What is API testing?

API testing verifies HTTP/gRPC (or similar) endpoints for correct status codes, payloads, auth, and side effects without driving the full UI. It is faster and more stable than UI tests because it skips rendering and browser timing. Teams test contracts - request/response schemas, error handling, rate limits, and idempotency - often in integration or contract-test suites. API tests suit microservices where the UI is thin and business logic lives behind services. They complement UI automation: APIs can set up data and assert backend state while UI confirms the user-visible path. Tools range from Postman collections to code-first frameworks in the same language as the app. Good API coverage reduces the number of slow E2E cases needed for the same confidence.