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API contract testing checks whether a consumer and provider agree on the messages exchanged at their boundary. Integration testing checks whether connected parts work together in the integrated setup being tested. Contract tests are focused evidence of message compatibility; they do not prove that a service performed the intended business operation or saved the right data. Teams often need both because compatibility and behavior are different risks.
What is API contract testing?
API contract testing verifies that messages exchanged between a consumer and a provider match an agreed contract. For an HTTP API, that usually means checking expected requests and responses; for a message-based integration, it means checking the messages exchanged. The contract captures the interactions a consumer depends on, rather than every possible behavior of the provider. Pact describes itself as “a code-first tool for testing HTTP and message integrations using contract tests.” Pact documentation explains this message-focused approach.
In Pact’s consumer-driven workflow, the consumer defines concrete interactions it needs. The resulting contract can then be verified against the provider. This gives independently developed applications a shared, executable description of their boundary without requiring all participating applications to be deployed together for that check. Pact’s workflow overview describes the consumer and provider roles.
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Integration testing checks whether connected components work together in an integrated setup. The scope is not fixed: a team might test a particular component boundary, a service path, real dependency wiring, or a broader system path. Depending on the test, it may exercise runtime behavior, data flow, business logic, and side effects.
That variability matters: “integration test” does not automatically mean a full end-to-end test, nor does every integration test have the same dependencies, speed, or cost. The useful question is what components and behaviors a specific test actually covers. Pact’s discussion of testing scope likewise distinguishes a focused contract boundary from broader tests. Pact’s testing-scope guidance provides further context.
Contract testing vs. integration testing
| Question | Contract testing | Integration testing |
|---|---|---|
| What does it ask? | Do the consumer and provider agree on the tested messages? | Do the connected components work together in the tested setup? |
| Typical scope | A specific consumer-provider interaction or message contract. | A component boundary, service path, or larger integrated system; scope varies by test. |
| What runs? | Consumer and provider can be checked separately for their respective contract responsibilities. | Often exercises connected components or dependencies, depending on the scope. |
| What evidence does it provide? | The tested request/response or message interactions match the contract, including provider verification. | The integrated components exhibit the runtime behavior covered by the test. |
| What might it miss? | Business logic, persistence, unmodeled interactions, or semantics beyond the contract. | Paths and behaviors outside the selected integration-test scope. |
| When is it useful? | When independent consumers and providers need confidence that a change preserves expected communication. | When real dependency wiring, data flow, business behavior, or side effects need checking. |
The decisive distinction is what a passing test proves. A contract test can show that a provider returns a response matching the consumer’s recorded expectation. By itself, that result does not show that the service calculated the correct result, persisted an order, or completed another intended side effect. Those behaviors need functional or integration coverage. Pact’s comparison of contract and functional tests explains this limitation.
How a Pact contract test works
- Define a consumer interaction. The consumer test specifies a request and the response or message the consumer needs.
- Run the consumer test against a mock provider. This checks the consumer’s assumptions without requiring the real provider to be available.
- Generate the Pact file. It records the consumer and provider names and the interactions under test.
- Verify the provider. Provider verification replays the expected requests against provider code and checks the returned responses against the contract.
- Share and coordinate, if needed. Teams can use a Pact Broker to share contract artifacts and coordinate verification in CI/CD. Pact describes the Broker as an externally hosted service with an API and UI; those facts do not establish its current pricing or commercial terms.
Pact’s consumer testing guidance describes consumer responsibilities, while its terminology guide defines Pact files, verification, and the Broker.
When should you use each approach?
Choose contract tests for compatibility risk
Use contract tests when a provider change might break a consumer’s expected request or response, or when separate teams need an explicit, executable view of their integration. The test focuses on interactions consumers actually rely on, rather than trying to establish every possible provider behavior.
Choose integration or functional tests for behavior risk
Use broader integration or functional tests when you need to verify business rules, actual dependencies, persistence, side effects, or a complete data path. For example, a matching response contract cannot alone prove that an order was committed to storage.
Use both when both risks matter
Contract checks and integration tests answer complementary questions: whether messages remain compatible, and whether the connected system behaves correctly. A focused contract suite may reduce the need for some costly integrated checks, but it does not replace behavioral coverage.
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Are API schemas and contracts the same thing?
Not necessarily. A static schema can describe an API’s allowed structure, while consumer-driven contract testing records concrete interactions that a consumer needs and verifies provider behavior against them. Pact’s documentation distinguishes consumer-driven contracts from static schemas and notes that generating Pact files by hand from a Swagger document defeats the consumer-driven purpose: the interactions should reflect consumer needs. Pact’s introduction and its FAQ discuss these distinctions.
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