Why API Testing is not only about only “200” OK?

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Why API Testing is not only about only “200” OK?

-> What I test in every API request <-

1) Response Structure
- Are all expected fields present?
- Are data types correct?
- Any unexpected extra fields?

2) Performance
- Response time under 2 seconds?
- Behavior under load
- Memory leaks with multiple requests

3) Security
- Authorization working correctly?
- Can I access someone else’s data?
- SQL injection through parameters?

4) Data Validation
- Invalid data handling
- Boundary values testing
- Encoding/decoding (UTF-8, special chars)

5) Integration
- Consistency between API and UI
- Cascade operations (delete user → delete posts)
- Third-party integrations

-> My favorite bug:** API returned 200, but response had an empty object instead of user data. Frontend showed “success” but displayed nothing