growing-outside-in-systems
Drive feature development using Outside-In TDD with Hexagonal Architecture. Design emerges through inline code, in-memory fakes, interface extraction, and deferred I/O. Use when building features, writing tests, or structuring backend services. Triggers on: TDD, outside-in, hexagonal, ports and adapters, emergent design, acceptance test, component test, walking skeleton, in-memory fakes, component, contract test, adapter, fast tests, sub-second feedback. Language-agnostic (Go, Rust, Python, TypeScript, Java, C#).
npx skills add a5c-ai/babysitter --skill growing-outside-in-systems --agent claude-code
Same command for any agent — swap --agent for codex, cursor, copilot.
Weekly change comes from our own snapshots, not the repository page — it measures attention, not adoption.
STARTER_CHARACTER = 🔴🟢 # Outside-In TDD with Hexagonal Architecture Build features by driving design from the outside in. Every feature starts with a failing acceptance test. Design emerges through disciplined Red-Green-Refactor cycles. Infrastructure is deferred until the domain API is proven. Unlike inside-out TDD with mocks, this approach tests behavior at the boundary — not implementation details — making the suite refactor-friendly by design. See [methodology.md](references/methodology.md#why-outside-in-over-regular-tdd). For canonical terms used throughout, see [references/glossary.md](references/glossary.md). --- ## The Two Loops **Outer loop (Acceptance test):** Write a failing test scoped at the service/system boundary. This test exercises integration across Bounded Contexts using in-memory adapters. It defines "done." **Inner loop (Red-Green-Refactor):** Cycles inside a Bounded Context to make the outer test pass. Drop into this loop only to implement what the acceptance test demands. Every feature starts from the outside in. The acceptance test drives the process. --- ## Test Priorities Acceptance and component tests are the primary instruments. Unit tests are the exce
- The Two Loops
- Test Priorities
- The Testing Matrix
- Emergent Design Workflow
- Hexagonal Architecture
- In-Memory Fakes, Not Mocks
- Walking Skeleton
- Composition Root (Configurator)
- Feature Implementation Order
- Listen to the Tests
- Reference Documentation
- Sources
What does the growing-outside-in-systems skill do?
Drive feature development using Outside-In TDD with Hexagonal Architecture. Design emerges through inline code, in-memory fakes, interface extraction, and deferred I/O. Use when building features, writing tests, or structuring backend services. Triggers on: TDD, outside-in, hexagonal, ports and adapters, emergent design, acceptance test, component test, walking skeleton, in-memory fakes, component, contract test, adapter, fast tests, sub-second feedback. Language-agnostic (Go, Rust, Python, TypeScript, Java, C#).
How do I install it?
Run `npx skills add a5c-ai/babysitter --skill growing-outside-in-systems --agent claude-code` — it drops the skill into your project so the agent can pick it up. Swap the --agent value for codex, cursor or copilot if you use one of those.
Where does this skill come from?
From a5c-ai/babysitter, a repository with 1,642 stars. We read it straight from the repository tree rather than a submitted listing, so what you see here is what is actually published.
Is a popular skill a good skill?
Not necessarily. Stars measure attention, not adoption — a repository can trend for a week and be abandoned. That is why we show the weekly change from our own snapshots next to the total, instead of a single flattering number.