Devcontainer Setup skill

Devcontainer Setup Skill

Creates a pre-configured devcontainer with Claude Code and language-specific tooling.

When to Use

• User asks to "set up a devcontainer" or "add devcontainer support"
• User wants a sandboxed Claude Code development environment
• User needs isolated development environments with persistent configuration

When NOT to Use

• User already has a devcontainer configuration and just needs modifications
• User is asking about general Docker or container questions
• User wants to deploy production containers (this is for development only)

Workflow

flowchart TB
    start([User requests devcontainer])
    recon[1. Project Reconnaissance]
    detect[2. Detect Languages]
    generate[3. Generate Configuration]
    write[4. Write files to .devcontainer/]
    done([Done])

    start --> recon
    recon --> detect
    detect --> generate
    generate --> write
    write --> done

Phase 1: Project Reconnaissance

Infer Project Name

Check in order (use first match):

1. package.json → name field
2. pyproject.toml → project.name
3. Cargo.toml → package.name
4. go.mod → module path (last segment after /)
5. Directory name as fallback

Convert to slug: lowercase, replace spaces/underscores with hyphens.

Detect Language Stack

Language	Detection Files
Python	pyproject.toml, *.py
Node/TypeScript	package.json, tsconfig.json
Rust	Cargo.toml
Go	go.mod, go.sum

Multi-Language Projects

If multiple languages are detected, configure all of them in the following priority order:

1. Python - Primary language, uses Dockerfile for uv + Python installation
2. Node/TypeScript - Uses devcontainer feature
3. Rust - Uses devcontainer feature
4. Go - Uses devcontainer feature

For multi-language postCreateCommand, chain all setup commands:

uv run /opt/post_install.py && uv sync && npm ci

Extensions and settings from all detected languages should be merged into the configuration.

Phase 2: Generate Configuration

Start with base templates from resources/ directory. Substitute:

• {{PROJECT_NAME}} → Human-readable name (e.g., "My Project")
• {{PROJECT_SLUG}} → Slug for volumes (e.g., "my-project")

Then apply language-specific modifications below.

Base Template Features

The base template includes:

• Claude Code with marketplace plugins (anthropics/skills, trailofbits/skills, trailofbits/skills-curated)
• Sandboxing via bubblewrap and socat
• Python 3.13 via uv (fast binary download)
• Node 22 via fnm (Fast Node Manager)
• ast-grep for AST-based code search
• Network isolation tools (iptables, ipset) with NET_ADMIN capability
• Security mounts: .devcontainer/ mounted read-only to prevent container escape
• Token forwarding: CLAUDE_CODE_OAUTH_TOKEN and ANTHROPIC_API_KEY via remoteEnv
• Modern CLI tools: ripgrep, fd, fzf, tmux, git-delta

---

Language-Specific Sections

Python Projects

Detection: pyproject.toml, requirements.txt, setup.py, or *.py files

Dockerfile additions:

The base Dockerfile already includes Python 3.13 via uv. If a different version is required (detected from pyproject.toml), modify the Python installation:

# Install Python via uv (fast binary download, not source compilation)
RUN uv python install <version> --default

devcontainer.json extensions:

Add to customizations.vscode.extensions:

"ms-python.python",
"ms-python.vscode-pylance",
"charliermarsh.ruff"

Add to customizations.vscode.settings:

"python.defaultInterpreterPath": ".venv/bin/python",
"[python]": {
  "editor.defaultFormatter": "charliermarsh.ruff",
  "editor.codeActionsOnSave": {
    "source.organizeImports": "explicit"
  }
}

postCreateCommand: If pyproject.toml exists, chain commands:

rm -rf .venv && uv sync && uv run /opt/post_install.py

---

Node/TypeScript Projects

Detection: package.json or tsconfig.json

No Dockerfile additions needed: The base template includes Node 22 via fnm (Fast Node Manager).

devcontainer.json extensions:

Add to customizations.vscode.extensions:

"dbaeumer.vscode-eslint",
"esbenp.prettier-vscode"

Add to customizations.vscode.settings:

"editor.defaultFormatter": "esbenp.prettier-vscode",
"editor.codeActionsOnSave": {
  "source.fixAll.eslint": "explicit"
}

postCreateCommand: Detect package manager from lockfile and chain with base command:

• pnpm-lock.yaml → uv run /opt/post_install.py && pnpm install --frozen-lockfile
• yarn.lock → uv run /opt/post_install.py && yarn install --frozen-lockfile
• package-lock.json → uv run /opt/post_install.py && npm ci
• No lockfile → uv run /opt/post_install.py && npm install

---

Rust Projects

Detection: Cargo.toml

Features to add:

"ghcr.io/devcontainers/features/rust:1": {}

devcontainer.json extensions:

Add to customizations.vscode.extensions:

"rust-lang.rust-analyzer",
"tamasfe.even-better-toml"

Add to customizations.vscode.settings:

"[rust]": {
  "editor.defaultFormatter": "rust-lang.rust-analyzer"
}

postCreateCommand: If Cargo.lock exists, use locked builds:

uv run /opt/post_install.py && cargo build --locked

If no lockfile, use standard build:

uv run /opt/post_install.py && cargo build

---

Go Projects

Detection: go.mod

Features to add:

"ghcr.io/devcontainers/features/go:1": {
  "version": "latest"
}

devcontainer.json extensions:

Add to customizations.vscode.extensions:

"golang.go"

Add to customizations.vscode.settings:

"[go]": {
  "editor.defaultFormatter": "golang.go"
},
"go.useLanguageServer": true

postCreateCommand:

uv run /opt/post_install.py && go mod download

---

Reference Material

For additional guidance, see:

• references/dockerfile-best-practices.md - Layer optimization, multi-stage builds, architecture support
• references/features-vs-dockerfile.md - When to use devcontainer features vs custom Dockerfile

---

Adding Persistent Volumes

Pattern for new mounts in devcontainer.json:

"mounts": [
  "source={{PROJECT_SLUG}}-<purpose>-${devcontainerId},target=<container-path>,type=volume"
]

Common additions:

• source={{PROJECT_SLUG}}-cargo-${devcontainerId},target=/home/vscode/.cargo,type=volume (Rust)
• source={{PROJECT_SLUG}}-go-${devcontainerId},target=/home/vscode/go,type=volume (Go)

---

Output Files

Generate these files in the project's .devcontainer/ directory:

1. Dockerfile - Container build instructions
2. devcontainer.json - VS Code/devcontainer configuration
3. post_install.py - Post-creation setup script
4. .zshrc - Shell configuration
5. install.sh - CLI helper for managing the devcontainer (devc command)

---

Validation Checklist

Before presenting files to the user, verify:

1. All {{PROJECT_NAME}} placeholders are replaced with the human-readable name
2. All {{PROJECT_SLUG}} placeholders are replaced with the slugified name
3. JSON syntax is valid in devcontainer.json (no trailing commas, proper nesting)
4. Language-specific extensions are added for all detected languages
5. postCreateCommand includes all required setup commands (chained with &&)

---

User Instructions

After generating, inform the user:

1. How to start: "Open in VS Code and select 'Reopen in Container'"
2. Alternative: devcontainer up --workspace-folder .
3. CLI helper: Run .devcontainer/install.sh self-install to add the devc command to PATH