Claude Agent Sdk Builder

Claude Agent SDK Builder

Create powerful agents using the Claude Agent SDK with custom tools, programmatic subagents, and production-ready patterns.

Overview

This skill provides comprehensive guidance for building agents with the Claude Agent SDK (TypeScript/Node.js). Use this skill when:

• Creating new SDK-based agents
• Adding custom tools to extend agent capabilities
• Configuring subagents programmatically (preferred over markdown)
• Building production agent applications with WebSocket streaming
• Implementing hooks for safety and permissions
• Managing multi-turn conversations and sessions

Prerequisites

To use this skill and build Claude Agent SDK applications, ensure the following are installed:

Required:

• Node.js 18+ or Bun runtime - For running TypeScript/JavaScript code
• npm or bun - Package manager for installing dependencies
• Git - For version control and using init-agent-project.sh script

Optional but Recommended:

• TypeScript - For type checking and better development experience
• Anthropic API Key - Required to run agents (get from console.anthropic.com)

Verify Installation:

node --version  # Should be 18.0.0 or higher
npm --version
git --version

Or with Bun:

bun --version

Helper Scripts

This skill includes powerful scripts to automate common tasks:

init-agent-project.sh - Project Scaffolding

Quickly create a new agent project from templates:

./scripts/init-agent-project.sh

The script will:

• Prompt for project name
• Let you choose a template (basic-agent, custom-tools, in-code-subagent, full-app)
• Copy template files
• Create .env and .gitignore
• Initialize git repository
• Install dependencies
• Create workspace directory

generate-tool.ts - Custom Tool Generator

Generate boilerplate for new custom tools:

bun run scripts/generate-tool.ts

Interactive prompts for:

• Tool name and description
• Parameters with types and descriptions
• Generates complete TypeScript code with Zod schemas

generate-subagent.ts - Subagent Config Generator

Generate in-code subagent configurations:

bun run scripts/generate-subagent.ts

Creates:

• Factory function or class-based subagent
• Proper TypeScript types
• System prompt template
• Tool selection
• Usage examples

validate-agent-config.ts - Configuration Validator

Validate agent configuration before running:

bun run scripts/validate-agent-config.ts config.ts

Checks for:

• Tool name typos and case sensitivity
• Missing MCP servers
• Invalid tool references
• Subagent configuration errors
• Common mistakes

add-hooks.ts - Hooks Generator

Generate common hook patterns:

bun run scripts/add-hooks.ts

Available templates:

• File path validation
• Command allowlist
• File type restrictions
• Rate limiting
• Logging hooks
• Environment-based restrictions

Quick Start Workflow

Step 1: Choose Your Starting Point

Select the appropriate template based on your needs:

1. basic-agent - Simple one-shot agent with basic message handling
2. custom-tools - Agent with custom MCP tools
3. in-code-subagent - Agent with programmatic subagent configuration
4. full-app - Production-ready app with WebSocket, sessions, and persistence

Access templates in assets/templates/.

Step 2: Core SDK Concepts

Before building, understand these core concepts:

Query Function

import { query } from "@anthropic-ai/claude-agent-sdk";

for await (const message of query({ prompt, options })) {
  // Handle messages
}

Message Types

• system - Init, context updates
• user - User messages
• assistant - AI responses and tool calls
• result - Final success/failure with cost

Multi-Turn Conversations

• Capture session_id from system init message
• Use resume: sessionId option for subsequent turns

Step 3: Implementation Patterns

Pattern A: Basic Agent

Copy from assets/templates/basic-agent/:

import { query } from "@anthropic-ai/claude-agent-sdk";

for await (const message of query({
  prompt: "Your task",
  options: {
    maxTurns: 20,
    model: "sonnet",
    allowedTools: ["Read", "Write", "Edit"]
  }
})) {
  if (message.type === 'assistant') {
    console.log(message.message.content);
  }
}

Pattern B: Agent with Custom Tools

Copy from assets/templates/custom-tools/:

import { tool, createSdkMcpServer } from "@anthropic-ai/claude-agent-sdk";
import { z } from "zod";

const myServer = createSdkMcpServer({
  name: "mytools",
  version: "1.0.0",
  tools: [
    tool("tool_name", "Description", {
      param: z.string().describe("Parameter description")
    }, async (args) => ({
      content: [{ type: "text", text: "Result" }]
    }))
  ]
});

// Use in query
options: {
  mcpServers: { "mytools": myServer },
  allowedTools: ["mcp__mytools__tool_name"]
}

Pattern C: Agent with In-Code Subagents (Preferred)

Copy from assets/templates/in-code-subagent/:

// Define subagents programmatically
function createSearchSubagent() {
  return {
    name: "searcher",
    description: "Finds and analyzes files",
    tools: ["Read", "Grep", "Glob"],
    systemPrompt: "Search specialist instructions...",
    maxTurns: 10
  };
}

// Register with main agent
options: {
  subagents: [createSearchSubagent()],
  allowedTools: ["Task"]  // Allow spawning subagents
}

Why in-code subagents?

• Type safety - catch errors at compile time
• Dynamic configuration - adjust at runtime
• Better testing - unit test configurations
• Code reuse - share configuration logic

When to use markdown subagents?

• Only when user explicitly requests markdown-based configuration
• For non-technical users who need to modify agents

Pattern D: Production App

Copy from assets/templates/full-app/:

Complete application with:

• WebSocket server for real-time streaming
• Session management for multi-turn conversations
• Database persistence
• Error handling and cleanup
• Built-in web UI

See assets/templates/full-app/README.md for details.

Creating Custom Tools

Custom tools extend agent capabilities with domain-specific functionality. Use tool() with Zod schemas to define parameters, then bundle into MCP servers with createSdkMcpServer().

Key Points:

• Tool naming format: mcp__<server-name>__<tool-name>
• Define parameters with Zod for type safety
• Return format: { content: [{ type: "text", text: "..." }] }

Quick Example:

const myTool = tool("tool_name", "Description", {
  param: z.string()
}, async (args) => ({
  content: [{ type: "text", text: "Result" }]
}));

Generator: Use bun run scripts/generate-tool.ts to create tool boilerplate.

Full Guide: See references/custom-tools-guide.md for:

• Complete tool creation examples
• MCP server configuration
• Real-world patterns (email, database, API integration)
• Error handling and testing

Configuring In-Code Subagents

Preferred Approach: Define subagents programmatically in code rather than markdown files for better type safety, testing, and dynamic configuration.

Key Patterns:

• Factory functions - Reusable config generators
• Class-based - For complex subagents with state
• Dynamic - Adjust behavior based on runtime conditions

Spawning: Main agent uses Task tool with subagent_type parameter.

Benefits over Markdown:

• Type safety (catch errors at compile time)
• Dynamic configuration (adjust at runtime)
• Better testing and code reuse

Generator: Use bun run scripts/generate-subagent.ts for boilerplate.

Full Guide: See references/in-code-subagents.md for:

• Factory and class-based patterns
• Spawning and orchestration
• Multi-level subagents
• Context passing and error handling

Implementing Safety Hooks

Hooks intercept tool execution for validation, permissions, and safety. Use PreToolUse hooks to block unsafe operations before they execute.

Common Use Cases:

• File path validation (restrict to specific directories)
• Command allowlisting (only safe bash commands)
• Environment-based restrictions (dev vs prod)
• Rate limiting and logging

Hook Return:

• Allow: { continue: true }
• Block: { decision: 'block', stopReason: '...', continue: false }

Generator: Use bun run scripts/add-hooks.ts for common patterns.

Full Guide: See references/hooks-guide.md for:

• Complete hook examples
• Real-world patterns (workspace safety, file types, rate limiting)
• Testing hooks
• PostToolUse hooks

Managing Sessions

Multi-turn conversations require capturing and resuming session state. Capture session_id from system init messages and use resume option for subsequent turns.

Basic Pattern:

1. Capture session_id from first system message
2. Use resume: sessionId option for follow-up queries
3. Session persists conversation context

Production Patterns:

• Session classes for encapsulation
• WebSocket streaming for real-time updates
• Database persistence for session data
• Cleanup strategies for inactive sessions

Full Guide: See references/session-patterns.md for:

• Multi-turn conversation patterns
• Streaming to WebSocket, console, or React
• SessionManager class for production
• Error handling and retry patterns

Reference Documentation

Detailed guides available in references/:

• sdk-api-reference.md - Complete API reference for query(), options, message types

  • Search: grep "query()" references/sdk-api-reference.md for query function details
  • Search: grep "tool()" references/sdk-api-reference.md for custom tool creation
  • Search: grep "ClaudeAgentOptions" references/sdk-api-reference.md for configuration options
  • Search: grep "Multi-Turn" references/sdk-api-reference.md for session management

• custom-tools-guide.md - Creating tools, MCP servers, tool patterns

  • Search: grep "createSdkMcpServer" references/custom-tools-guide.md for MCP server creation
  • Search: grep "Zod" references/custom-tools-guide.md for parameter schema examples
  • Search: grep "Real-World Examples" references/custom-tools-guide.md for complete tool examples

• in-code-subagents.md - Programmatic subagent configuration (preferred approach)

  • Search: grep "Factory" references/in-code-subagents.md for factory pattern examples
  • Search: grep "Class-Based" references/in-code-subagents.md for class-based patterns
  • Search: grep "Spawning" references/in-code-subagents.md for Task tool delegation

• hooks-guide.md - PreToolUse and PostToolUse hooks for safety

  • Search: grep "PreToolUse" references/hooks-guide.md for pre-execution hooks
  • Search: grep "File Path" references/hooks-guide.md for path validation examples
  • Search: grep "Command Allowlist" references/hooks-guide.md for bash restrictions

• session-patterns.md - Session management, streaming, multi-turn, production patterns

  • Search: grep "Multi-Turn" references/session-patterns.md for conversation patterns
  • Search: grep "WebSocket" references/session-patterns.md for streaming examples
  • Search: grep "SessionManager" references/session-patterns.md for production patterns

Template Usage

Quick Start (Recommended)

Use the automated project scaffolder:

./scripts/init-agent-project.sh

This interactive script will guide you through creating a new project from any template.

Manual Template Copy

Alternatively, copy templates manually:

Basic Agent

cp -r assets/templates/basic-agent ./my-agent
cd my-agent
npm install
npm start

Custom Tools

cp -r assets/templates/custom-tools ./my-agent
cd my-agent
npm install
npm start "Calculate 5 + 3 times 2"

In-Code Subagents

cp -r assets/templates/in-code-subagent ./my-agent
cd my-agent
npm install
npm start

Full Production App

cp -r assets/templates/full-app ./my-agent
cd my-agent
npm install
npm start
# Open browser to http://localhost:3000

Best Practices

1. Start simple - Begin with basic-agent, add complexity as needed
2. Use in-code subagents - Prefer programmatic configuration over markdown
3. Validate with hooks - Add PreToolUse hooks for safety
4. Capture session IDs - Always capture for multi-turn conversations
5. Handle all message types - Process system, user, assistant, and result messages
6. Set appropriate maxTurns - Prevent runaway execution
7. Use specific tool allowlists - Only enable tools the agent needs
8. Test thoroughly - Unit test tools, hooks, and configurations
9. Log extensively - Log tool calls, sessions, errors for debugging
10. Monitor costs - Track total_cost_usd from result messages

Common Scenarios

Building a Code Assistant

1. Start with custom-tools template
2. Create tools for:
   • Reading codebases
   • Running tests
   • Generating code
3. Add in-code subagents for:
   • Code search
   • Code analysis
   • Documentation generation
4. Add hooks to restrict file operations

Building a Data Assistant

1. Start with custom-tools template
2. Create tools for:
   • Database queries
   • API calls
   • Data processing
3. Add in-code subagents for:
   • Data retrieval
   • Data analysis
   • Report generation

Building a Production App

1. Start with full-app template
2. Customize AIClient with your tools
3. Add authentication to WebSocket
4. Configure database for persistence
5. Add monitoring and logging
6. Deploy with proper security

Troubleshooting

Agent not using tools

• Check tool is in allowedTools array
• Verify MCP server is in mcpServers object
• Check tool naming: mcp__<server>__<tool>

Multi-turn not working

• Verify session ID is captured from init message
• Use resume: sessionId in options
• Don't recreate session between messages

Hooks not firing

• Check matcher regex pattern matches tool name
• Ensure hooks are in correct format
• Verify hook returns proper HookOutput

WebSocket disconnecting

• Check for errors in message broadcasting
• Verify session cleanup logic
• Add error handling to WebSocket handlers

Next Steps

1. Review templates in assets/templates/
2. Read detailed guides in references/
3. Start with basic-agent and iterate
4. Add custom tools for your use case
5. Configure in-code subagents for specialization
6. Implement hooks for safety
7. Build production app with full-app template

For specific implementation details, consult the reference documentation.