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Build software with test-driven development and design discipline

Test-Driven Development workflow that writes behavior-focused tests through vertical slicing, one test-implementation cycle at a time.

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mattpocock
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78
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Updated 26 days ago
Version 1.0.1
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Why it matters

Engineers hire this skill collection to maintain code quality and architectural integrity while using AI coding agents, preventing the common failure modes of misalignment, verbosity, broken code, and architectural decay through structured workflows based on decades of software engineering best practices.

Outcomes

What it gets done

01

Align on requirements through grilling sessions before writing code

02

Build shared language documentation to reduce agent verbosity and improve consistency

03

Enforce red-green-refactor TDD loops to ensure code actually works

04

Maintain clean architecture through regular codebase improvement sessions

Install

Add it to your toolbox

Run in your project directory:

curl -fsSL https://spark.entire.vc/get/ag-tdd | bash

Overview

Test-Driven Development

A Test-Driven Development workflow from mattpocock/skills that enforces vertical slicing: one test, minimal implementation, repeat. Tests verify behavior through public interfaces, not implementation details, so they survive refactors and document what the system does. Use when building features where behavior verification matters and you want tests that act as specifications. Focus on critical paths and complex logic. Never write all tests first then all implementation (horizontal slicing) - this produces brittle tests coupled to imagined behavior rather than actual capabilities.

What it does

This workflow implements Test-Driven Development (TDD) through vertical slicing: writing one test, implementing minimal code to pass it, then repeating. Tests verify behavior through public interfaces rather than implementation details, ensuring they survive refactors and act as living specifications.

When to use - and when NOT to

Use this workflow when building new features or modifying existing code where behavior verification matters. Focus testing effort on critical paths and complex logic.

Do NOT use horizontal slicing (writing all tests first, then all implementation). This produces tests that verify imagined behavior and data structure shapes rather than actual user-facing capabilities. Avoid writing bulk tests before understanding the implementation - this creates brittle tests that fail on refactors but pass when behavior breaks.

Inputs and outputs

The workflow requires confirmation of interface changes and test priorities before writing code. It reads CONTEXT.md (if present) to match test names and interface vocabulary to the project's domain language and respect Architecture Decision Records (ADRs).

The workflow provides a planning phase, tracer bullet guidance, an incremental loop for each behavior, and refactor candidates after all tests pass. Each cycle includes a checklist ensuring tests describe behavior (not implementation), use public interfaces only, and would survive internal refactors.

Workflow example

The workflow enforces vertical slicing over horizontal:

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Each cycle follows:

RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes

Then for remaining behaviors:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Every cycle validates:

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added

Who it's for

Developers building features where correctness matters and tests should serve as specifications. The workflow is valuable when working in codebases with CONTEXT.md files and Architecture Decision Records (ADRs) that define domain language and design constraints.

Source README

Test-Driven Development

When to Use

Use when this workflow matches the user request: Use this skill for its documented workflow.

Source: mattpocock/skills (MIT).

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.

See tests.md for examples and mocking.md for mocking guidelines.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code."

This produces crap tests:

  • Tests written in bulk test imagined behavior, not actual behavior
  • You end up testing the shape of things (data structures, function signatures) rather than user-facing behavior
  • Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
  • You outrun your headlights, committing to test structure before understanding the implementation

Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Workflow

1. Planning

When exploring the codebase, read CONTEXT.md (if it exists) so that test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching.

Before writing any code:

  • Confirm with user what interface changes are needed
  • Confirm with user which behaviors to test (prioritize)
  • Identify opportunities for deep modules (small interface, deep implementation) - run the /codebase-design skill for the vocabulary and the testability checks
  • List the behaviors to test (not implementation steps)
  • Get user approval on the plan

Ask: "What should the public interface look like? Which behaviors are most important to test?"

You can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes

This is your tracer bullet - proves the path works end-to-end.

3. Incremental Loop

For each remaining behavior:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

  • One test at a time
  • Only enough code to pass current test
  • Don't anticipate future tests
  • Keep tests focused on observable behavior

4. Refactor

After all tests pass, look for refactor candidates:

  • Extract duplication
  • Deepen modules (move complexity behind simple interfaces)
  • Apply SOLID principles where natural
  • Consider what new code reveals about existing code
  • Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added

Limitations

  • Requires the upstream tool, account, API key, or local setup when the workflow names one.
  • Does not authorize destructive, production, paid, or external-message actions without explicit user approval.
  • Validate generated artifacts or recommendations against the user's real sources before treating them as final.

FAQ

Common questions

Discussion

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