Protecting API compatibility with coding agents

API compatibility checks help prevent service changes that pass unit tests yet break clients through altered fields, inputs, defaults, authentication, or error responses. Maintaining an OpenAPI specification in version control provides coding agents with a baseline contract, but effective reviews also require explicit compatibility policies, deprecation rules, consumer tests, SDKs, gateway behavior, and evidence of how real clients use the API. Proposed changes should be compared against the baseline with tools such as OpenAPI Diff, while agents also trace behavioral effects across validation, authorization, serialization, storage, events, generated SDKs, and documentation. The recommended approach favors additive changes, explicit and owned compatibility shims, consumer migration before removal, and a defined deprecation period. Pull requests should document contract differences, affected consumers, migration sequencing, and focused test evidence, with provider tests run against old requests and consumer tests against the new service, particularly for authentication and failure paths. Human approval remains important for accepted breaking changes, and agents should report blocked or incomplete evidence rather than make unsupported claims when they cannot establish a baseline, identify consumers, or reproduce prior behavior.

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