DevContainer Setup

# 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

```mermaid 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)
• 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
• 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:

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

devcontainer.json extensions:

Add to `customizations.vscode.extensions`: ```json "ms-python.python", "ms-python.vscode-pylance", "charliermarsh.ruff" ```

Add to `customizations.vscode.settings`: ```json "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`: ```json "dbaeumer.vscode-eslint", "esbenp.prettier-vscode" ```

Add to `customizations.vscode.settings`: ```json "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:

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

devcontainer.json extensions:

Add to `customizations.vscode.extensions`: ```json "rust-lang.rust-analyzer", "tamasfe.even-better-toml" ```

Add to `customizations.vscode.settings`: ```json "[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:

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

devcontainer.json extensions:

Add to `customizations.vscode.extensions`: ```json "golang.go" ```

Add to `customizations.vscode.settings`: ```json "[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`:

```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