# TDD London Chicago

Apply London (mock-based) and Chicago (state-based) TDD schools.

- Repository: https://github.com/proffesor-for-testing/agentic-qe (521 stars)
- Category: Testing
- License: MIT
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add proffesor-for-testing/agentic-qe --skill tdd-london-chicago -g -a claude-code -y`
Codex: `npx skills add proffesor-for-testing/agentic-qe --skill tdd-london-chicago -g -a codex -y`
Cursor: `npx skills add proffesor-for-testing/agentic-qe --skill tdd-london-chicago -g -a cursor -y`
Gemini CLI: `npx skills add proffesor-for-testing/agentic-qe --skill tdd-london-chicago -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add proffesor-for-testing/agentic-qe`

## Description

Apply London (mock-based) and Chicago (state-based) TDD schools. Use when practicing test-driven development or choosing testing style for your context.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Test-Driven Development: London & Chicago Schools

<default_to_action>
When implementing TDD or choosing testing style:
1. IDENTIFY code type: domain logic → Chicago, external deps → London
2. WRITE failing test first (Red phase)
3. IMPLEMENT minimal code to pass (Green phase)
4. REFACTOR while keeping tests green (Refactor phase)
5. REPEAT cycle for next functionality

**Quick Style Selection:**
- Pure functions/calculations → Chicago (real objects, state verification)
- Controllers/services with deps → London (mocks, interaction verification)
- Value objects → Chicago (test final state)
- API integrations → London (mock external services)
- Mix both in practice (London for controllers, Chicago for domain)

**Critical Success Factors:**
- Tests drive design, not just verify it
- Make tests fail first to ensure they test something
- Write minimal code - no features beyond what's tested
</default_to_action>

## Quick Reference Card

### When to Use
- Starting new feature with test-first approach
- Refactoring legacy code with test coverage
- Teaching TDD practices to team
- Choosing between mocking vs real objects

### TDD Cycle
| Phase | Action | Discipline |
|-------|--------|------------|
| **Red** | Write failing test | Verify it fails, check message is clear |
| **Green** | Minimal code to pass | No extra features, don't refactor |
| **Refactor** | Improve structure | Keep tests passing, no new functionality |

### School Comparison
| Aspect | Chicago (Classicist) | London (Mockist) |
|--------|---------------------|------------------|
| Collaborators | Real objects | Mocks/stubs |
| Verification | State (assert outcomes) | Interaction (assert calls) |
| Isolation | Lower (integrated) | Higher (unit only) |
| Refactoring | Easier | Harder (mocks break) |
| Design feedback | Emerges from use | Explicit from start |

The rest: https://github.com/proffesor-for-testing/agentic-qe/blob/HEAD/.claude/skills/tdd-london-chicago/SKILL.md