APIs are the connective tissue of modern software. Whether you are building a feature that calls an external service, debugging why a request is failing, or exploring a new API before writing integration code, the ability to test API endpoints quickly and accurately is an essential skill.
What API Testing Involves
An API test sends an HTTP request to an endpoint and examines the response. At minimum, you check the status code (is it the expected 200, or is it a 404 or 500?). You check the response body (does it contain the data you expect in the format you expect?). You check the response headers (is the Content-Type correct? Are CORS headers present?). You check the response time (is the endpoint responding fast enough?).
For write operations (POST, PUT, PATCH, DELETE), you also check that the change was applied correctly by reading the data back afterward.
HTTP Methods
GET retrieves data and should have no side effects. POST creates a new resource or triggers an action. PUT replaces an existing resource entirely. PATCH partially updates an existing resource. DELETE removes a resource.
The correct method is part of the API contract. Using GET for operations that change data violates REST conventions and causes caching problems because caches assume GET responses can be safely cached.
Request Headers
Authorization is the most important header for authenticated APIs. REST APIs commonly use Bearer token authorization, where you include a JWT or API key in the header: Authorization: Bearer your_token_here.
Content-Type tells the server what format the request body is in. For JSON APIs, use Content-Type: application/json.
Accept tells the server what formats the client can handle in the response. For JSON responses, use Accept: application/json.
Using the DevHexLab API Tester
Open the tool at /tools/developer/api-tester. Enter the URL and select the method. Add any required headers in the Headers tab. For POST or PUT requests, add the JSON body in the Body tab. Click Send. The response status, headers, and body all appear in the output panel.
This lets you test any public or local API endpoint directly from your browser without installing Postman or any other tool.
Frequently Asked Questions
What is the difference between the API Tester and Postman?
Both send HTTP requests and display responses. Postman is a full desktop application with collections, environments, test scripting, and team collaboration features. The DevHexLab API Tester is a quick browser-based tool for one-off testing with no installation or account required.
Can I test APIs that require cookies?
Cookie-based authentication is harder to test in a browser tool because of same-origin restrictions. For cookie-based APIs, use the browser's built-in developer tools Network tab, which has access to all cookies for the current site.
Why do I get CORS errors in the API Tester?
CORS restrictions apply when a browser makes cross-origin requests. If the API does not allow requests from the DevHexLab origin, the browser blocks the response. Use the DevHexLab CORS Tester to check the server's CORS configuration, or test the API from a non-browser context.
Test before you code. Understanding the API response shape saves hours of integration debugging.