API Test Suite Builder

Tier: POWERFUL Category: Engineering / Testing Maintainer: Claude Skills Team

Overview

Scan API route definitions across frameworks (Next.js App Router, Express, FastAPI, Django REST, Go net/http), analyze request/response schemas, and generate comprehensive test suites covering authentication, authorization, input validation, error handling, pagination, file uploads, rate limiting, contract testing, and load testing. Outputs ready-to-run test files for Vitest+Supertest (Node), Pytest+httpx (Python), or k6 (load testing).

Keywords

API testing, test generation, contract testing, load testing, Pact, k6, Supertest, httpx, auth testing, input validation, error matrix, OpenAPI testing, regression suite

Core Capabilities

1. Route Detection and Analysis

Scan source files to extract all API endpoints with HTTP methods
Parse request body schemas from types, validators, and decorators
Detect authentication middleware and authorization rules
Identify response types and status codes from handler implementations

2. Test Matrix Generation

Authentication: valid/invalid/expired tokens, missing headers, wrong roles
Input validation: missing fields, wrong types, boundary values, injection
Error paths: 400/401/403/404/409/422/429/500 for each route
Pagination: first/last/empty/oversized pages, cursor-based and offset
File uploads: valid, oversized, wrong MIME, empty, path traversal
Rate limiting: burst detection, per-user vs global limits

3. Contract Testing

OpenAPI spec to test generation
Pact consumer-driven contract tests
Schema snapshot testing for breaking change detection
Response shape validation with JSON Schema

4. Load Testing

k6 scripts with ramp-up patterns and SLA thresholds
Artillery scenarios for sustained load profiles
Latency percentile tracking (P50, P95, P99)
Concurrent user simulation with realistic data

When to Use

New API added — generate test scaffold before implementation (TDD)
Legacy API with no tests — scan and generate baseline coverage
Pre-release — ensure all routes have at least smoke tests
API contract change — detect and test breaking changes
Security audit — generate adversarial input tests
Performance validation — create load test baselines

Route Detection Commands

Next.js App Router

# Find all route handlers and extract HTTP methods
find ./app/api -name "route.ts" -o -name "route.js" | while read f; do
  route=$(echo "$f" | sed 's|./app||; s|/route\.[tj]s||')
  methods=$(grep -oE "export (async )?function (GET|POST|PUT|PATCH|DELETE)" "$f" | \
    grep -oE "(GET|POST|PUT|PATCH|DELETE)" | tr '\n' ',')
  echo "$methods $route"
done

Express / Fastify

grep -rn "router\.\(get\|post\|put\|delete\|patch\)\|app\.\(get\|post\|put\|delete\|patch\)" \
  src/ --include="*.ts" --include="*.js" | \
  grep -oE "\.(get|post|put|delete|patch)\(['\"][^'\"]+['\"]" | \
  sed "s/\.\(.*\)('\(.*\)'/\U\1 \2/"

FastAPI

grep -rn "@\(app\|router\)\.\(get\|post\|put\|delete\|patch\)" . --include="*.py" | \
  grep -oE "(get|post|put|delete|patch)\(['\"][^'\"]*['\"]"

Go (net/http, Chi, Gin)

grep -rn "\.HandleFunc\|\.Handle\|\.GET\|\.POST\|\.PUT\|\.DELETE" . --include="*.go" | \
  grep -oE "(GET|POST|PUT|DELETE|HandleFunc)\(['\"][^'\"]*['\"]"

Test Generation Framework

Auth Test Matrix

For every authenticated endpoint, generate these test cases:

// tests/api/[resource]/auth.test.ts
import { describe, it, expect } from 'vitest'
import request from 'supertest'
import { createTestApp } from '../../helpers/app'
import { createTestUser, generateToken, generateExpiredToken } from '../../helpers/auth'

describe('GET /api/v1/projects - Authentication', () => {
  const app = createTestApp()

  it('returns 401 when no Authorization header is sent', async () => {
    const res = await request(app).get('/api/v1/projects')
    expect(res.status).toBe(401)
    expect(res.body.error).toMatchObject({
      code: 'UNAUTHORIZED',
      message: expect.any(String),
    })
  })

  it('returns 401 when token format is invalid', async () => {
    const res = await request(app)
      .get('/api/v1/projects')
      .set('Authorization', 'InvalidFormat')
    expect(res.status).toBe(401)
  })

  it('returns 401 when token is expired', async () => {
    const token = generateExpiredToken({ userId: 'user_123' })
    const res = await request(app)
      .get('/api/v1/projects')
      .set('Authorization', `Bearer ${token}`)
    expect(res.status).toBe(401)
    expect(res.body.error.code).toBe('TOKEN_EXPIRED')
  })

  it('returns 403 when user lacks required role', async () => {
    const user = await createTestUser({ role: 'viewer' })
    const token = generateToken(user)
    const res = await request(app)
      .get('/api/v1/projects')
      .set('Authorization', `Bearer ${token}`)
    expect(res.status).toBe(403)
  })

  it('returns 401 when token belongs to a deleted user', async () => {
    const user = await createTestUser()
    const token = generateToken(user)
    await deleteUser(user.id)
    const res = await request(app)
      .get('/api/v1/projects')
      .set('Authorization', `Bearer ${token}`)
    expect(res.status).toBe(401)
  })

  it('returns 200 with valid token and correct role', async () => {
    const user = await createTestUser({ role: 'member' })
    const token = generateToken(user)
    const res = await request(app)
      .get('/api/v1/projects')
      .set('Authorization', `Bearer ${token}`)
    expect(res.status).toBe(200)
    expect(res.body).toHaveProperty('data')
  })
})

Input Validation Matrix

// tests/api/[resource]/validation.test.ts
describe('POST /api/v1/projects - Input Validation', () => {
  const validPayload = {
    name: 'My Project',
    description: 'A test project',
    visibility: 'private',
  }

  it('returns 422 when body is empty', async () => {
    const res = await authedRequest('POST', '/api/v1/projects', {})
    expect(res.status).toBe(422)
    expect(res.body.error.details).toEqual(
      expect.arrayContaining([
        expect.objectContaining({ field: 'name', rule: 'required' }),
      ])
    )
  })

  it.each([
    ['name', undefined, 'required'],
    ['name', '', 'min_length'],
    ['name', 'a'.repeat(256), 'max_length'],
    ['name', 123, 'type'],
    ['visibility', 'invalid', 'enum'],
    ['description', 'a'.repeat(10001), 'max_length'],
  ])('returns 422 when %s is %s (%s)', async (field, value, rule) => {
    const payload = { ...validPayload, [field]: value }
    if (value === undefined) delete payload[field]
    const res = await authedRequest('POST', '/api/v1/projects', payload)
    expect(res.status).toBe(422)
    expect(res.body.error.details).toEqual(
      expect.arrayContaining([
        expect.objectContaining({ field, rule }),
      ])
    )
  })

  it('rejects SQL injection in string fields', async () => {
    const res = await authedRequest('POST', '/api/v1/projects', {
      ...validPayload,
      name: "'; DROP TABLE projects; --",
    })
    // Should either reject (422) or sanitize and succeed (201)
    expect([201, 422]).toContain(res.status)
    if (res.status === 201) {
      expect(res.body.data.name).not.toContain('DROP TABLE')
    }
  })

  it('rejects XSS payloads in string fields', async () => {
    const res = await authedRequest('POST', '/api/v1/projects', {
      ...validPayload,
      name: '<script>alert("xss")</script>',
    })
    if (res.status === 201) {
      expect(res.body.data.name).not.toContain('<script>')
    }
  })

  it('accepts valid payload and returns 201 with created resource', async () => {
    const res = await authedRequest('POST', '/api/v1/projects', validPayload)
    expect(res.status).toBe(201)
    expect(res.body.data).toMatchObject({
      id: expect.any(String),
      name: validPayload.name,
      visibility: validPayload.visibility,
      created_at: expect.any(String),
    })
    // Verify sensitive fields are NOT in response
    expect(res.body.data).not.toHaveProperty('internal_id')
  })
})

Pagination Testing

describe('GET /api/v1/projects - Pagination', () => {
  beforeAll(async () => {
    await seedProjects(25) // Create 25 test projects
  })

  it('returns first page with default limit', async () => {
    const res = await authedRequest('GET', '/api/v1/projects')
    expect(res.status).toBe(200)
    expect(res.body.data.length).toBeLessThanOrEqual(20) // default limit
    expect(res.body.meta).toMatchObject({
      total: 25,
      page: 1,
      has_more: true,
    })
  })

  it('returns second page correctly', async () => {
    const res = await authedRequest('GET', '/api/v1/projects?page=2&limit=10')
    expect(res.status).toBe(200)
    expect(res.body.data.length).toBe(10)
    expect(res.body.meta.page).toBe(2)
  })

  it('returns empty array for page beyond data', async () => {
    const res = await authedRequest('GET', '/api/v1/projects?page=100')
    expect(res.status).toBe(200)
    expect(res.body.data).toEqual([])
    expect(res.body.meta.has_more).toBe(false)
  })

  it('rejects limit above maximum', async () => {
    const res = await authedRequest('GET', '/api/v1/projects?limit=1000')
    expect(res.status).toBe(422)
  })

  it('returns consistent results with cursor-based pagination', async () => {
    const page1 = await authedRequest('GET', '/api/v1/projects?limit=5')
    const cursor = page1.body.meta.next_cursor
    const page2 = await authedRequest('GET', `/api/v1/projects?limit=5&cursor=${cursor}`)
    // No overlapping items between pages
    const ids1 = new Set(page1.body.data.map(p => p.id))
    const ids2 = new Set(page2.body.data.map(p => p.id))
    const overlap = [...ids1].filter(id => ids2.has(id))
    expect(overlap).toHaveLength(0)
  })
})

Contract Testing with Pact

// tests/contracts/projects.pact.test.ts
import { PactV3, MatchersV3 } from '@pact-foundation/pact'

const { like, eachLike, string, integer, iso8601DateTimeWithMillis } = MatchersV3

const provider = new PactV3({
  consumer: 'frontend-app',
  provider: 'projects-api',
})

describe('Projects API Contract', () => {
  it('returns a list of projects', async () => {
    provider
      .given('projects exist')
      .uponReceiving('a request for projects')
      .withRequest({
        method: 'GET',
        path: '/api/v1/projects',
        headers: { Authorization: like('Bearer token123') },
      })
      .willRespondWith({
        status: 200,
        headers: { 'Content-Type': 'application/json' },
        body: {
          data: eachLike({
            id: string('proj_abc123'),
            name: string('My Project'),
            visibility: string('private'),
            created_at: iso8601DateTimeWithMillis('2026-01-15T10:30:00.000Z'),
            owner: {
              id: string('user_xyz'),
              name: string('Jane Doe'),
            },
          }),
          meta: {
            total: integer(1),
            page: integer(1),
            has_more: false,
          },
        },
      })

    await provider.executeTest(async (mockServer) => {
      const response = await fetch(`${mockServer.url}/api/v1/projects`, {
        headers: { Authorization: 'Bearer token123' },
      })
      expect(response.status).toBe(200)
      const body = await response.json()
      expect(body.data[0]).toHaveProperty('id')
      expect(body.data[0]).toHaveProperty('name')
      expect(body.meta).toHaveProperty('total')
    })
  })
})

Load Testing with k6

// tests/load/api-load.k6.js
import http from 'k6/http'
import { check, sleep } from 'k6'
import { Rate, Trend } from 'k6/metrics'

const errorRate = new Rate('errors')
const listLatency = new Trend('list_projects_duration')
const createLatency = new Trend('create_project_duration')

export const options = {
  stages: [
    { duration: '30s', target: 10 },   // ramp up to 10 users
    { duration: '1m',  target: 50 },   // ramp up to 50 users
    { duration: '2m',  target: 50 },   // sustain 50 users
    { duration: '30s', target: 0 },    // ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<200', 'p(99)<500'],  // SLA: P95 < 200ms
    errors: ['rate<0.01'],                            // Error rate < 1%
    list_projects_duration: ['p(95)<150'],
    create_project_duration: ['p(95)<300'],
  },
}

const BASE_URL = __ENV.BASE_URL || 'http://localhost:3000'
const AUTH_TOKEN = __ENV.AUTH_TOKEN || 'test-token'

const headers = {
  Authorization: `Bearer ${AUTH_TOKEN}`,
  'Content-Type': 'application/json',
}

export default function () {
  // GET /api/v1/projects
  const listRes = http.get(`${BASE_URL}/api/v1/projects?limit=20`, { headers })
  listLatency.add(listRes.timings.duration)
  check(listRes, {
    'list: status 200': (r) => r.status === 200,
    'list: has data array': (r) => JSON.parse(r.body).data !== undefined,
  }) || errorRate.add(1)

  sleep(1)

  // POST /api/v1/projects
  const createRes = http.post(
    `${BASE_URL}/api/v1/projects`,
    JSON.stringify({
      name: `Load Test Project ${Date.now()}`,
      description: 'Created by k6 load test',
      visibility: 'private',
    }),
    { headers }
  )
  createLatency.add(createRes.timings.duration)
  check(createRes, {
    'create: status 201': (r) => r.status === 201,
    'create: has id': (r) => JSON.parse(r.body).data.id !== undefined,
  }) || errorRate.add(1)

  sleep(1)
}

Run Load Tests

# Local
k6 run tests/load/api-load.k6.js

# With environment variables
k6 run -e BASE_URL=https://staging.app.com -e AUTH_TOKEN=$STAGING_TOKEN tests/load/api-load.k6.js

# Output to cloud dashboard
k6 cloud tests/load/api-load.k6.js

Test Generation Process

When given a codebase, follow this workflow:

Scan routes using detection commands for the detected framework
Read each route handler to understand: request schema, auth middleware, response types, business rules
Generate test file per resource (not per route) using the matrices above
Name tests descriptively: "returns 401 when token is expired" not "auth test 3"
Use factories/fixtures for test data — never hardcode IDs or tokens
Assert response shape, not just status codes
Include negative tests — error paths catch 80% of production bugs
Add contract tests for any API consumed by external services
Add load tests for any endpoint expected to handle >100 RPM

Test Helper Patterns

// tests/helpers/auth.ts — reusable auth utilities
import jwt from 'jsonwebtoken'

export function generateToken(user: { id: string; role: string }, expiresIn = '1h') {
  return jwt.sign({ sub: user.id, role: user.role }, process.env.JWT_SECRET!, { expiresIn })
}

export function generateExpiredToken(user: { id: string }) {
  return jwt.sign({ sub: user.id }, process.env.JWT_SECRET!, { expiresIn: '-1h' })
}

// tests/helpers/request.ts — authed request helper
export async function authedRequest(
  method: string,
  path: string,
  body?: any,
  userOverrides?: Partial<User>,
) {
  const user = await createTestUser(userOverrides)
  const token = generateToken(user)
  const req = request(app)[method.toLowerCase()](path)
    .set('Authorization', `Bearer ${token}`)
  if (body) req.send(body)
  return req
}

// tests/helpers/factory.ts — test data factories
export function buildProject(overrides = {}) {
  return {
    name: `Project ${Date.now()}`,
    description: 'Test project',
    visibility: 'private',
    ...overrides,
  }
}

Common Pitfalls

Testing only happy paths — 80% of production bugs live in error paths; test those first
Hardcoded IDs and tokens — use factories; data changes between environments
Shared state between tests — always clean up; one test's data should not affect another
Testing implementation, not behavior — assert what the API returns, not how it does it
Missing boundary tests — off-by-one errors are the most common bug in pagination and limits
Ignoring Content-Type — test that the API rejects wrong content types
Not testing token expiry separately from invalid tokens — they produce different error codes
Flaky tests from timing — never depend on clock time; use deterministic test data

Best Practices

One describe block per endpoint, nested by concern (auth, validation, business logic)
Seed only the minimal data each test needs — do not load the entire database
Assert specific error codes and field names, not just HTTP status
Test that sensitive fields (password, secret_key) are never present in responses
For contract tests, run them in CI against both consumer and provider
For load tests, set SLA thresholds (p(95)<200) and fail the build if violated
Keep test files colocated with the code they test or in a parallel tests/ tree

Troubleshooting

Problem	Cause	Solution
Generated tests fail with `Cannot find module` errors	Test helper imports reference paths that don't exist in target project	Update import paths in generated files to match the project's `tsconfig.json` paths or Jest `moduleNameMapper` configuration
Auth tests all return 200 instead of 401/403	Test app instance is not using the same auth middleware as production	Ensure `createTestApp()` loads the full middleware stack including auth guards; check that `JWT_SECRET` env var is set in the test environment
Pact contract verification fails on CI but passes locally	Provider state callbacks are missing or the provider is running a different version	Pin the provider version in CI, ensure all `given()` states have matching provider state handlers, and verify the Pact broker URL is correct
k6 load tests report 0 requests or instant completion	`BASE_URL` environment variable is not set or points to an unreachable host	Pass `-e BASE_URL=http://localhost:3000` explicitly and verify the server is running before starting the k6 run
Pagination tests fail with inconsistent ordering	The API does not enforce a default sort order, so results vary between runs	Add an explicit `ORDER BY` clause to the API query or include `?sort=created_at` in test requests to guarantee deterministic ordering
Input validation tests pass but miss real-world edge cases	Generated boundary values use generic limits (256 chars) that don't match actual schema constraints	Read the schema or validator definitions (Zod, Joi, Pydantic) and adjust boundary values to match declared `maxLength`, `minimum`, and `enum` values
Tests are flaky due to database state leakage between runs	Tests share a database and don't clean up after themselves	Wrap each test in a transaction that rolls back, or use `beforeEach` to truncate relevant tables; avoid relying on auto-increment IDs

Success Criteria

Route coverage >= 95%: Every API endpoint in the codebase has at least one generated test file covering auth, validation, and happy path scenarios
Error path ratio >= 3:1: At least three negative/error test cases exist for every happy-path test case per endpoint
Test execution time < 60s: The full generated unit/integration test suite runs in under 60 seconds (excluding load tests)
Zero hardcoded secrets: No test file contains hardcoded API keys, tokens, or passwords; all credentials come from environment variables or factories
Contract test coverage for all external APIs: Every endpoint consumed by an external service or frontend client has a corresponding Pact or schema snapshot test
Load test SLA thresholds defined: Every load-tested endpoint has explicit P95 and P99 latency thresholds and an error rate ceiling configured in the k6 script
CI integration complete: Generated tests run automatically in the CI pipeline with clear pass/fail reporting and no manual intervention required

Scope & Limitations

This skill covers:

Generating test suites from route definitions for REST APIs across Node.js, Python, and Go frameworks
Authentication, authorization, input validation, pagination, and error-path test generation
Consumer-driven contract testing with Pact and schema snapshot validation
Load and performance testing script generation with k6 and Artillery

This skill does NOT cover:

GraphQL API testing (see engineering/api-design-reviewer for schema review patterns)
End-to-end browser testing or UI interaction testing (see engineering/playwright-pro)
Database migration testing or schema validation (see engineering/database-schema-designer)
Security penetration testing beyond input sanitization checks (see engineering/skill-security-auditor)

Integration Points

Skill	Integration	Data Flow
`engineering/api-design-reviewer`	Validate API design before generating tests	Design review output defines the endpoint contracts that this skill generates tests for
`engineering/ci-cd-pipeline-builder`	Embed generated tests into CI/CD pipelines	Generated test files and k6 scripts are added as pipeline stages with pass/fail gates
`engineering/playwright-pro`	Complement API tests with E2E browser tests	API test suite validates backend behavior; Playwright tests validate the frontend consuming those APIs
`engineering/database-schema-designer`	Align test fixtures with database schema	Schema definitions inform factory functions and seed data used in generated test helpers
`engineering/observability-designer`	Monitor test-covered endpoints in production	Load test thresholds (P95, P99) feed into alerting rules for the same endpoints in production dashboards
`engineering/performance-profiler`	Investigate endpoints that fail load test thresholds	k6 results identify slow endpoints; the profiler skill traces root causes at the code level

api-test-suite-builder