Core TDD philosophy and testing principles. Use when writing tests interactively or need testing guidance. Emphasizes tests as contracts, mock boundaries not internals, and implementation fixes over test changes.
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What the file says
# Test-Driven Development
**For system-level design principles and architectural boundaries, see the `software-engineer` skill.**
## Core Philosophy
<core_philosophy>
**Foundation**: TDD is an application of contract-based design. Tests define the contract an implementation must fulfill—they are specifications, not afterthoughts. This connects directly to the abstraction principles in the `software-engineer` skill.
**Tests = Contracts**: Tests represent business requirements and expected behavior. They are contracts the implementation must fulfill. Never compromise test integrity to achieve green tests.
**Tests are source of truth**: When tests fail, fix the implementation. Only change tests when requirements change or test has a verified bug.
</core_philosophy>
## Testing Approach
### Architectural Boundaries and Mocking Strategy
<architectural_boundaries>
**Identify architectural boundaries** in your system:
- Layer boundaries (e.g., data layer, service layer, presentation layer)
- Module boundaries (e.g., namespaces, packages, crates)
- Process boundaries (e.g., Erlang processes, Elixir GenServers)
…
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