API Testing Services

Why test APIs directly

APIs carry the business logic behind web apps, mobile apps and partner integrations. Testing through the user interface alone is slow and misses many cases; testing the API directly is faster, more thorough and catches problems before they reach any client.

API tests also protect your partners. When an endpoint changes in a way that breaks an integration, contract tests flag it in the pipeline instead of in a customer’s production system.

Problems we solve

• Mobile or front-end releases breaking because the API changed unexpectedly
• Partners reporting integration failures after your deployments
• Edge cases and error responses never tested
• Slow end-to-end UI suites used to test business logic
• No confidence about API performance under load

What’s included

Building an API test suite

We start from the API specification, or create one if it does not exist. Tests cover the happy path, validation errors, permission boundaries between users and tenants, pagination, idempotency and rate limits. Test data is created through the API itself so suites run reliably in any environment. The suite runs in your pipeline in minutes, giving developers fast feedback.

Testing authorisation properly

Broken object-level authorisation — where one user can access another user’s data by changing an ID — is the most common serious API vulnerability. Automated tools rarely catch it because it depends on business rules. We write explicit tests for every sensitive endpoint, checking that each role and each tenant can access only what it should.

Types of API tests

Managing test data

Unreliable test data is the most common reason API test suites become flaky. A few principles keep them stable:

• Each test creates the data it needs and cleans up afterwards, rather than relying on shared records.
• Use factories or seed scripts so data is consistent and easy to change.
• Isolate tests by tenant or unique identifiers so they can run in parallel.
• Mock unreliable third-party services, but keep a smaller set of tests against their sandboxes.
• Never copy production personal data into test environments without anonymisation.

With data handled well, API suites can run in minutes on every pull request, giving developers fast confidence that changes have not broken clients or integrations.

Testing GraphQL and event-driven APIs

REST is not the only API style worth testing. GraphQL APIs need tests for query depth and complexity limits, field-level authorisation and error handling for partial results, because one request can touch many resolvers. Event-driven systems — webhooks, message queues, streaming — need tests that verify message schemas, handle duplicates and out-of-order delivery, and confirm that consumers recover after downtime. Contract testing tools and schema registries help both styles evolve safely without breaking the clients that depend on them.

API testing tools compared

There is no single best API testing tool. The right choice depends on who writes the tests, which language your services use and whether the tests must run in a pipeline. We usually recommend keeping API tests in the same language and repository as the service, so developers can run and fix them without switching context.

We pick tools with your team; many projects combine two, for example pytest for functional tests and Pact for contracts.

What a mature API test suite contains

• Happy-path tests for every public endpoint and method
• Validation tests: missing fields, wrong types, oversized payloads and boundary values
• Authentication tests: missing, expired and tampered tokens
• Authorisation tests proving one tenant or user cannot read or change another’s data
• Error-format tests so every failure returns a consistent, documented structure
• Idempotency and retry tests for payment, order and other critical writes
• Pagination, sorting and filtering tests on list endpoints
• Contract or schema checks that fail the build when the response shape changes unexpectedly

A typical engagement

1. Inventory the API2–4 days

   Map endpoints, consumers, authentication methods and the most business-critical flows. Where no OpenAPI document exists, we generate one from traffic or code.

2. Prioritise risk1–2 days

   Rank endpoints by business impact and change frequency so the first tests protect what matters most.

3. Build the framework1 week

   Set up the runner, environments, test data factories, reporting and CI integration.

4. Write and expand testsongoing

   Cover critical flows first, then broaden coverage sprint by sprint alongside new features.

5. Hand over

   Document conventions and pair with your developers so the suite stays healthy after we step back.

Common API testing mistakes

• Testing only status codes. A 200 response with the wrong body is still a bug; assert on the data that matters.
• Sharing state between tests. Tests that depend on each other’s data become flaky and impossible to run in parallel.
• Hard-coding environment URLs and secrets. Keep configuration outside the tests and secrets in your CI vault.
• Ignoring negative cases. Most security issues hide in requests the developer never expected.
• Running the suite only before releases. API tests are fast enough to run on every pull request; that is where they deliver most value.

Metrics worth tracking

API tests sit at the heart of a healthy test automation strategy and run best inside a well-designed CI/CD pipeline.

Related services

Part of QA & testing. Pairs with API development and test automation.

This page is part of QA & Software Testing Services — see the full overview for related services, approach and pricing.