Agent Builder

Agent Builder - Claude SDK & CLI Expert

Triggers:

• "build agent", "create agent", "agent sdk", "claude sdk"
• "headless mode", "programmatic", "cli command"
• "claude -p", "query programmatically"
• "custom tools", "mcp server", "sdk mcp"
• "claude code api", "agent options"

Prefer Claude Code Headless Mode

This skill covers Claude Code's headless mode and the Agent SDK — not the raw Anthropic API.

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Preferred: Claude Code CLI headless mode (claude -p)
Preferred: Claude Agent SDK (wraps Claude Code CLI)
Avoid:     Raw Anthropic API (anthropic.Anthropic())
Avoid:     Direct API calls with API keys

Why this matters:

• The Agent SDK wraps the Claude Code CLI — it does not use the raw Anthropic API
• Authentication comes from existing ~/.claude/ config — no API keys needed
• All settings, MCP servers, and configuration inherit from your Claude Code setup
• You get Claude Code's full agent loop, tools, and context management

Rule: If code uses anthropic.Anthropic(), anthropic.messages.create(), or requires ANTHROPIC_API_KEY, that is the wrong approach. Use claude -p or claude_agent_sdk instead.

Core Capabilities

1. Claude Agent SDK Python - Programmatic agent creation with custom tools and hooks (wraps Claude Code CLI)
2. Headless/Print Mode - Run Claude Code from CLI, scripts, CI/CD (claude -p)
3. CLI Reference - All commands, flags, and configuration options

All use Claude Code's existing authentication from ~/.claude/ — no API keys required.

Documentation Sources

• Agent SDK: https://github.com/anthropics/claude-agent-sdk-python
• Headless Mode: https://code.claude.com/docs/en/headless
• CLI Reference: https://code.claude.com/docs/en/cli-reference

Workflow

1. Understand the requirement - What type of agent/automation is needed?
2. Check dependencies - Verify Python version, packages, Claude Code installation
3. Reference live docs - Fetch relevant documentation if details are unclear
4. Build confidently - Write production-ready code with proper error handling
5. Test immediately - Run and validate the implementation

Reference Files

Load only the reference file relevant to the current task.

Security

• Do not use --dangerously-skip-permissions in production
• Validate inputs in custom tools
• Use permission rules to block destructive commands
• Set budget limits with --max-budget-usd
• Implement hooks for sensitive operations
• Review allowed tools before automation

Quick Decision Tree

Need to...

• Run one-off query → claude -p "query"
• Build custom agent → Agent SDK with ClaudeSDKClient
• Add custom tools → SDK MCP server with @tool decorator
• Control execution → Hooks with HookMatcher
• Stream responses → --output-format stream-json
• Get structured data → --json-schema
• Continue conversation → --continue or --resume
• Run in CI/CD → claude -p with --allowedTools
• Integrate with app → Python SDK with query() or ClaudeSDKClient

When to Fetch Docs

Fetch live documentation when:

• API signatures or parameters are unclear
• New features or flags are mentioned
• Error messages reference unknown configuration
• Implementing complex hooks or MCP servers
• User asks about specific capabilities

Use WebFetch tool to pull latest documentation from source URLs.

Implementation Checklist

Before writing code:

• Verify: Using Claude Code headless mode (claude -p or claude_agent_sdk) — not raw Anthropic API
• Verify: No API keys in code — authentication comes from ~/.claude/ config
• Confirm Python version (3.10+)
• Verify Claude Code CLI installation (which claude)
• Check required packages with uv pip list
• Understand the specific use case
• Determine if SDK or CLI approach is needed

After writing code:

• Verify: Code uses claude_agent_sdk or claude -p — not anthropic.Anthropic()
• Verify: No ANTHROPIC_API_KEY or api_key= parameters in code
• Add error handling for all SDK exceptions
• Test with actual Claude Code installation
• Validate tool permissions and security
• Set appropriate limits (turns, budget)
• Document custom tools and their purposes

Skill Invocation

This skill provides:

• Complete Agent SDK implementation guidance
• Headless/CLI command construction
• Custom tool creation with MCP servers
• Hook implementation for control flow
• Security best practices
• Live documentation fetching when needed

Ask specific questions about building agents, running headless commands, or CLI usage.

Limitations

• The Agent SDK is Python-only (Python 3.10+); no JavaScript or other language bindings are covered here.
• Requires Claude Code CLI installed and authenticated via ~/.claude/ before any SDK or headless usage works.
• This skill does not cover raw Anthropic API usage (anthropic.Anthropic()); see the Anthropic API docs for that.
• Agent processes launched via the SDK are ephemeral by default; session persistence requires explicit --resume or session ID capture.
• MCP server patterns in this skill use stdio transport; HTTP-based MCP transports require separate configuration not covered here.