Part of Eventide's Useful-Objects doctrine. A dependency is defaulted to an inert null-object implementation of its interface (via Mimic), optionally specialised via a `Substitute` module that adds domain-level telemetry methods (e.g. `http_client.posted?(content)`). Used in tests to assert that a collaborator was interacted with correctly, without invoking live services. Bellware insists this is *not* a mock object: mock objects retrofit transparency into the test file after design is finished, whereas a diagnostic substitute is part of the class's own design and lives with the production code. A diagnostic substitute can behave like a spy in spirit, but the architectural choice differs.