4. Negative tests & edge cases
API Testing · 44 pages source format
Happy paths are table stakes. Pro API testers ship negative cases: invalid payloads, missing auth, wrong methods, boundary values.
What you'll learn
- Negative case categories
- Error body assertions
- Boundary testing
Negative categories
Invalid JSON, missing required fields, wrong types, unauthorized, forbidden, not found, method not allowed.
Do this now
Add 5 negative requests to collection. Assert 4xx status AND error message shape.
Assert error contracts
Errors should be predictable: { "error": "...", "code": "..." }. Assert keys exist, not just status.
Do this now
Document expected error schema for 401 and 404 in ERROR-CONTRACTS.md.
Boundary values
Empty string, max length, zero, negative numbers, special characters in strings.
Do this now
Test POST with empty title, 10k character body (if API allows), invalid email format.
Test data isolation
Unique titles with timestamp prevent collisions. Delete in teardown or use disposable resources.
Do this now
Pre-request script: pm.environment.set("uniqueTitle", "post-" + Date.now());