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What is an API contract?
An API contract describes what a service offers and how consumers interact with it. It is more than explanatory prose: it is a structured definition that software tools and teams can use to validate requests and responses, generate code, or create test doubles.
Amazon Web Services describes service contracts as “documented agreements between API producers and consumers defined in a machine-readable API definition” in its REL03-BP03 guidance.
For example, imagine one team owns an order service while another builds a storefront. Their contract can define the operations available to the storefront and the typed fields expected in requests and returned in responses. Both teams can implement and release independently as long as their systems continue to meet that shared agreement.
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Choose a format that fits the interface
OpenAPI is one option for describing HTTP APIs. AWS also identifies GraphQL schemas and event schemas as ways to describe service interfaces. There is no single format implied for every API style; choose one that represents how your service and consumers actually communicate.
Why do I need an API contract?
A contract replaces assumptions scattered across teams with a reference they can inspect and use. That can reduce coordination bottlenecks: a consumer team can build against an agreed shape while the provider team implements it, and each side can check its work against the same definition.
Because a strongly typed schema can be validated programmatically, a contract may support generated validation, client code, test cases, and mock implementations. AWS recommends using contracts to develop test cases and mocks. These uses are only as reliable as the contract: if the definition does not reflect the interface consumers actually need, generated artifacts will reproduce that mismatch.
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What should an API contract include?
At minimum, describe the service capabilities consumers can call and the shapes of the inputs and outputs. Make the data types and required or optional fields explicit so teams can validate whether an implementation conforms.
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For a useful contract in a real service, decide how it will describe these interface details:
- Operations or capabilities: what a consumer can ask the service to do or what events it can exchange.
- Requests and responses: their fields, types, and which values are required or optional.
- Errors: which failure cases consumers should handle and how those failures are represented.
- Authentication: what credentials or authorization requirements apply.
- Behavioral guarantees: any timing, ordering, or other behavior consumers rely on beyond the data shape.
The last three are design questions to resolve for your interface, not a universal checklist prescribed by a single standard. A schema can be precise about structure while leaving important behavior unclear, so document the expectations that matter to consumers.
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How do API contract tests work?
Contract-based testing checks whether implementations meet defined expectations. The term can describe related checks with different scopes, so distinguish shape validation, consumer-driven checks, and provider functional tests.
Schema or conformance checks
These checks ask whether requests, responses, or an implementation conform to the declared contract—for example, whether a response contains the required fields with the expected types.
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In consumer-driven testing, a particular consumer records the requests it makes and the responses it expects; the provider is then checked against those expectations. Pact describes contract testing as ensuring that consumer and provider teams have “a shared understanding of what the requests and responses will be in each possible scenario” in its consumer testing documentation.
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Pact advises that consumer tests exercise the actual consumer code and focus on its assumptions about provider responses. They are not a substitute for checking whether the provider’s business logic is functionally correct.
Provider functional tests
Functional tests check that the provider performs its intended behavior. A contract check may show that a response has the expected structure without proving that the underlying business result is correct. The approaches complement one another: contract checks address specified interactions, while provider tests address service behavior.
No contract test guarantees that integration failures cannot happen. Tests can miss cases that neither the contract nor the tests cover.
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How do I change an API without breaking clients?
Give consumers a clear way to remain on an existing contract while they prepare to migrate. AWS recommends a contract versioning strategy that allows consumers to keep using an existing API and move when ready. The exact approach and support policy depend on the service; no universal deprecation period follows from that recommendation.
Define what counts as compatible
One published example is the Government of Canada’s API standard, which uses major, minor, and patch changes:
- Major: changes likely to break backward compatibility.
- Minor: backward-compatible additions, such as optional attributes or functionality.
- Patch: internal fixes that should not affect the schema or contract.
This is one policy, not a universal versioning rule. Whatever scheme you choose, state which changes consumers can safely absorb and which require migration.
Make migration predictable
Document how consumers select a contract version, how long older versions remain available, and how you will communicate migration steps and dates. Before changing a published contract, check it against the expectations of known consumers; after a change, test both the new behavior and any versions that remain supported. These practices help manage compatibility, but cannot guarantee that every client has been identified or every dependency captured.
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