API Contract

Overview

API Contract is the documented agreement that defines an API’s operations, inputs, outputs, errors, security requirements, and behavioral guarantees. It is used to keep providers and consumers aligned as implementations and versions change. It differs from an API schema, which describes structure but may not capture all semantics, timing, or side effects.

A typical implementation works as follows: Teams express the contract through specifications, prose, examples, validation rules, and tests, then use it during implementation and release review. Testing API Contract should cover malformed input, denied access, rate limits, timeouts, duplicates, asynchronous completion, and version changes.

Common risks include ambiguous fields and undocumented side effects. Important failure modes include ambiguous fields, undocumented side effects, incompatible changes, and stale examples. These failures can break integrations without obvious compile-time failure.

Core controls require teams to version the contract, test consumers, define compatibility policy, publish change notices, and connect examples to executable validation. For API Contract, the application should preserve a correlation path from request construction through response, asynchronous completion, and final business state.

Operational evidence should include contract version, change history, compatibility result, owner, approval, and affected consumers. API Contract should be documented alongside API Definition, API Schema, and API Contract Diff. The API Contract owner should publish stable error semantics and guidance on whether correction, retry, status lookup, or escalation is appropriate.

Data handled by API Contract should use documented types, units, timestamp rules, nullable fields, and enum behavior to prevent silent interpretation errors. The API Contract contract should define identifiers, validation, authorization, status semantics, errors, limits, and version behavior at the same boundary.