api-test-suite-builder — Claude Code skill: what it does

---
name: "api-test-suite-builder"
description: "Use when the user asks to generate API tests, create integration test suites, test REST endpoints, or build contract tests."
---

# API Test Suite Builder

**Tier:** POWERFUL
**Category:** Engineering
**Domain:** Testing / API Quality

---

## Overview

Scans API route definitions across frameworks (Next.js App Router, Express, FastAPI, Django REST) and
auto-generates comprehensive test suites covering auth, input validation, error codes, pagination, file
uploads, and rate limiting. Outputs ready-to-run test files for Vitest+Supertest (Node) or Pytest+httpx
(Python).

---

## Core Capabilities

- **Route detection** — scan source files to extract all API endpoints
- **Auth coverage** — valid/invalid/expired tokens, missing auth header
- **Input validation** — missing fields, wrong types, boundary values, injection attempts
- **Error code matrix** — 400/401/403/404/422/500 for each route
- **Pagination** — first/last/empty/oversized pages
- **File uploads** — valid, oversized, wrong MIME type, empty
- **Rate limiting** — burst detection, per-user vs global limits

---

## When to Use

- New API added — generate test scaffold before writing implementation (TDD)
- Legacy API with no tests — scan and generate baseline coverage
- API contract review — verify existing tests match current route definitions
- Pre-release regression check — ensure all routes have at least smoke tests
- Security audit prep — generate adversarial input tests

---

## Route Detection

### Next.js App Router
```bash
# Find all route handlers
find ./app/api -name "route.ts" -o -name "route.js" | sort

# Extract HTTP methods from each route file
grep -rn "export async function\|export function" app/api/**/route.ts | \
  grep -oE "(GET|POST|PUT|PATCH|DELETE|HEAD|OPTIONS)" | sort -u

# Full route map
find ./app/api -name "route.ts" | while read f; do
  route=$(echo $f | sed 's|./app||' | sed 's|/route.ts||')
  methods=$(grep -oE "export (async )?function (GET|POST|PUT|PATCH|DELETE)" "$f" | \
    grep -oE "(GET|POST|PUT|PATCH|DELETE)")
  echo "$methods $route"
done
```

### Express
```bash
# Find all router files
find ./src -name "*.ts" -o -name "*.js" | xargs grep -l "router\.\(get\|post\|put\|delete\|patch\)" 2>/dev/null

# Extract routes with line numbers
grep -rn "router\.\(get\|post\|put\|delete\|patch\)\|app\.\(get\|post\|put\|delete\|patch\)" \
  src/ --include="*.ts" | grep -oE "(get|post|put|delete|patch)\(['\"][^'\"]*['\"]"

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

### FastAPI
```bash
# Find all route decorators
grep -rn "@app\.\|@router\." . --include="*.py" | \
  grep -E "@(app|router)\.(get|post|put|delete|patch)"

# Extract with path and function name
grep -rn "@\(app\|router\)\.\(get\|post\|put\|delete\|patch\)" . --include="*.py" | \
  grep -oE "@(app|router)\.(get|post|put|delete|patch)\(['\"][^'\"]*['\"]"
```

### Django REST Framework
```bash
# urlpatterns extraction
grep -rn "path\|re_path\|url(" . --include="*.py" | grep "urlpatterns" -A 50 | \
  grep -E "path\(['\"]" | grep -oE "['\"][^'\"]+['\"]" | head -40

# ViewSet router registration
grep -rn "router\.register\|DefaultRouter\|SimpleRouter" . --include="*.py"
```

---

## Test Generation Patterns

### Auth Test Matrix

For every authenticated endpoint, generate:

| Test Case | Expected Status |
|-----------|----------------|
| No Authorization header | 401 |
| Invalid token format | 401 |
| Valid token, wrong user role | 403 |
| Expired JWT token | 401 |
| Valid token, correct role | 2xx |
| Token from deleted user | 401 |

### Input Validation Matrix

For every POST/PUT/PATCH endpoint with a request body:

| Test Case | Expected Status |
|-----------|----------------|
| Empty body `{}` | 400 or 422 |
| Missing required fields (one at a time) | 400 or 422 |
| Wrong type (string where int expected) | 400 or 422 |
| Boundary: value at min-1 | 400 or 422 |
| Boundary: value at min | 2xx |
| Boundary: value at max | 2xx |
| Boundary: value at max+1 | 400 or 422 |
| SQL injection in string field | 400 or 200 (sanitized) |
| XSS payload in string field | 400 or 200 (sanitized) |
| Null values for required fields | 400 or 422 |

---

## Example Test Files
→ See references/example-test-files.md for details

## Generating Tests from Route Scan

When given a codebase, follow this process:

1. **Scan routes** using the detection commands above
2. **Read each route handler** to understand:
   - Expected request body schema
   - Auth requirements (middleware, decorators)
   - Return types and status codes
   - Business rules (ownership, role checks)
3. **Generate test file** per route group using the patterns above
4. **Name tests descriptively**: `"returns 401 when token is expired"` not `"auth test 3"`
5. **Use factories/fixtures** for test data — never hardcode IDs
6. **Assert response shape**, not just status code

---

## Common Pitfalls

- **Testing only happy paths** — 80% of bugs live in error paths; test those first
- **Hardcoded test data IDs** — use factories/fixtures; IDs change between environments
- **Shared state between tests** — always clean up in afterEach/afterAll
- **Testing implementation, not behavior** — test what the API returns, not how it does it
- **Missing boundary tests** — off-by-one errors are extremely common in pagination and limits
- **Not testing token expiry** — expired tokens behave differently from invalid ones
- **Ignoring Content-Type** — test that API rejects wrong content types (xml when json expected)

---

## Best Practices

1. One describe block per endpoint — keeps failures isolated and readable
2. Seed minimal data — don't load the entire DB; create only what the test needs
3. Use `beforeAll` for shared setup, `afterAll` for cleanup — not `beforeEach` for expensive ops
4. Assert specific error messages/fields, not just status codes
5. Test that sensitive fields (password, secret) are never in responses
6. For auth tests, always test the "missing header" case separately from "invalid token"
7. Add rate limit tests last — they can interfere with other test suites if run in parallel