Before releasing a new API version, what kind of testing directly checks if the new API still works correctly with existing client applications by verifying their agreed-upon interactions?

The kind of testing that directly checks if a new API version still works correctly with existing client applications by verifying their agreed-upon interactions is called Backward Compatibility Testing. Backward Compatibility Testing is a crucial process to ensure that a newly updated or modified Application Programming Interface, or API, which is a set of defined rules and methods that allow different software components to communicate with each other, can still be used without issues by older versions of client applications. Client applications are the software programs that consume or interact with the API to perform their functions. This testing specifically verifies that the *contract– the established set of expected inputs, outputs, data formats, and behaviors that the API and client applications have agreed upon for their interaction – remains valid and functional for existing clients. For example, if a new API version adds an optional field to an existing response, Backward Compatibility Testing confirms that older client applications, which do not expect this new field, can still successfully parse the response and continue their operations without errors. This type of testing is a specialized form of Regression Testing, which generally aims to ensure that new code changes or modifications have not introduced new defects or adversely affected existing functionality. In the context of APIs, Backward Compatibility Testing focuses on preserving the stable interface for existing consumers, preventing breaking changes that would force clients to update their own code.