Mock what you must. Understand what you mock.
Every substitute encodes assumptions about a dependency that may drift from reality.
Interpretation
Mocks buy speed and isolation by replacing reality with a model. If the model omits latency, errors, serialization, or version behavior, a green test can defend a fiction. Use substitutes at deliberate boundaries and verify the contract against the real dependency often enough to keep it honest.
Practice
- Mock at stable boundaries rather than every method.
- Run contract tests against real implementations.
- Model meaningful failure modes in substitutes.
Anti-pattern
A mock returning values the real service can never produce.