API testing is the practice of verifying an API directly — sending a request and checking that the response, status code, headers, and payload shape are all correct — without going through a UI at all.
Because it skips the UI, API testing catches bugs closer to where they're introduced (in the backend logic and data contracts) and runs an order of magnitude faster than end-to-end UI tests.
What API testing actually checks
- Correctness — does the response match the expected schema, status code, and values?
- Reliability — does the API behave consistently across repeated calls?
- Performance — how fast does it respond, and does that hold up under load?
- Security — is the endpoint protected against common issues like broken auth or IDOR (see our IDOR testing guide)?
Where it fits in your workflow
Most teams run API tests at three points: locally while building an endpoint, in CI on every pull request, and as scheduled monitoring against production. OptraFlow supports all three from a single test suite — see Run Testing from CI/CD.
Next step
If you are new to this, our step-by-step guide to performing API testing walks through building your first test suite.