What This Is
API testing is the work of verifying that the contracts your API exposes — the endpoints, the request shapes, the response shapes, the status codes, the headers, the error formats — behave correctly across every condition that callers will encounter. It is testing the API as a product in its own right, not just as a layer behind a UI.
This matters because APIs almost always have callers you do not control. A mobile app, a partner integration, a third-party tool, a JavaScript front-end, an internal microservice — each of them has assumptions baked into how it calls your API. A change that looks innocuous from inside (rename a field, change a status code, add a required header) can silently break every consumer the moment it deploys.
API testing finds those breakages before deploy. The work covers contract validation (does the response match the documented schema), behaviour validation (does the API do the right thing for valid input), error validation (does it fail correctly for invalid input), and version compatibility (does v1 still work the way v1 callers expect, even after v2 has shipped).
We design and operate seven distinct product APIs across the Beacon ecosystem — WP Beacon, Lens, Pulse, Workbench, Bits, Agents, and the public website — each with its own contract, its own auth scheme, and its own consumer base. That experience shapes how we approach client API testing: every breaking change has a downstream cost, so the test suite must catch them before the release notes do.
When You Need This
API testing is essential when:
- You have a public or partner-facing API with consumers you cannot redeploy in lockstep — mobile apps, third-party integrations, customer scripts
- You maintain multiple API versions simultaneously and need confidence that v1 still works after every v2 change
- Your mobile app depends on API stability, and an unintended change to the API breaks every installed copy
- You are building an API as a product — a developer-facing platform where the API is the deliverable — and contract drift would damage the relationship with paying customers
- Your team has had a breaking change reach production and now wants a way to make sure it cannot happen again
This is not the right service if you only have one consumer (your own front-end) and you can deploy them together. In that case, integration tests over the controllers cover the same ground without the additional layer of contract assertions.
How We Work
API testing engagements start with the contract, not the implementation. We document what the API is supposed to do (in OpenAPI, in markdown, in whatever format you have), and the test suite asserts the implementation matches the contract. If the contract changes, the tests fail; if the implementation drifts from the contract, the tests fail. Either way, the team finds out before consumers do.
We test happy paths and error paths with equal weight. Most API bugs in the wild are not in the happy path — they are in how the API responds to malformed input, missing fields, invalid auth, expired tokens, rate limits, and the long tail of error conditions. We test the error format, the status codes, and the consistency of error responses across endpoints, because consumers handle errors based on those.
We version-test explicitly. When v2 ships, the v1 tests do not get deleted — they keep running against the v1 routes. The day v1 stops behaving the way v1 consumers expect is the day a customer’s app breaks. We make that day visible by failing CI instead.
We test authentication exhaustively. Public APIs are attacked. Token expiry, token revocation, scope enforcement, rate limits, CORS, and authentication failure responses all need explicit tests. The cost of getting any of these wrong is much higher than the cost of getting a feature wrong.
We integrate with consumer test suites where possible. When a consumer is something we control (a mobile app, a browser extension), we run contract tests from both sides — the API asserts it produces the documented shape, and the consumer asserts it expects the documented shape. If either drifts, both fail.
What You Get
- Contract test suite asserting the documented API shape against the running implementation
- Behaviour tests for happy paths, edge cases, and error conditions
- Authentication and authorisation tests for every endpoint and role combination
- Version compatibility tests ensuring older API versions remain stable as newer versions ship
- Rate limit and throttling tests verifying limits are enforced and communicated correctly
- OpenAPI or Postman collections kept in sync with the test suite where useful
- CI integration so contract drift fails the pipeline before merge
Technologies We Use
- PHPUnit feature tests for Laravel APIs — direct framework testing with full request and response coverage
- Pest where preferred for the syntax
- Postman / Newman for collection-based testing that doubles as living documentation
- k6 for combined functional and load testing of API endpoints
- OpenAPI / Swagger as the contract source of truth
- Sanctum and API key auth helpers for testing the various authentication patterns
Related Systems
API testing applies to any system that exposes endpoints to callers outside the same deploy unit. A customer self-service portal with a mobile app needs API stability to survive App Store review cycles. An outbound automation system called by external tools cannot afford to break the integrations its callers depend on.
Talk to Us About Testing Your API
If your API is consumed by anything you cannot redeploy at the same time — mobile, partners, customer scripts — get in touch and we will scope a contract testing engagement that prevents breaking changes from reaching consumers.
