Why I Built My Own API Testing Habit
Early in my career, I treated API testing like a chore. Hit the endpoint, see if it returns 200, move on. Then I spent three days debugging an integration that worked perfectly in my tests but failed catastrophically in production. The issue? I had been testing against a mock server with different response schemas.
Now, API testing is part of my development workflow. Not at the end—throughout. Every endpoint I build gets tested immediately. Every third-party API I integrate gets explored thoroughly before I write a single line of integration code.
What Is API Testing?
API testing involves sending HTTP requests to endpoints and validating the responses. Unlike UI testing, you're testing the application logic directly—the data layer, business rules, authentication, and error handling. It's faster, more reliable, and catches issues that UI tests might miss.
REST APIs use standard HTTP methods:
- GET: Retrieve data (should not modify anything)
- POST: Create new resources
- PUT: Replace a resource entirely
- PATCH: Partially update a resource
- DELETE: Remove a resource
- HEAD: GET without the body (check if resource exists)
- OPTIONS: Check what methods are allowed
đź“– Developer Story #1: The Silent 201
Background: A developer was integrating with a payment processor. The POST request returned 201 Created, so she assumed success and moved on.
The Problem: Payments were failing in production, but the integration "worked." No error handling triggered.
The Cause: The 201 response included a "status" field in the body: "pending_verification". The payment wasn't actually processed—it was queued for manual review. She had only checked the HTTP status, not the response body.
Lesson: Always read the full response body. HTTP status codes tell you if the request was received correctly, not if the business logic succeeded.
Key Things to Test in Every API
1. Response Structure
Does the response match the documented schema? Are all expected fields present? Are the data types correct? A string where you expect a number will break your code.
2. Error Handling
What happens with invalid input? Missing required fields? Invalid authentication? Bad IDs? You need to know what errors look like before you can handle them.
3. Edge Cases
Empty arrays vs null. Zero quantities. Unicode characters. Very long strings. Dates at year boundaries. These are where bugs hide.
4. Authentication Flow
How do tokens expire? What's the refresh flow? What error do you get with an invalid token? Test these before writing your auth handling code.
đź“– Developer Story #2: The Paginated Nightmare
Background: An app needed to display all user orders. The API returned them, so the developer displayed them. Simple.
The Problem: Users with more than 100 orders only saw their most recent 100. Complaints started rolling in.
The Cause: The API paginated results at 100 items by default. The developer never tested with enough data to trigger pagination. There was a "next_page" field he never noticed.
Lesson: Test with realistic data volumes. Pagination, rate limiting, and timeout behaviors only show up at scale. Ask: "What happens with 1,000 items? 10,000?"
Understanding HTTP Status Codes
Status codes are grouped by their first digit:
| Range | Meaning | Common Codes |
|---|---|---|
| 2xx | Success | 200 OK, 201 Created, 204 No Content |
| 3xx | Redirection | 301 Moved, 302 Found, 304 Not Modified |
| 4xx | Client Error | 400 Bad Request, 401 Unauthorized, 404 Not Found, 429 Rate Limited |
| 5xx | Server Error | 500 Internal Error, 502 Bad Gateway, 503 Unavailable |
Don't just check for "200 or error." A 200 might have an empty body indicating no results. A 201 might have important metadata you need to capture. A 204 explicitly has no content—that's intentional.
đź’ˇ Pro Tip:
Pay special attention to 429 (Too Many Requests). Know the rate limits before you hit production. The response usually includes Retry-After headers telling you when to try again.
đź“– Developer Story #3: The Cached Disaster
Background: A dashboard showed real-time inventory data from an API. The data looked correct during development.
The Problem: In production, inventory showed as available even after items sold out. The "real-time" dashboard was serving stale data.
The Cause: The API had aggressive caching (Cache-Control: max-age=3600). The CDN and browser were both caching responses. Nobody checked the response headers during testing.
Lesson: Check response headers, especially caching headers. Cache-Control, ETag, Last-Modified—these determine how "fresh" your data really is.
Best Practices for API Testing
1. Test Against Real APIs (Eventually)
Mocks are great for unit tests, but nothing replaces testing against the actual API. Response times, error messages, authentication quirks—these only show up with real requests.
2. Save Test Cases
When you find an edge case, save that request. Build a collection of test cases you can run before deployments.
3. Check Headers, Not Just Body
Rate limit info, pagination links, content types, caching directives—important data lives in headers.
4. Test Failure Scenarios
What happens when the API is down? Times out? Returns malformed JSON? Your app needs to handle these gracefully.
5. Document What You Find
API documentation is often incomplete. When you discover undocumented behavior (and you will), document it for your team.
How This Tool Works
This API tester sends requests from the server side using PHP's cURL. This means:
- No CORS issues: Unlike browser-based tools, we can hit any public API
- Real HTTP requests: Exactly what your server would send
- Full headers: See all response headers, not just those exposed to browsers
- Accurate timing: Measure actual request/response time
The tool blocks requests to internal/private IP addresses for security—you can't probe internal networks through this.
Final Thoughts
API testing isn't glamorous, but it's essential. An issue caught during API testing is an issue that never makes it to production. Every hour spent testing saves days of debugging later.
Keep this tool bookmarked. The next time you're integrating with a new API, spend an hour exploring it before writing code. Understand the responses, the errors, the edge cases. Your future self will thank you.
