API testing checks whether a service behaves correctly at its boundary. Begin with the request method, URL, headers, body, authentication, and expected response. A successful status code is not enough; the response shape, data rules, timing, and side effects also matter.
Write positive tests for valid requests and negative tests for missing fields, invalid permissions, malformed data, and unavailable dependencies. Contract tests can verify that clients and services agree on fields and types. Use isolated test data and clean it up so tests remain repeatable.
Automated tests should produce useful failure messages and run in continuous integration. Avoid relying on execution order or live external services when a controlled substitute is sufficient. Add security checks for authorization boundaries and sensitive data exposure. Good API testing gives teams confidence that a change works for both normal users and unexpected input.