Audience member pushes back that checking `http_client.posted?(content)` in a test looks exactly like a mock or a spy. Bellware agrees the behaviour is similar in spirit but insists on the architectural distinction: a mock is retrofitted transparency added to the test file after design is finished, violating the system to make it observable. A diagnostic substitute is designed-in — the production class ships a `Substitute` that exposes telemetry through the same interface, so the transparency lives with the design rather than in the test. Also covers how to handle dependencies that return data (specialise the substitute to allow setting return values — not ideal but sometimes necessary for HTTP client libraries).