Claude Agent SDK

Overview

The Claude Agent SDK enables building autonomous AI agents with Claude through a feedback loop architecture. Available for Python (3.10+) and TypeScript (Node 18+).

Repository:

• Python: https://github.com/anthropics/claude-agent-sdk-python
• TypeScript: https://github.com/anthropics/claude-agent-sdk-typescript

Documentation: https://platform.claude.com/docs/en/agent-sdk/overview

Installation

# Python
pip install claude-agent-sdk

# TypeScript
npm install @anthropic-ai/agent-sdk

Core Architecture: Feedback Loop Pattern

Every agent follows this cycle:

1. Gather Context → filesystem navigation, subagents, tools
2. Take Action → tools, bash, code generation, MCP
3. Verify Work → rules-based, visual, LLM-as-judge
4. Repeat → iterate until completion

This pattern applies whether you're building a simple script or a complex multi-agent system.

Execution Mechanisms (Priority Order)

Choose mechanisms based on task requirements:

1. Custom Tools → Primary workflows (appear prominently in context)
2. Bash → Flexible one-off operations
3. Code Generation → Complex, reusable outputs (prefer TypeScript for linting feedback)
4. MCP → Pre-built external integrations (Slack, GitHub, databases)

Rule: Use tools for repeatable operations, bash for exploration, code generation when you need structured output that can be validated.

Quick Start Patterns

Python: Basic Query

from claude_agent_sdk import query

result = await query(
    model="claude-sonnet-4-5",
    system_prompt="You are a helpful coding assistant.",
    user_message="List files in current directory",
    working_dir=".",
)
print(result.final_message)

TypeScript: Session Management

import { ClaudeSdkClient } from '@anthropic-ai/agent-sdk';

const client = new ClaudeSdkClient({ apiKey: process.env.ANTHROPIC_API_KEY });

const result = await client.query({
  model: 'claude-sonnet-4-5',
  systemPrompt: 'You are a helpful coding assistant.',
  userMessage: 'List files in current directory',
  workingDir: '.',
});

console.log(result.finalMessage);

Key Components

1. Custom Tools (SDK MCP Servers)

In-process tools with no subprocess overhead. Primary building block for agents.

Python:

from claude_agent_sdk.mcp import tool, create_sdk_mcp_server

@tool(
    name="calculator",
    description="Perform calculations",
    input_schema={"expression": str}
)
async def calculator(args):
    result = eval(args["expression"])  # Use safe eval in production
    return {"content": [{"type": "text", "text": str(result)}]}

server = create_sdk_mcp_server(name="math", tools=[calculator])

TypeScript:

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

const calculator = tool({
  name: 'calculator',
  description: 'Perform calculations',
  inputSchema: z.object({ expression: z.string() }),
  async execute({ expression }) {
    const result = eval(expression); // Use safe eval in production
    return { content: [{ type: 'text', text: String(result) }] };
  },
});

const server = createSdkMcpServer({ name: 'math', tools: [calculator] });

Benefits over external MCP: Better performance, easier debugging, shared memory space, no IPC overhead.

2. Hooks (Lifecycle Callbacks)

Intercept and modify agent behaviour at specific points.

Available hooks:

• PreToolUse → Validate/modify/deny tool calls before execution
• PostToolUse → Process/log/modify tool results
• Stop → Handle completion events

Python validation example:

async def validate_command(input_data, tool_use_id, context):
    if "rm -rf" in input_data["tool_input"].get("command", ""):
        return {
            "hookSpecificOutput": {
                "permissionDecision": "deny",
                "permissionDecisionReason": "Dangerous command blocked"
            }
        }

TypeScript logging example:

const loggingHook = {
  matcher: (input) => input.toolName === 'bash',
  async handler(input, toolUseId, context) {
    console.log(`Executing: ${input.toolInput.command}`);
  }
};

3. Permission System

Four modes with progressively less restriction:

• default → Prompt for each tool use
• plan → Agent can read/explore freely, prompts for modifications
• acceptEdits → Auto-approve file edits, prompt for bash/destructive ops
• bypassPermissions → Fully autonomous (use carefully)

Dynamic control with canUseTool:

async def permission_callback(tool_name, tool_input, context):
    if tool_name == "bash" and "git push" in tool_input.get("command", ""):
        return False  # Deny
    return True  # Allow

4. Subagents

Isolated agents with separate context windows and specialised capabilities.

When to use:

• Parallel processing of independent tasks
• Context isolation (prevent one task from bloating main context)
• Specialised agents with different tools/models

Python:

from claude_agent_sdk import ClaudeAgentOptions

options = ClaudeAgentOptions(
    subagent_definitions={
        "researcher": {
            "tools": ["read", "grep", "glob"],
            "model": "claude-haiku-4",
            "description": "Fast research agent"
        }
    }
)

TypeScript:

const options = {
  subagentDefinitions: {
    researcher: {
      tools: ['read', 'grep', 'glob'],
      model: 'claude-haiku-4',
      description: 'Fast research agent'
    }
  }
};

5. Context Management

Agentic Search (Preferred): Use bash + filesystem navigation (grep, ls, tail) before reaching for semantic search. Simpler and more reliable.

Automatic Compaction: SDK automatically summarises messages when approaching token limits. Transparent and automatic.

Folder Structure as Context Engineering: Organise files intentionally—directory structure is visible to the agent and influences its understanding.

Verification Patterns

Rules-Based (Preferred)

Explicit validation enables self-correction:

# In PostToolUse hook
if tool_name == "write":
    # Run linter on generated file
    lint_result = run_linter(tool_output)
    if lint_result.has_errors:
        return {"continue": True}  # Let agent fix errors

Visual Feedback

For UI tasks, screenshot and re-evaluate:

@tool(name="check_ui", description="Verify UI matches requirements")
async def check_ui(args):
    screenshot = take_screenshot(args["url"])
    # Return screenshot to agent for evaluation
    return {"content": [{"type": "image", "source": screenshot}]}

LLM-as-Judge

Only for fuzzy criteria where rules don't work (higher latency):

judge_result = await secondary_model.evaluate(
    criteria="Does output match tone guidelines?",
    output=agent_output
)

Common Pitfalls & Solutions

1. System Prompt Not Loading

Symptom: CLAUDE.md ignored, custom prompts not applied Solution: Set setting_sources=["project"] or ["user", "project"]

# Python
options = ClaudeAgentOptions(setting_sources=["project"])

# TypeScript
const options = { settingSources: ['project'] };

2. Tool Not Available

Symptom: "Tool not found" errors Solution: Check MCP tool naming: mcp__{server_name}__{tool_name}

3. Permission Denied

Symptom: Agent can't access directories Solution: Add directories explicitly:

options = ClaudeAgentOptions(add_dirs=["/path/to/data"])

4. Python Keyword Conflicts

Symptom: Syntax errors with async or continue parameters Solution: Use async_ and continue_ (SDK auto-converts)

# Use async_ not async
hook_result = {"async_": True, "continue_": False}

5. Context Overflow

Symptom: Token limit errors Solution: Use subagents for isolation or let automatic compaction handle it

6. Tool Execution Failures

Symptom: Tools fail silently or with unclear errors Solution: Return structured error messages in tool responses:

return {
    "content": [{
        "type": "text",
        "text": "Error: Invalid input. Expected format: ...",
        "isError": True
    }]
}

7. External MCP Server Not Connecting

Symptom: stdio/SSE MCP servers timeout Solution: Verify server is executable and logs are accessible:

# Check server stderr in context.mcp_server_logs
async def debug_hook(input_data, tool_use_id, context):
    print(context.mcp_server_logs.get("server_name"))

Language-Specific Considerations

Python vs TypeScript

Aspect	Python	TypeScript
Runtime	anyio.run(main)	Native async/await
Min Version	Python 3.10+	Node.js 18+
Type Safety	Type hints optional	Strict types with Zod
Hook Fields	async_, continue_	async, continue
CLI	Bundled (no install)	Separate install needed
Tool Validation	Dict-based schemas	Zod schemas

TypeScript Advantages

• Linting provides extra feedback layer for generated code
• Stronger type safety catches errors earlier
• Better IDE integration

Python Advantages

• Simpler setup for data science workflows
• Direct integration with ML/data tools
• More concise for scripting tasks

Decision Frameworks

When to Use Claude Agent SDK

✅ Use when:

• Building autonomous agents that need computer access
• Iterative workflows with verification loops
• Multi-step tasks requiring context and tool use
• Custom tool integration requirements
• Need for permission control and safety

❌ Don't use when:

• Simple API calls sufficient (use Messages API)
• No tool/computer access needed
• Purely conversational applications
• Real-time streaming responses critical

Tool vs Bash vs Code Generation

Use Custom Tools when:

• Operation repeats frequently
• Need structured input/output validation
• Want prominent placement in agent context
• Require error handling and retry logic

Use Bash when:

• One-off exploration or debugging
• System operations (git, file management)
• Flexible scripting without formal structure

Use Code Generation when:

• Need structured, reusable output
• Can validate with linting/compilation
• Building components or modules
• TypeScript preferred for feedback quality

SDK MCP vs External MCP

Use SDK MCP (in-process) when:

• Building custom tools for your agent
• Performance matters (no subprocess overhead)
• Need shared state with main process
• Debugging tool logic

Use External MCP (stdio/SSE) when:

• Integrating third-party services
• Tool needs isolation
• Using pre-built MCP servers
• Cross-language tool requirements

Session Management

Resuming Sessions

Python:

# First run
result1 = await query(user_message="Create a file", working_dir=".")

# Resume with new message
result2 = await query(
    user_message="Now modify it",
    working_dir=".",
    session_id=result1.session_id
)

TypeScript:

// First run
const result1 = await client.query({ userMessage: 'Create a file' });

// Resume
const result2 = await client.query({
  userMessage: 'Now modify it',
  sessionId: result1.sessionId
});

Forking Sessions

Create alternative branches from a point:

# Fork for different approach
result_fork = await query(
    user_message="Try different implementation",
    session_id=original_result.session_id,
    fork_session=True
)

Budget Control

Set USD spending limits:

options = ClaudeAgentOptions(budget={"usd": 5.00})

Agent stops when budget exceeded. Useful for cost control in production.

Testing Patterns

Mock Tools for Testing

Python:

import pytest
from unittest.mock import AsyncMock

@pytest.fixture
def mock_tool():
    return AsyncMock(return_value={
        "content": [{"type": "text", "text": "mocked"}]
    })

async def test_agent(mock_tool):
    server = create_sdk_mcp_server(name="test", tools=[mock_tool])
    # Test with mocked tool

TypeScript:

import { jest } from '@jest/globals';

const mockTool = {
  name: 'test',
  execute: jest.fn().mockResolvedValue({
    content: [{ type: 'text', text: 'mocked' }]
  })
};

Integration Testing

Test with real tools in isolated environment:

import tempfile
import os

async def test_file_operations():
    with tempfile.TemporaryDirectory() as tmpdir:
        result = await query(
            user_message="Create test.txt with content 'hello'",
            working_dir=tmpdir,
            permission_mode="bypassPermissions"
        )
        assert os.path.exists(f"{tmpdir}/test.txt")

Migration from Claude Code SDK

If migrating from the deprecated claude-code-sdk:

1. Package renamed: claude-code-sdk → claude-agent-sdk
2. System prompt not default: Must explicitly set or enable via setting_sources
3. Type renamed (Python): ClaudeCodeOptions → ClaudeAgentOptions
4. Settings sources not automatic: Must set setting_sources=["project"]

See migration guide: https://platform.claude.com/docs/en/agent-sdk/migration-guide

Performance Optimisation

1. Use Haiku for simple tasks → 5x cheaper, faster for research/exploration
2. SDK MCP over external → No subprocess overhead
3. Batch operations → Combine file operations when possible
4. Set turn limits → Prevent infinite loops (turn_limit parameter)
5. Monitor token usage → Use budget controls in production

Security Best Practices

1. Always validate tool inputs → Never trust unchecked input
2. Use permission callbacks → Deny dangerous operations dynamically
3. Restrict filesystem access → Use add_dirs to limit scope
4. Sandbox external MCP servers → Isolate third-party tools
5. Set budgets → Prevent runaway costs
6. Log all tool uses → Audit trail via PostToolUse hooks
7. Never hardcode API keys → Use environment variables

Key Principles

1. Folder structure is context engineering → Organise intentionally
2. Rules-based feedback enables self-correction → Add linting and validation
3. Start with agentic search → Bash navigation before semantic search
4. Tools are primary, bash is secondary → Use tools for repeatable operations
5. TypeScript for generated code → Extra feedback layer improves quality
6. Verification closes the loop → Always validate agent work
7. Use subagents for isolation → Prevent context bloat and enable parallelism

Additional Resources

• API Reference (Python): https://platform.claude.com/docs/en/agent-sdk/python
• API Reference (TypeScript): https://platform.claude.com/docs/en/agent-sdk/typescript
• Examples Repository: https://github.com/anthropics/claude-agent-sdk-demos
• Engineering Blog: https://www.anthropic.com/engineering/building-agents-with-the-claude-agent-sdk