API Compatibility

Overview

API Compatibility is the degree to which an API change, client, or implementation continues to work with existing consumers and expected behavior. It is used to allow independent releases without unexpectedly breaking integrations. It differs from API version compatibility, which specifically compares behavior across named API versions.

A typical implementation works as follows: Teams compare request and response structures, semantics, errors, authentication, timing, and side effects against supported consumer expectations. For API Compatibility, the application should preserve a correlation path from request construction through response, asynchronous completion, and final business state.

Common risks include a schema-compatible change can still break consumers through changed defaults and ordering. Important failure modes include a schema-compatible change. These failures can still break consumers through changed defaults, ordering, precision, performance, or undocumented behavior.

Core controls require teams to define compatibility rules, run contract and consumer tests, use additive evolution, publish changes, and monitor old-client behavior. A successful transport result from API Compatibility should not be interpreted beyond the exact API state documented by the provider.

Operational evidence should include provider release, consumer version, tested operations, contract diff, behavioral result, exception, and support decision. API Compatibility should be documented alongside API Version Compatibility, API Contract Diff, and API Migration. Data handled by API Compatibility should use documented types, units, timestamp rules, nullable fields, and enum behavior to prevent silent interpretation errors.

The API Compatibility contract should define identifiers, validation, authorization, status semantics, errors, limits, and version behavior at the same boundary. Changes to API Compatibility should be checked against older clients, stored payloads, enum growth, default behavior, and retry assumptions.