);
  });

  it('validates ranges', async () => {
    expect((await api.post('/orders', { productId: 'abc', quantity: -5 })).status).toBe(400);
  });
});

Idempotency

it('prevents duplicates with idempotency key', async () => {
  const key = 'unique-123';
  const data = { productId: 'abc', quantity: 2 };

  const r1 = await api.post('/orders', data, { headers: { 'Idempotency-Key': key } });
  const r2 = await api.post('/orders', data, { headers: { 'Idempotency-Key': key } });

  expect(r1.body.orderId).toBe(r2.body.orderId); // Same order
});

Concurrency

it('handles race condition on inventory', async () => {
  const promises = Array(10).fill().map(() =>
    api.post('/orders', { productId: 'abc', quantity: 1 })
  );
  const responses = await Promise.all(promises);
  const successful = responses.filter(r => r.status === 201);

  const inventory = await db.inventory.findById('abc');
  expect(inventory.quantity).toBe(initialQuantity - successful.length);
});

---

REST CRUD Pattern

describe('Product CRUD', () => {
  let productId;

  it('CREATE', async () => {
    const r = await api.post('/products', { name: 'Widget', price: 10 });
    expect(r.status).toBe(201);
    productId = r.body.id;
  });

  it('READ', async () => {
    const r = await api.get(`/products/${productId}`);
    expect(r.body.name).toBe('Widget');
  });

  it('UPDATE', async () => {
    const r = await api.put(`/products/${productId}`, { price: 12 });
    expect(r.body.price).toBe(12);
  });

  it('DELETE', async () => {
    expect((await api.delete(`/products/${productId}`)).status).toBe(204);
    expect((await api.get(`/products/${productId}`)).status).toBe(404);
  });
});

---

Best Practices

✅ Do This

• Test from consumer perspective
• Use schema validation (not exact values)
• Test error scenarios extensively
• Version API tests
• Automate in CI/CD

❌ Avoid This

• Testing implementation, not contract
• Ignoring HTTP semantics (status codes)
• No negative testing
• Asserting on field order or extra fields
• Slow tests (mock external services)

---

Agent-Assisted API Testing

// Validate contracts
await Task("Contract Validation", {
  spec: 'openapi.yaml',
  endpoint: '/orders',
  checkBreakingChanges: true
}, "qe-api-contract-validator");

// Generate tests from spec
await Task("Generate API Tests", {
  spec: 'openapi.yaml',
  coverage: 'comprehensive',
  include: ['happy-paths', 'input-validation', 'auth-scenarios', 'error-handling']
}, "qe-test-generator");

// Load test
await Task("API Load Test", {
  endpoint: '/orders',
  rps: 1000,
  duration: '5min'
}, "qe-performance-tester");

// Security scan
await Task("API Security Scan", {
  spec: 'openapi.yaml',
  checks: ['sql-injection', 'xss', 'broken-auth', 'rate-limiting']
}, "qe-security-scanner");

---

Agent Coordination Hints

Memory Namespace

aqe/api-testing/
├── contracts/*        - API contract definitions
├── generated-tests/*  - Generated test suites
├── validation/*       - Contract validation results
└── performance/*      - Load test results

Fleet Coordination

const apiFleet = await FleetManager.coordinate({
  strategy: 'contract-testing',
  agents: ['qe-api-contract-validator', 'qe-test-generator', 'qe-test-executor'],
  topology: 'mesh'
});

await apiFleet.execute({
  services: [
    { name: 'orders-api', consumers: ['checkout-ui', 'admin-api'] },
    { name: 'payment-api', consumers: ['orders-api'] }
  ]
});

---

Related Skills

• agentic-quality-engineering - API testing with agents
• tdd-london-chicago - London school for API testing
• performance-testing - API load testing
• security-testing - API security validation
• contract-testing - Consumer-driven contracts deep dive

---

Remember

API testing = verifying contracts and behavior, not implementation. Focus on what matters to consumers: correct responses, proper error handling, acceptable performance.

With Agents: Agents automate contract validation, generate comprehensive test suites from specs, and monitor production APIs for drift. Use agents to maintain API quality at scale.