tdd-vertical-slice

name	tdd-vertical-slice
description	Build features or fixes one behavior-first vertical slice at a time using a red-green-refactor loop.

TDD Vertical Slice

Use this skill when implementing behavior that can be verified with tests or another repeatable feedback loop.

Native Integration

Use native file-editing, terminal, browser, run, verify, and test tools for the mechanics. This skill owns the implementation cadence: one observable behavior, one feedback signal, one minimal implementation, then cleanup.

Principles

• Test public behavior, not implementation details.
• Prefer one end-to-end or integration-style behavior test over many shallow implementation tests.
• Work in vertical slices: one behavior, one failing signal, one minimal implementation, one cleanup pass.
• Do not write all tests first and all implementation second.

Workflow

1. Find the feedback loop.

   • Use existing test commands when available.
   • If no test seam exists, create the smallest meaningful seam at the highest useful level.
   • For UI work, include browser or DOM verification when practical.

2. Pick one behavior.

   • Choose the smallest observable behavior that moves the feature forward.
   • Name the behavior in user or caller language.

3. Red.

   • Add one failing test or repeatable check for that behavior.
   • Run it and confirm it fails for the expected reason.

4. Green.

   • Add the smallest implementation that passes the check.
   • Run the focused check again.

5. Refactor.

   • Improve names, remove duplication, and align with local patterns.
   • Keep the feedback loop green after each meaningful cleanup.

6. Repeat.

   • Move to the next behavior only after the current slice is green.

Done When

• Every implemented behavior has a meaningful feedback signal.
• All relevant checks pass.
• The final code follows local patterns and avoids speculative features.