Agent Sandboxes

Agent Sandboxes

This skill provides access to E2B sandboxes through a streamlined CLI for safe code execution and file operations in isolated environments.

Variables

• E2B_API_KEY: The environment variable containing the E2B API key (stored in the environment file)
• SANDBOX_CLI_PATH: .claude/skills/agent-sandboxes/sandbox_cli/
• ENVIRONMENT_FILE_PATH: ../../../../.env
• TIMEOUT_DURATION_IN_SECONDS: 43200 (12 hours)

Prerequisites

Before using sandbox operations, validate the environment:

1. Check for E2B_API_KEY:

   # Verify the API key is set
   grep "E2B_API_KEY" `ENVIRONMENT_FILE_PATH`
   

   If missing, instruct the user:

   Error: E2B_API_KEY not found in .env file
   
   Please add your E2B API key to the .env file in the project root:
   echo "E2B_API_KEY=your_api_key_here" >> .env
   
   Get your API key from: https://e2b.dev/docs
   

2. Verify CLI is available: The sandbox CLI is located at SANDBOX_CLI_PATH The .env file is automatically loaded from the project root.

Instructions

• IMPORTANT: Don't create files locally, always use the sandbox. Only do this if the user explicitly asks you to do so.
• ALWAYS USE --timeout TIMEOUT_DURATION_IN_SECONDS
• CAPTURE AND REMEMBER SANDBOX ID - Store it in your context, don't use shell variables or files
• Multi-agent safe - Each agent tracks its own sandbox ID independently
• Always validate E2B_API_KEY first - Don't proceed without it
• Change directory to SANDBOX_CLI_PATH - All commands must be run from there
• Use --shell for complex commands - Enables pipes, redirections, wildcards
• Use --cwd instead of cd - More reliable for working directory changes
• Binary files - Use upload/download for images, PDFs, executables
• Never delete the sandbox unless you're explicitly asked to do so - Sandboxes auto-timeout after TIMEOUT_DURATION_IN_SECONDS

Template Tiers

Pre-built templates with different resource levels. Use --template flag with sbx init:

Build new templates: uv run build_template.py --tier <tier> or --list-tiers to see options.

When to Use Sandboxes

Use agent sandboxes when the user needs to:

• Run untrusted or experimental code safely
• Test packages or dependencies without affecting the local system
• Execute system commands in an isolated environment
• Work with binary files (images, PDFs, executables)
• Clone and test repositories in isolation
• Install and test different package versions
• Run long-running processes without blocking

CLI Overview

The sandbox CLI has four core command groups:

1. sbx init - Quick sandbox initialization
2. sbx sandbox - Lifecycle management (create, connect, kill, pause, extend-lifetime, info)
3. sbx files - File operations (ls, read, write, upload, download, rm, mkdir, mv)
4. sbx exec - Unified command execution (the most powerful command)
5. sbx browser - Browser automation for UI validation (visual testing)

Get CLI Help: Use sbx --help to see all available commands and options:

cd SANDBOX_CLI_PATH
uv run sbx --help           # Main help
uv run sbx init --help      # Help for init command
uv run sbx sandbox --help   # Help for sandbox commands
uv run sbx files --help     # Help for file operations
uv run sbx exec --help      # Help for exec command

Key Command: sbx exec

The exec command is the primary interface for running commands in sandboxes:

uv run sbx exec $SANDBOX_ID "command" [options]

Options:
  --cwd PATH          Working directory
  --env KEY=VALUE     Environment variables (multiple allowed)
  --root              Run as root user
  --shell             Enable shell features (pipes, redirections, wildcards)
  --timeout SECONDS   Command timeout (default: 60)
  --background        Run command in background

Important: All commands must be run from SANDBOX_CLI_PATH directory.

Key Command: sbx browser

Browser automation for visual validation of sandbox applications using Playwright's isolated Chromium.

Commands (all support --port PORT for parallel agents):

sbx browser init                        # First-time setup (once per machine)
sbx browser start [--headed]            # Start browser (--headed shows window)
sbx browser nav <url>                   # Navigate to URL
sbx browser eval <code>                 # Run JavaScript, returns result
sbx browser screenshot [--path P] [--full]  # Screenshot (--full for entire page)
sbx browser click <selector>            # Click element by CSS selector
sbx browser type <selector> <text>      # Type text into input field
sbx browser scroll <direction>          # up | down | top | bottom
sbx browser a11y                        # Get accessibility tree (JSON)
sbx browser dom [--full]                # Get DOM (--full for raw HTML)
sbx browser status                      # Check if browser is running
sbx browser close                       # Close browser and kill process

For parallel agents: Use --port flag with unique ports (9222-9999).

Multi-Agent Considerations

CRITICAL: Multiple agents may be running sandboxes simultaneously. Each agent MUST:

1. Always use --timeout TIMEOUT_DURATION_IN_SECONDS when initializing (unless the user specifies a different timeout)
2. Capture the sandbox ID from sbx init output and remember it in your context
3. DO NOT use shell variables like export SANDBOX_ID=... (conflicts with other agents)
4. DO NOT rely on .sandbox_id file (gets overwritten by other agents)
5. Track the sandbox ID yourself and use it directly in all subsequent commands
6. Report the sandbox ID to the user when done

Example of proper ID handling:

# When you run this:
uv run sbx init

# Capture the sandbox ID from output (e.g., "sbx_abc123def456")
# Store it in YOUR context/memory as: sandbox_id = "sbx_abc123def456"

# Then use it directly in all commands:
uv run sbx exec sbx_abc123def456 "python --version"
uv run sbx files write sbx_abc123def456 /home/user/test.py "print('hello')"

Cookbook

Extended documentation for specific features. Read these when you need to use the feature.

Workflow

Step 1: Validate Environment

Always start by checking for the E2B_API_KEY:

cd SANDBOX_CLI_PATH
grep "E2B_API_KEY" `ENVIRONMENT_FILE_PATH`

If not found, stop and request the user to add their API key.

Step 2: Initialize Sandbox and Capture ID

Create a new sandbox and capture the sandbox ID from the output:

cd SANDBOX_CLI_PATH
uv run sbx init --timeout TIMEOUT_DURATION_IN_SECONDS

CRITICAL:

• Always use --timeout TIMEOUT_DURATION_IN_SECONDS
• The command will output a sandbox ID (e.g., sbx_abc123def456)
• Capture this ID and remember it in your context
• DO NOT use environment variables or files to store it
• Use this exact ID in all subsequent commands

Additional options (optional):

• --template NAME - Use a specific template (default: base)
• --env KEY=VALUE - Set environment variables

Example:

uv run sbx init --timeout TIMEOUT_DURATION_IN_SECONDS
# Output: Created sandbox: sbx_abc123def456
# YOU remember: sandbox_id = "sbx_abc123def456"

Step 3: Perform Operations

Use the appropriate command based on the task, using the sandbox ID you captured:

Run commands (replace <sandbox_id> with your captured ID):

uv run sbx exec <sandbox_id> "python --version"
uv run sbx exec <sandbox_id> "pip list" --cwd /home/user/project

File operations:

uv run sbx files write <sandbox_id> /home/user/script.py "print('hello')"
uv run sbx files read <sandbox_id> /home/user/output.txt
uv run sbx files upload <sandbox_id> ./local.png /home/user/image.png
uv run sbx files edit <sandbox_id> /home/user/config.ts --old "old text" --new "new text"
uv run sbx files edit <sandbox_id> /home/user/config.ts --old "find" --new "replace" --all

Download directories (clone sandbox code locally):

# Download directory (excludes .venv, node_modules, .git, __pycache__ by default)
uv run sbx files download-dir <sandbox_id> /home/user/project ./local/project

# Include everything (no exclusions)
uv run sbx files download-dir <sandbox_id> /home/user/project ./local/project --all

IMPORTANT - Writing Files with Special Characters: The sbx files write command has limitations with special characters like brackets [] when passing content as an argument due to shell glob expansion. Use one of these workarounds:

Option 1: Use --stdin flag (recommended):

# Pipe content through stdin to avoid shell escaping issues
echo 'const arr = [1, 2, 3]; const val = obj["key"];' | uv run sbx files write <sandbox_id> /home/user/file.js --stdin

# Or with heredoc
uv run sbx files write <sandbox_id> /home/user/file.js --stdin << 'EOF'
const arr = [1, 2, 3];
const value = obj["key"];
EOF

Option 2: Write locally, then upload:

# Write file locally first (using Write tool)
# Then upload to sandbox - bypasses shell escaping entirely
uv run sbx files upload <sandbox_id> ./local/file.js /home/user/file.js

Option 3: Use sbx exec with heredoc or Python:

# Use cat heredoc for complex content
uv run sbx exec <sandbox_id> "cat > /home/user/file.js << 'EOF'
const arr = [1, 2, 3];
const value = obj['key'];
EOF
" --shell

# Or use Python to write files
uv run sbx exec <sandbox_id> "python3 << 'EOF'
with open('/home/user/file.js', 'w') as f:
    f.write('''const arr = [1, 2, 3];
const value = obj[\"key\"];''')
EOF
" --shell

Install packages:

# Install uv package manager
uv run sbx exec <sandbox_id> "curl -LsSf https://astral.sh/uv/install.sh | sh" --shell --timeout 120

# Install Python packages
uv run sbx exec <sandbox_id> "/home/user/.local/bin/uv pip install --system requests"

Step 4: Expose Frontend (If Applicable)

When building applications with a frontend/UI, create a lightweight server to host your work and expose it:

4.1: Start the Server

Always default to port 5173 and ensure your frontend is configured to use this port:

# For Python/Flask
uv run sbx exec <sandbox_id> "python -m http.server 5173" --background --cwd /home/user/project

# For Node/React (Vite)
uv run sbx exec <sandbox_id> "npm run dev -- --port 5173" --background --cwd /home/user/project

# For static HTML/CSS/JS
uv run sbx exec <sandbox_id> "python -m http.server 5173" --background --cwd /home/user/dist

# For a custom Python server
uv run sbx files write <sandbox_id> /home/user/server.py "from flask import Flask; app = Flask(__name__); app.run(host='0.0.0.0', port=5173)"
uv run sbx exec <sandbox_id> "python /home/user/server.py" --background

Key points:

• Use --background flag to keep server running
• Port 5173 is the default - use this unless you have a specific reason not to
• Ensure your frontend code is configured for the same port

4.2: Get the Exposed URL

CRITICAL: Always use the get-host command to retrieve the actual public URL. Do NOT try to construct or infer the URL.

uv run sbx sandbox get-host <sandbox_id> --port 5173

This command returns the authoritative public URL (format: https://5173-<sandbox_id>.e2b.app).

Example:

uv run sbx sandbox get-host sbx_abc123def456 --port 5173
# Output: https://5173-sbx_abc123def456.e2b.app
# YOU capture and remember this URL in your context

Important:

• Always use --port 5173 to match your server port
• Capture the URL in your context/memory (not shell variables)
• Use the exact URL returned by this command
• Do NOT construct URLs manually (they will fail)

4.3: Verify It's Working

Get the URL using get-host and test it:

# Get the URL (captures output: https://5173-<sandbox_id>.e2b.app)
uv run sbx sandbox get-host <sandbox_id> --port 5173

# YOU remember the URL in your context, then test it
curl https://5173-<sandbox_id>.e2b.app

Note: Capture the URL from get-host output and remember it in your context. Use the exact URL in subsequent commands.

Important:

• The server must listen on 0.0.0.0 (not localhost or 127.0.0.1)
• Port must match between server and frontend configuration
• The sandbox will remain alive until TIMEOUT_DURATION_IN_SECONDS

Pausing, Resuming, and Extending Sandboxes

Pause a sandbox (beta feature):

uv run sbx sandbox pause <sandbox_id>

Resume a paused sandbox (use connect):

uv run sbx sandbox connect <sandbox_id>

Extend sandbox lifetime (adds time to remaining):

uv run sbx sandbox extend-lifetime <sandbox_id> <seconds_to_add>

Examples:

# Add 1 hour to remaining time
uv run sbx sandbox extend-lifetime <sandbox_id> 3600

# Add 3 hours
uv run sbx sandbox extend-lifetime <sandbox_id> 10800

# Add 12 hours
uv run sbx sandbox extend-lifetime <sandbox_id> 43200

The extend-lifetime command adds the specified seconds to the remaining lifetime. If a sandbox has 30m left and you add 1h, it will have 1h 30m remaining.

Step 5: Report Results

Report the results to the user with the sandbox information:

5.1: Get the Sandbox URL (if frontend/web app)

If you built a frontend or web application, use get-host to retrieve the public URL:

uv run sbx sandbox get-host <sandbox_id> --port 5173

This returns the actual URL (e.g., https://5173-<sandbox_id>.e2b.app).

Do NOT construct the URL manually - always use the get-host command.

5.2: Report to User

Provide the user with:

1. The sandbox ID - So they can reference it if needed
2. The URL (if applicable) - So they can access the application
3. Timeout information - Let them know the sandbox timeout (TIMEOUT_DURATION_IN_SECONDS)

Example report:

✓ Sandbox created successfully!

Sandbox ID: sbx_abc123def456
Application URL: [Use: uv run sbx sandbox get-host sbx_abc123def456 --port 5173]

Your sandbox will automatically terminate after TIMEOUT_DURATION_IN_SECONDS.

Note: Always get the actual URL using sbx sandbox get-host <sandbox_id> --port 5173 - never construct it manually.

IMPORTANT:

• Never delete the sandbox unless you're explicitly asked to do so
• Sandboxes will automatically timeout after TIMEOUT_DURATION_IN_SECONDS

Examples

Progressive Disclosure: Read only the example you need for your specific task.

Example 1: Run Python Code Safely

Read when: User needs to run/test Python code in isolation. See: examples/01_run_python_code.md

Covers: Basic sandbox workflow, writing scripts, executing Python code, capturing sandbox ID.

Example 2: Test a Package

Read when: User needs to install and test Python packages. See: examples/02_test_package.md

Covers: Installing uv package manager, installing Python packages, testing package functionality.

Example 3: Clone and Test Repository

Read when: User needs to clone a GitHub repo and run tests or commands in it. See: examples/03_clone_and_test_repo.md

Covers: Git operations, using --cwd flag, running commands in repository context, longer timeouts.

Example 4: Process Binary Files

Read when: User needs to upload, process, or download binary files (images, PDFs). See: examples/04_process_binary_files.md

Covers: Binary file upload/download, image processing, using appropriate file operations for binary vs text.

Example 5: Host Frontend Application

Read when: User wants to build a web app, UI, dashboard, or any frontend accessible via browser. See: examples/05_host_frontend.md

Covers: Exposing frontends, using port 5173, starting servers in background, getting public URLs, keeping sandboxes alive.

Reference

Built-in CLI Help:

cd SANDBOX_CLI_PATH
uv run sbx --help       # Overview of all commands
uv run sbx <command> --help  # Detailed help for specific command

For complete command reference and advanced usage, see:

• CLI Help: Run uv run sbx --help for interactive command reference
• Full documentation: SANDBOX_CLI_PATH/README.md
• Architecture details: Command groups, modules, and design principles
• Advanced examples: Multi-step workflows, git operations, package management

Troubleshooting

"Need help with a command":

• Run uv run sbx --help for all available commands
• Run uv run sbx <command> --help for specific command details
• All commands have built-in help documentation

"E2B_API_KEY not found":

• Check .env file exists in project root
• Verify key is set: grep E2B_API_KEY .env
• Add key if missing: echo "E2B_API_KEY=key" >> .env

"Command not found: sbx":

• Ensure you're in the sandbox_cli directory
• Run with uv run sbx instead of just sbx

"Sandbox timeout":

• Increase timeout: sbx init --timeout 900
• Use --timeout flag on long-running exec commands

"Permission denied":

• Use --root flag for system operations
• Check file paths are in /home/user/ directory

"Forgot command syntax":

• Use uv run sbx --help to see command structure
• Each command group has detailed help with examples

Browser issues: See cookbook/browser.md for troubleshooting.