Dev Environment Skill

"Works on my machine" is a solved problem. Consistent environments across the team eliminate entire classes of bugs and onboarding friction.

When to Activate

• Setting up a new project's development environment (/setup-dev)
• Onboarding a new developer
• Debugging "it works on CI but not locally" problems
• Standardizing the team's Node/Python/Go version
• Setting up pre-commit hooks for automatic quality checks
• Configuring a Dev Container so the project opens identically in VS Code and GitHub Codespaces
• Running local email testing with Mailpit so email flows can be verified without a real SMTP server

---

Runtime Version Pinning (mise)

mise (formerly rtx) manages Node, Python, Go, Java, and 200+ other runtimes per-project:

# .mise.toml — commit this to git
[tools]
node = "22"
python = "3.12"
go = "1.24"

[env]
NODE_ENV = "development"

# Install mise
curl https://mise.run | sh

# Install all tools defined in .mise.toml
mise install

# Activate (add to shell profile)
mise activate zsh >> ~/.zshrc

Everyone on the team runs the same versions. CI should use the same .mise.toml:

# .github/workflows/ci.yml
- uses: jdx/mise-action@v2  # Reads .mise.toml automatically

---

Environment Variables (direnv)

direnv automatically loads .env when you cd into a project directory:

# Install
brew install direnv
echo 'eval "$(direnv hook zsh)"' >> ~/.zshrc

# Project setup
echo 'dotenv' > .envrc      # Load .env automatically
direnv allow                 # Approve this project

.env is gitignored. .env.example is committed with all variables documented.

# .env (local, gitignored)
DATABASE_URL=postgresql://app:app@localhost:5432/appdb
REDIS_URL=redis://localhost:6379

# .envrc (committed — just loads .env)
dotenv

---

Dev Container (VS Code / GitHub Codespaces)

Dev Containers give every developer (and CI) the exact same environment in Docker:

// .devcontainer/devcontainer.json
{
  "name": "myapp",
  "dockerComposeFile": ["../docker-compose.dev.yml", "docker-compose.devcontainer.yml"],
  "service": "app",
  "workspaceFolder": "/workspace",

  "features": {
    "ghcr.io/devcontainers/features/git:1": {},
    "ghcr.io/devcontainers/features/github-cli:1": {}
  },

  "postCreateCommand": "npm install && npm run db:migrate",
  "postStartCommand": "docker compose up -d postgres redis",

  "customizations": {
    "vscode": {
      "settings": {
        "editor.formatOnSave": true,
        "editor.defaultFormatter": "biomejs.biome"
      },
      "extensions": [
        "biomejs.biome",
        "prisma.prisma",
        "bradlc.vscode-tailwindcss"
      ]
    }
  }
}

# .devcontainer/docker-compose.devcontainer.yml
services:
  app:
    build:
      context: ..
      dockerfile: .devcontainer/Dockerfile
    volumes:
      - ..:/workspace:cached
    command: sleep infinity
    environment:
      DATABASE_URL: postgresql://app:app@postgres:5432/appdb
      REDIS_URL: redis://redis:6379

---

Docker Compose for Local Services

Run only infrastructure services locally — run the app code natively (better hot reload, debugger):

# docker-compose.dev.yml
services:
  postgres:
    image: postgres:18-alpine
    environment: { POSTGRES_DB: appdb, POSTGRES_USER: app, POSTGRES_PASSWORD: app }
    ports: ['5432:5432']
    volumes: [postgres_data:/var/lib/postgresql/data]
    healthcheck:
      test: [CMD, pg_isready, -U, app]
      interval: 5s
      retries: 5

  redis:
    image: redis:8-alpine
    ports: ['6379:6379']

  mailpit:
    image: axllent/mailpit
    ports: ['1025:1025', '8025:8025']  # SMTP + Web UI for local email testing
    # Test emails visible at http://localhost:8025

volumes:
  postgres_data:

# Start services
docker compose -f docker-compose.dev.yml up -d

# Stop (keeps data)
docker compose -f docker-compose.dev.yml stop

# Reset data
docker compose -f docker-compose.dev.yml down -v && docker compose -f docker-compose.dev.yml up -d

---

Pre-Commit Hooks (automatic quality gates)

These already exist via hooks.json — document the key ones:

Hook	Trigger	What It Does
Post-edit format	Edit any TS/JS file	Runs Biome/Prettier
Post-edit typecheck	Edit any .ts file	Runs tsc --noEmit
Console.log warning	Edit any file	Warns if console.log added
Pre-push reminder	git push	Prompts to review changes

For teams without Claude Code, use pre-commit:

# .pre-commit-config.yaml
repos:
  - repo: local
    hooks:
      - id: lint
        name: Lint
        entry: npm run lint
        language: system
        pass_filenames: false

      - id: typecheck
        name: Type Check
        entry: npm run typecheck
        language: system
        pass_filenames: false

---

Checklist

• .mise.toml committed with pinned runtime versions
• .env.example committed with all variables documented
• .env is in .gitignore
• docker-compose.dev.yml for local services
• README Getting Started section < 5 steps to running locally
• Local email testing (Mailpit) for features that send email
• Dev Container (optional but valuable for onboarding and Codespaces)