Mock-based API tests can pass after the live API changes because they check your code against the behavior configured in the mock—not whether the provider still behaves that way. To catch incompatibilities, verify the consumer’s recorded expectations against the provider, or validate the provider against a maintained API schema. Those checks answer different questions, and neither replaces provider functional tests.
Why mock tests pass when the API changes
A mock supplies a test with a simulated response. If that mock still returns the fields and status codes your client expects, the test can pass even when the real provider has changed its response, removed a field, or altered an endpoint.
In other words, a passing mock test shows that the consumer behaves as expected for the configured interaction. It does not, by itself, show that the current provider remains compatible. That gap is the reason to connect consumer expectations to a check of the provider implementation. Pact’s introduction to contract testing explains this distinction.
Three ways to check API compatibility
| Approach | What it checks | Useful when | Limitation |
|---|---|---|---|
| Mock-based unit test alone | Consumer behavior for the interactions configured in the mock | You want fast feedback on client logic and response handling | Does not establish that the current provider meets the mock’s assumptions. |
| Consumer-driven contract with provider verification | Concrete request/response interactions that consumers rely on, replayed against the provider implementation | Consumer and provider teams need a shared compatibility check | Verifies recorded interactions, not every provider behavior or whether the provider is functionally correct. |
| OpenAPI or schema validation | Whether provider requests and responses conform to documented API schemas | The API description is maintained and conformance to its broader documented surface matters | A schema may not capture every consumer-specific expectation or semantic behavior. |
Pact describes consumer-driven contracts as concrete interactions used by particular consumers, while schema validation checks conformance to a documented API description. Pact’s overview and MockServer’s contract-testing guide describe these related but distinct approaches.
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How consumer-driven contract testing closes the gap
A consumer-driven contract captures requests and responses that a client actually depends on. The consumer test still runs against a mock, but the interaction is recorded in a contract that can be shared and checked against the provider. This makes the mock a way to define an expectation—not evidence, on its own, that the provider meets it.
- Test meaningful consumer interactions. Cover the method, path, request details, response fields, and behavior the client relies on. Keep examples focused: Pact advises making tests as loose as possible while still catching real breaking changes. Pact’s consumer-test guidance explains how to choose useful examples.
- Record and share the interactions. In Pact’s documented HTTP workflow, consumer tests run against a mock and produce a JSON contract. The consumer publishes or otherwise shares that contract.
- Verify the contract against the provider. The provider replays the contract’s requests against a running implementation. A mismatch can reveal that the provider no longer supports behavior a consumer depends on. Pact’s JavaScript consumer guide describes this workflow.
Consumer contract tests are not provider functional tests. They help expose incorrect consumer requests, response-handling mistakes, and misunderstandings between consumer and provider. Provider functional tests remain responsible for checking whether the provider does the right thing for a request. Pact’s consumer guidance makes that distinction.
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When OpenAPI validation is the better fit
If your team maintains an OpenAPI description and wants to check the provider against that documented API surface, schema validation can test whether actual responses conform to defined schemas. MockServer documents a workflow that generates representative requests from OpenAPI and validates responses against the specified response schemas. Its contract-testing guide also describes importing Pact contracts.
Choose according to the compatibility risk you need to cover: consumer contracts protect concrete interactions current clients rely on; schema validation checks conformance to the documented schema. A schema does not automatically capture every client-specific expectation, while a consumer contract does not cover every possible provider behavior.
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Roll out incompatible API changes in stages
When a change will break existing consumers, use an expand-and-contract rollout rather than removing the old interface immediately:
- Add and deploy the replacement field or endpoint while the old one remains available.
- Migrate consumers to the replacement.
- Remove the old field or endpoint only after consumers have moved.
Pact’s FAQ also describes using a Pact Broker to check provider changes against production and the latest consumer contracts. Pact’s FAQ covers these staged changes and contract checks.
Quick Recap
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