Skill: Decide Mock

"Mock at the boundaries, not everywhere."

The Decision

Mock when:

• External service (payment gateway, email provider)
• Slow dependency (file system, network)
• Non-deterministic (time, randomness)
• You don't control it

Don't mock when:

• In-process dependency (your own classes)
• Database (use transactions instead)
• The mock is more complex than the real thing
• You're testing integration, not isolation

The Heuristic

Ask: "Does this cross a process boundary?"

Yes → Mock it. No → Use the real thing.

The Quick Test

Ask	Answer	Action
External API?	Yes	Mock
Your own class?	Yes	Real
Database?	Yes	Real + transaction
Time-dependent?	Yes	Freeze time
Non-deterministic?	Yes	Mock or seed

Real-World Examples

See examples.md.