Subagents in the SDK

Define and invoke subagents to isolate context, run tasks in parallel, and apply specialized instructions in your Claude Agent SDK applications.

Subagents are separate agent instances that your main agent can spawn to handle focused subtasks. Use subagents to isolate context for focused subtasks, run multiple analyses in parallel, and apply specialized instructions without bloating the main agent’s prompt.

This guide explains how to define and use subagents in the SDK using the agents parameter.

Overview

You can create subagents in three ways:

• Programmatically: use the agents parameter in your query() options (TypeScript, Python)
• Filesystem-based: define agents as markdown files in .claude/agents/ directories (see defining subagents as files)
• Built-in general-purpose: Claude can invoke the built-in general-purpose subagent at any time via the Agent tool without you defining anything

This guide focuses on the programmatic approach, which is recommended for SDK applications.

When you define subagents, Claude determines whether to invoke them based on each subagent’s description field. Write clear descriptions that explain when the subagent should be used, and Claude will automatically delegate appropriate tasks. You can also explicitly request a subagent by name in your prompt (for example, “Use the code-reviewer agent to…”).

Benefits of using subagents

Context isolation

Each subagent runs in its own fresh conversation. Intermediate tool calls and results stay inside the subagent; only its final message returns to the parent. See What subagents inherit for exactly what’s in the subagent’s context.

Example: a research-assistant subagent can explore dozens of files without any of that content accumulating in the main conversation. The parent receives a concise summary, not every file the subagent read.

Parallelization

Multiple subagents can run concurrently, dramatically speeding up complex workflows.

Example: during a code review, you can run style-checker, security-scanner, and test-coverage subagents simultaneously, reducing review time from minutes to seconds.

Specialized instructions and knowledge

Each subagent can have tailored system prompts with specific expertise, best practices, and constraints.

Example: a database-migration subagent can have detailed knowledge about SQL best practices, rollback strategies, and data integrity checks that would be unnecessary noise in the main agent’s instructions.

Tool restrictions

Subagents can be limited to specific tools, reducing the risk of unintended actions.

Example: a doc-reviewer subagent might only have access to Read and Grep tools, ensuring it can analyze but never accidentally modify your documentation files.

Creating subagents

Programmatic definition (recommended)

Define subagents directly in your code using the agents parameter. This example creates two subagents: a code reviewer with read-only access and a test runner that can execute commands. The Agent tool must be included in allowedTools since Claude invokes subagents through the Agent tool.

Python

import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, AgentDefinition


async def main():
    async for message in query(
        prompt="Review the authentication module for security issues",
        options=ClaudeAgentOptions(
            # Agent tool is required for subagent invocation
            allowed_tools=["Read", "Grep", "Glob", "Agent"],
            agents={
                "code-reviewer": AgentDefinition(
                    # description tells Claude when to use this subagent
                    description="Expert code review specialist. Use for quality, security, and maintainability reviews.",
                    # prompt defines the subagent's behavior and expertise
                    prompt="""You are a code review specialist with expertise in security, performance, and best practices.

When reviewing code:
- Identify security vulnerabilities
- Check for performance issues
- Verify adherence to coding standards
- Suggest specific improvements

Be thorough but concise in your feedback.""",
                    # tools restricts what the subagent can do (read-only here)
                    tools=["Read", "Grep", "Glob"],
                    # model overrides the default model for this subagent
                    model="sonnet",
                ),
                "test-runner": AgentDefinition(
                    description="Runs and analyzes test suites. Use for test execution and coverage analysis.",
                    prompt="""You are a test execution specialist. Run tests and provide clear analysis of results.

Focus on:
- Running test commands
- Analyzing test output
- Identifying failing tests
- Suggesting fixes for failures""",
                    # Bash access lets this subagent run test commands
                    tools=["Bash", "Read", "Grep"],
                ),
            },
        ),
    ):
        if hasattr(message, "result"):
            print(message.result)


asyncio.run(main())

AgentDefinition configuration

Field	Type	Required	Description
description	string	Yes	Natural language description of when to use this agent
prompt	string	Yes	The agent’s system prompt defining its role and behavior
tools	string[]	No	Array of allowed tool names. If omitted, inherits all tools
model	'sonnet' | 'opus' | 'haiku' | 'inherit'	No	Model override for this agent. Defaults to main model if omitted
skills	string[]	No	List of skill names available to this agent
memory	'user' | 'project' | 'local'	No	Memory source for this agent (Python only)
mcpServers	(string | object)[]	No	MCP servers available to this agent, by name or inline config

Subagents cannot spawn their own subagents. Don’t include Agent in a subagent’s tools array.

Filesystem-based definition (alternative)

You can also define subagents as markdown files in .claude/agents/ directories. See the Claude Code subagents documentation for details on this approach. Programmatically defined agents take precedence over filesystem-based agents with the same name.

Even without defining custom subagents, Claude can spawn the built-in general-purpose subagent when Agent is in your allowedTools. This is useful for delegating research or exploration tasks without creating specialized agents.

What subagents inherit

A subagent’s context window starts fresh (no parent conversation) but isn’t empty. The only channel from parent to subagent is the Agent tool’s prompt string, so include any file paths, error messages, or decisions the subagent needs directly in that prompt.

The subagent receives	The subagent does not receive
Its own system prompt (AgentDefinition.prompt) and the Agent tool’s prompt	The parent’s conversation history or tool results
Project CLAUDE.md (loaded via settingSources)	Skills (unless listed in AgentDefinition.skills)
Tool definitions (inherited from parent, or the subset in tools)	The parent’s system prompt

The parent receives the subagent’s final message verbatim as the Agent tool result, but may summarize it in its own response. To preserve subagent output verbatim in the user-facing response, include an instruction to do so in the prompt or systemPrompt option you pass to the main query() call.

Invoking subagents

Automatic invocation

Claude automatically decides when to invoke subagents based on the task and each subagent’s description. For example, if you define a performance-optimizer subagent with the description “Performance optimization specialist for query tuning”, Claude will invoke it when your prompt mentions optimizing queries.

Write clear, specific descriptions so Claude can match tasks to the right subagent.

Explicit invocation

To guarantee Claude uses a specific subagent, mention it by name in your prompt:

"Use the code-reviewer agent to check the authentication module"

This bypasses automatic matching and directly invokes the named subagent.

Dynamic agent configuration

You can create agent definitions dynamically based on runtime conditions. This example creates a security reviewer with different strictness levels, using a more powerful model for strict reviews.

Python

import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, AgentDefinition


# Factory function that returns an AgentDefinition
# This pattern lets you customize agents based on runtime conditions
def create_security_agent(security_level: str) -> AgentDefinition:
    is_strict = security_level == "strict"
    return AgentDefinition(
        description="Security code reviewer",
        # Customize the prompt based on strictness level
        prompt=f"You are a {'strict' if is_strict else 'balanced'} security reviewer...",
        tools=["Read", "Grep", "Glob"],
        # Key insight: use a more capable model for high-stakes reviews
        model="opus" if is_strict else "sonnet",
    )


async def main():
    # The agent is created at query time, so each request can use different settings
    async for message in query(
        prompt="Review this PR for security issues",
        options=ClaudeAgentOptions(
            allowed_tools=["Read", "Grep", "Glob", "Agent"],
            agents={
                # Call the factory with your desired configuration
                "security-reviewer": create_security_agent("strict")
            },
        ),
    ):
        if hasattr(message, "result"):
            print(message.result)


asyncio.run(main())

Detecting subagent invocation

Subagents are invoked via the Agent tool. To detect when a subagent is invoked, check for tool_use blocks where name is "Agent". Messages from within a subagent’s context include a parent_tool_use_id field.

The tool name was renamed from "Task" to "Agent" in Claude Code v2.1.63. Current SDK releases emit "Agent" in tool_use blocks but still use "Task" in the system:init tools list and in result.permission_denials[].tool_name. Checking both values in block.name ensures compatibility across SDK versions.

This example iterates through streamed messages, logging when a subagent is invoked and when subsequent messages originate from within that subagent’s execution context.

The message structure differs between SDKs. In Python, content blocks are accessed directly via message.content. In TypeScript, SDKAssistantMessage wraps the Claude API message, so content is accessed via message.message.content.

Python

import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, AgentDefinition


async def main():
    async for message in query(
        prompt="Use the code-reviewer agent to review this codebase",
        options=ClaudeAgentOptions(
            allowed_tools=["Read", "Glob", "Grep", "Agent"],
            agents={
                "code-reviewer": AgentDefinition(
                    description="Expert code reviewer.",
                    prompt="Analyze code quality and suggest improvements.",
                    tools=["Read", "Glob", "Grep"],
                )
            },
        ),
    ):
        # Check for subagent invocation. Match both names: older SDK
        # versions emitted "Task", current versions emit "Agent".
        if hasattr(message, "content") and message.content:
            for block in message.content:
                if getattr(block, "type", None) == "tool_use" and block.name in (
                    "Task",
                    "Agent",
                ):
                    print(f"Subagent invoked: {block.input.get('subagent_type')}")

        # Check if this message is from within a subagent's context
        if hasattr(message, "parent_tool_use_id") and message.parent_tool_use_id:
            print("  (running inside subagent)")

        if hasattr(message, "result"):
            print(message.result)


asyncio.run(main())

Resuming subagents

Subagents can be resumed to continue where they left off. Resumed subagents retain their full conversation history, including all previous tool calls, results, and reasoning. The subagent picks up exactly where it stopped rather than starting fresh.

When a subagent completes, Claude receives its agent ID in the Agent tool result. To resume a subagent programmatically:

1. Capture the session ID: Extract session_id from messages during the first query
2. Extract the agent ID: Parse agentId from the message content
3. Resume the session: Pass resume: sessionId in the second query’s options, and include the agent ID in your prompt

You must resume the same session to access the subagent’s transcript. Each query() call starts a new session by default, so pass resume: sessionId to continue in the same session.

If you’re using a custom agent (not a built-in one), you also need to pass the same agent definition in the agents parameter for both queries.

The example below demonstrates this flow: the first query runs a subagent and captures the session ID and agent ID, then the second query resumes the session to ask a follow-up question that requires context from the first analysis.

TypeScript

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

// Helper to extract agentId from message content
// Stringify to avoid traversing different block types (TextBlock, ToolResultBlock, etc.)
function extractAgentId(message: SDKMessage): string | undefined {
  if (!("message" in message)) return undefined;
  // Stringify the content so we can search it without traversing nested blocks
  const content = JSON.stringify(message.message.content);
  const match = content.match(/agentId:\s*([a-f0-9-]+)/);
  return match?.[1];
}

let agentId: string | undefined;
let sessionId: string | undefined;

// First invocation - use the Explore agent to find API endpoints
for await (const message of query({
  prompt: "Use the Explore agent to find all API endpoints in this codebase",
  options: { allowedTools: ["Read", "Grep", "Glob", "Agent"] }
})) {
  // Capture session_id from ResultMessage (needed to resume this session)
  if ("session_id" in message) sessionId = message.session_id;
  // Search message content for the agentId (appears in Agent tool results)
  const extractedId = extractAgentId(message);
  if (extractedId) agentId = extractedId;
  // Print the final result
  if ("result" in message) console.log(message.result);
}

// Second invocation - resume and ask follow-up
if (agentId && sessionId) {
  for await (const message of query({
    prompt: `Resume agent ${agentId} and list the top 3 most complex endpoints`,
    options: { allowedTools: ["Read", "Grep", "Glob", "Agent"], resume: sessionId }
  })) {
    if ("result" in message) console.log(message.result);
  }
}

Subagent transcripts persist independently of the main conversation:

• Main conversation compaction: When the main conversation compacts, subagent transcripts are unaffected. They’re stored in separate files.
• Session persistence: Subagent transcripts persist within their session. You can resume a subagent after restarting Claude Code by resuming the same session.
• Automatic cleanup: Transcripts are cleaned up based on the cleanupPeriodDays setting (default: 30 days).

Tool restrictions

Subagents can have restricted tool access via the tools field:

• Omit the field: agent inherits all available tools (default)
• Specify tools: agent can only use listed tools

This example creates a read-only analysis agent that can examine code but cannot modify files or run commands.

Python

import asyncio
from claude_agent_sdk import query, ClaudeAgentOptions, AgentDefinition


async def main():
    async for message in query(
        prompt="Analyze the architecture of this codebase",
        options=ClaudeAgentOptions(
            allowed_tools=["Read", "Grep", "Glob", "Agent"],
            agents={
                "code-analyzer": AgentDefinition(
                    description="Static code analysis and architecture review",
                    prompt="""You are a code architecture analyst. Analyze code structure,
identify patterns, and suggest improvements without making changes.""",
                    # Read-only tools: no Edit, Write, or Bash access
                    tools=["Read", "Grep", "Glob"],
                )
            },
        ),
    ):
        if hasattr(message, "result"):
            print(message.result)


asyncio.run(main())

Common tool combinations

Use case	Tools	Description
Read-only analysis	Read, Grep, Glob	Can examine code but not modify or execute
Test execution	Bash, Read, Grep	Can run commands and analyze output
Code modification	Read, Edit, Write, Grep, Glob	Full read/write access without command execution
Full access	All tools	Inherits all tools from parent (omit tools field)

Troubleshooting

Claude not delegating to subagents

If Claude completes tasks directly instead of delegating to your subagent:

1. Include the Agent tool: subagents are invoked via the Agent tool, so it must be in allowedTools
2. Use explicit prompting: mention the subagent by name in your prompt (for example, “Use the code-reviewer agent to…”)
3. Write a clear description: explain exactly when the subagent should be used so Claude can match tasks appropriately

Filesystem-based agents not loading

Agents defined in .claude/agents/ are loaded at startup only. If you create a new agent file while Claude Code is running, restart the session to load it.

Windows: long prompt failures

On Windows, subagents with very long prompts may fail due to command line length limits (8191 chars). Keep prompts concise or use filesystem-based agents for complex instructions.

Related documentation

• Claude Code subagents: comprehensive subagent documentation including filesystem-based definitions
• SDK overview: getting started with the Claude Agent SDK