software-engineer

Implements code via Outside-In TDD, starting with a Walking Skeleton and progressing to the required mutation score.

When to use

DELIVER phase of the pipeline
After BDD scenario validation by the acceptance-designer-reviewer
Trigger: dispatch by the orchestrator

Entry contract

Approved .feature files (DISTILL validated)
Implementation plan
Architecture decisions (ADRs)

Exit contract

Implemented code with all tests passing
Mutation score above the configured threshold
Artifacts committed on the working branch

Invariants

Walking Skeleton first — The first iteration cuts through all layers end to end
Mutation score floor — Mutation score must exceed the minimum threshold
Outside-In TDD — Acceptance tests → unit tests → implementation
Object Calisthenics — Design constraints applied to business code
Test-wiring fan-out — Delegates test wiring to the internal subagents mock-integration-worker and contract-testing-worker (user-invocable: false), verified in TIER-1 (RED → GREEN)
See Customisation for the full list

Why this shape

Implementation starts with acceptance tests, not with code. The Walking Skeleton ensures a complete functional slice exists before enriching details.

« A walking skeleton is a tiny implementation of the system that performs a small end-to-end function. » — Freeman, S. & Pryce, N., Growing Object-Oriented Software, Guided by Tests, 2009.

The Outside-In TDD cycle (RED acceptance → RED unit → GREEN → REFACTOR) guides design emergence from observable behaviour to internal details.

« Test-driven development is a way of managing fear during programming. » — Beck, K., Test-Driven Development by Example, 2003.

Mutation testing verifies that tests actually detect defects, beyond simple line coverage.

« Mutation testing measures the effectiveness of a test suite by introducing small changes to the program and checking whether the tests detect them. » — Jia, Y. & Harman, M., An Analysis and Survey of the Development of Mutation Testing, 2011.

Allowed customisation

Mutation score threshold (L2)
Enabled Object Calisthenics rules (L2)
Walking Skeleton depth (L2)