Contract testing

A contract is the set of interactions a consumer relies on: these requests, answered with responses of this shape and these status codes. Contract testing checks each side against that contract separately — the consumer against a mock built from it, the provider against the requests recorded in it — so incompatibilities are found before deployment rather than in an end-to-end environment.

How it works

In consumer-driven contract testing (Pact is the best-known tool), each consumer's tests produce a contract file; the provider's build replays every contract against the real service and fails if any expectation breaks. In schema-based contract testing, responses are validated against the API's OpenAPI document instead.

The misconception

That contract tests check behaviour. They check shape and protocol — that a field exists, has a type, comes with a status. Whether the price in that field is correct is a functional test's job. Teams that replace functional tests with contract tests ship well-formed wrong answers.

Try it here

Download any API's OpenAPI document from its docs page and validate the responses you get from Try it out against it; the assertion language can also pin a contract point by point, such as body.data.0.id exists.

Related

• What OpenAPI is
• JSON Schema

Sources

• HTTP semantics — RFC 9110
• OpenAPI Initiative

Other terms

• OpenAPI — A machine-readable description of an HTTP API — its paths, parameters, bodies and responses — in JSON or YAML.
• JSON Schema — A JSON vocabulary for describing what a valid JSON document looks like, so documents can be validated automatically.
• Mock API — A stand-in for a real API that returns realistic responses without the real system behind it.
• API sandbox — An isolated copy of an API where calls have no real-world effect and data can be reset.
• Mocks, stubs and fakes — Three kinds of test double — stand-ins for a real dependency — that differ in what they do and what they check.
• Test fixture — The known data and state a test starts from, set up the same way on every run.

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Last updated 16 October 2026