Acceptance Test-Driven Development (Outside-In TDD)

Philosophy

Core principle: We only build what the outside needs. Start at the user-facing boundary — an API endpoint, CLI command, or UI interaction — and let the outer behavior drive every design decision inward.

Two feedback loops run simultaneously: an outer acceptance test that defines "done" for the feature, and inner unit tests that drive each layer's implementation. Mocks aren't just test isolation — they're the mechanism for discovering what the next layer inward needs to do before it exists.

See double-loop.md for the loop mechanics and acceptance-tests.md for what makes a good acceptance test.

Anti-Pattern: Inside-Out Guessing

Don't start by building domain logic or infrastructure hoping it'll compose into the right feature. Building bottom-up means you're guessing what the outer layer will need.

The acceptance test is the anchor. Every design decision flows from it.

Workflow

1. Planning