MANDATORY when ANY of these appear: (1) writing/editing/refactoring code that calls any third-party SDK, API, or library method, (2) debugging ANY issue where an API call succeeds but the expected side-effect doesn't happen, (3) installing or importing external packages, (4) user reports something 'doesn't work' and the code involves an external service. This skill MUST be loaded BEFORE proposing any fix — never diagnose from memory.