Set Up a Test-Driven Development Flow in VS Code

Summary

GitHub Copilot's agent mode in VS Code can enforce test-driven development discipline through custom agent configurations. The approach creates three specialized agents—one for each TDD phase—with explicit handoff rules that prevent the common pitfall of skipping straight to implementation.

The TDD Cycle with AI Agents

The traditional Red-Green-Refactor cycle gains structure when each phase has a dedicated agent with constrained permissions.

flowchart LR
    R[Red Agent
Write failing test] --> G[Green Agent
Minimal code to pass]
    G --> RF[Refactor Agent
Improve structure]
    RF -.->|Next feature| R

Key Concepts

• Red phase agent: Writes tests that initially fail, establishing requirements before any implementation exists
• Green phase agent: Creates minimal code to make tests pass—explicitly avoids over-engineering
• Refactor phase agent: Improves code quality while maintaining passing tests
• Handoff mechanism: Agents transfer control explicitly, preventing phase-skipping

Agent Configuration

Each agent gets a dedicated markdown file defining its behavior and available tools.

TDD Red Agent

# TDD-red.agent.md
"Write the minimal code change needed so that the test passes"

Tools: read, edit, search
Handoff: → TDD-green when test written

TDD Green Agent

# TDD-green.agent.md
Implements changes then runs tests automatically

Tools: read, edit, execute
Handoff: → TDD-refactor when tests pass

Testing Guidelines

The guide recommends establishing testing standards in a custom instructions file. Key patterns:

• Arrange-Act-Assert: Structure every test with clear setup, action, and verification sections
• Independent tests: Each test should run in isolation without depending on others
• Behavior focus: Test what the code does, not how it does it

Connections

• copilot-sdk - The programmatic interface for building Copilot integrations, which these custom agents build upon
• how-to-build-a-coding-agent - Explains the fundamental agent loop pattern that these TDD agents implement