Individual skill
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#).
library/specializations/backend-development/skills/growing-outside-in-systems/SKILL.md
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README.mddoc · 4,268 bytesSKILL.mdcore · 8,423 bytesreferences/folder-structure.mddoc · 6,040 bytesreferences/glossary.mddoc · 1,049 bytesreferences/methodology.mddoc · 9,748 bytesreferences/testing-strategy.mddoc · 3,990 bytes
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# 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).
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## The Two Loops
