automating-api-testing

What this skill does

# API Test Automation

## Overview

Automate comprehensive API endpoint testing for REST and GraphQL APIs including request generation, response validation, schema compliance, authentication flows, and error handling. Supports Supertest (Node.js), REST-assured (Java), httpx/pytest (Python), Postman/Newman collections, and Pact for consumer-driven contract testing.

## Prerequisites

- API testing library installed (Supertest, REST-assured, httpx, or Postman/Newman)
- API specification file (OpenAPI/Swagger YAML/JSON or GraphQL SDL)
- Target API running in a test environment with seeded data
- Authentication credentials or API keys for protected endpoints
- JSON Schema validator (Ajv, jsonschema, or built-in framework assertions)

## Instructions

1. Read the API specification and extract all endpoints:
   - Parse OpenAPI spec to catalog every path, HTTP method, request schema, and response schema.
   - For GraphQL APIs, introspect the schema to list queries, mutations, and subscriptions.
   - Document authentication requirements per endpoint (API key, Bearer token, OAuth, none).
2. Generate test cases for each endpoint:
   - **Success cases**: Send valid requests matching the schema and assert 200/201 responses.
   - **Validation errors**: Send requests with missing required fields, wrong types, and out-of-range values; assert 400 responses.
   - **Authentication**: Test with valid, expired, and missing credentials; assert 200, 401, and 403 respectively.
   - **Not found**: Request non-existent resources; assert 404 responses.
   - **Idempotency**: Send the same PUT/DELETE request twice and verify consistent behavior.
3. Validate response structure against schemas:
   - Assert response Content-Type matches expected (application/json, etc.).
   - Validate response body against the OpenAPI response schema using JSON Schema validation.
   - Check response headers (Cache-Control, Rate-Limit headers, CORS headers).
   - Verify pagination metadata (total count, page number, next/previous links).
4. Test CRUD lifecycle for resource endpoints:
   - Create a resource (POST) and capture the ID.
   - Read it back (GET) and verify all fields match.
   - Update it (PUT/PATCH) and verify changes persisted.
   - Delete it (DELETE) and verify subsequent GET returns 404.
5. Test error handling and edge cases:
   - Send excessively large payloads and verify 413 or graceful rejection.
   - Send requests with unsupported Content-Types and verify 415.
   - Test rate limiting by sending rapid sequential requests.
   - Verify error response format is consistent (standard error schema).
6. For GraphQL APIs, test specifically:
   - Valid queries return expected data shapes.
   - Invalid queries return descriptive error messages.
   - Query depth limiting prevents deeply nested abuse queries.
   - Mutation input validation matches schema constraints.
7. Generate a test coverage report mapping endpoints to test cases.

## Output

- API test files organized by resource in `tests/api/`
- Request/response examples for API documentation
- Schema compliance report for each endpoint
- Endpoint coverage matrix showing tested vs. untested endpoints and methods
- CI pipeline step running API tests against staging environment

## Error Handling

| Error | Cause | Solution |
|-------|-------|---------|
| Connection refused | API server not running or wrong base URL | Verify server is up with a health check before test suite starts; check `BASE_URL` config |
| 401 on all requests | Authentication token expired or misconfigured | Refresh token in test setup; verify `Authorization` header format; check token scopes |
| Schema validation fails unexpectedly | API response includes extra fields not in spec | Update OpenAPI spec to include new fields; use `additionalProperties: true` if expected |
| Test data conflicts | Another test modified or deleted the resource | Use unique test data per test; create resources in `beforeEach`; avoid shared fixtures |
| Rate limit hit during test run | Too many requests in quick succession | Add delays between requests or use authenticated sessions with higher limits; run tests serially |

## Examples

**Supertest REST API test suite:**

```typescript
import request from 'supertest';
import { app } from '../src/app';

describe('GET /api/products', () => {
  it('returns a paginated product list', async () => {
    const res = await request(app)
      .get('/api/products?page=1&limit=10')
      .set('Authorization', `Bearer ${token}`)
      .expect(200)  # HTTP 200 OK
      .expect('Content-Type', /json/);

    expect(res.body.data).toBeInstanceOf(Array);
    expect(res.body.data.length).toBeLessThanOrEqual(10);
    expect(res.body.meta).toMatchObject({ page: 1, limit: 10 });
  });

  it('returns 401 without authentication', async () => {  # HTTP 401 Unauthorized
    await request(app).get('/api/products').expect(401);  # HTTP 401 Unauthorized
  });
});

describe('POST /api/products', () => {
  it('creates a product with valid data', async () => {
    const res = await request(app)
      .post('/api/products')
      .set('Authorization', `Bearer ${token}`)
      .send({ name: 'Widget', price: 9.99, category: 'tools' })
      .expect(201);  # HTTP 201 Created

    expect(res.body).toMatchObject({ name: 'Widget', price: 9.99 });
    expect(res.body.id).toBeDefined();
  });

  it('returns 400 for missing required fields', async () => {  # HTTP 400 Bad Request
    await request(app)
      .post('/api/products')
      .set('Authorization', `Bearer ${token}`)
      .send({ name: 'Widget' }) // missing price
      .expect(400);  # HTTP 400 Bad Request
  });
});
```

**GraphQL API test:**

```typescript
it('fetches user by ID', async () => {
  const query = `query { user(id: "1") { id name email } }`;
  const res = await request(app)
    .post('/graphql')
    .send({ query })
    .expect(200);  # HTTP 200 OK

  expect(res.body.data.user).toMatchObject({ id: '1', name: 'Alice' });
  expect(res.body.errors).toBeUndefined();
});
```

## Resources

- Supertest: https://github.com/ladjs/supertest
- REST-assured (Java): https://rest-assured.io/
- httpx (Python): https://www.python-httpx.org/
- Newman (Postman CLI):
- OpenAPI specification: https://spec.openapis.org/oas/v3.1.0
- Ajv JSON Schema validator: https://ajv.js.org/