API testing is one of the most critical skills in modern software development. Every frontend application communicates with a backend via APIs, and bugs in APIs directly translate to bugs in user-facing features.
HTTP Status Codes Every Tester Must Know
| Code | Meaning | When to Expect |
|---|---|---|
| 200 | OK | Successful GET, PUT, PATCH |
| 201 | Created | Successful POST creating resource |
| 400 | Bad Request | Invalid input |
| 401 | Unauthorized | Missing or invalid auth |
| 403 | Forbidden | Authenticated but no permission |
| 404 | Not Found | Resource doesn't exist |
| 429 | Too Many Requests | Rate limit exceeded |
| 500 | Server Error | Backend bug |
Testing Authentication
// Bearer Token (JWT)
curl -X GET https://api.example.com/users/me \
-H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."
Common API Testing Mistakes
1. Not Testing the Full Payload: Always validate the actual response data, not just the status code.
2. Ignoring Response Time: API response time directly affects user experience.
3. Not Testing Edge Cases: Empty arrays, maximum limits, and special characters reveal bugs.
The Bottom Line
Start with manual testing using curl or Postman. Graduate to automated tests for critical endpoints. Add performance and contract testing as your API matures.
Try our API Tester for manual debugging, JSON Formatter for response analysis.
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