Creating Subagents

This Skill provides comprehensive knowledge about creating effective subagents in Claude Code.

What Are Subagents?

Subagents are specialized AI assistants that Claude Code can delegate tasks to. Each subagent:

• Has a specific purpose and expertise area
• Uses its own context window separate from the main conversation
• Can be configured with specific tools it's allowed to use
• Includes a custom system prompt that guides its behavior

Critical Insight - The Primary Value: While specialization is important, the primary value of subagents is token context isolation. Subagents allow the main conversation to delegate "token-heavy" or "noisy" tasks (like extensive research, analyzing large codebases, or processing logs) to a separate context window. This keeps the main orchestrator's context clean and focused, enabling much longer-running and more complex workflows.

Think of subagents as allowing the main agent to "go on side quests" without polluting its main context with unnecessary details.

Mental Model: Orchestrator and Specialists

The most effective way to think about subagents is as a distributed team:

Main Conversation = Project Manager / Orchestrator

• Stays "token-light" - focuses on coordination, not execution
• Delegates clearly defined tasks to specialists
• Aggregates results from subagents
• Makes high-level decisions
• Writes the "final report" to the user

Subagents = Specialists

• Handle specific, token-intensive tasks
• Work in isolated context windows (their own "side quests")
• Return structured results to the orchestrator
• Examples: code reviewer, research analyst, test runner, database engineer

The Token Budget Strategy:

• Main thread has limited context window
• Many tasks are "token-heavy" (large file analysis, extensive research)
• Executing these in main thread "pollutes" the orchestrator's context
• Subagents solve this by isolating the noise
• Result: Main thread can handle much longer, more complex workflows

File Structure

Location: .claude/agents/{name}.md

Format:

---
name: agent-name
description: When to invoke this agent
tools: Read, Write, Edit  # Optional
model: sonnet             # Optional
---

# System Prompt

Agent's instructions, approach, and constraints.

Creating and Managing Subagents

The /agents Command (Recommended)

The /agents command is now the preferred way to create and manage subagents (as of October 2025).

Why Use /agents:

• Interactive guided creation with prompts
• Lists all available tools including MCP server tools
• Easier to discover which tools to grant
• Quick prototyping without manual YAML editing

Usage:

/agents create    # Interactive creation
/agents list      # Show all subagents
/agents edit      # Modify existing subagent
/agents delete    # Remove subagent

Manual File Editing: You can still manually create .claude/agents/{name}.md files for:

• Version control (git tracking)
• Templating and automation
• Fine-grained control

Both approaches create the same .md files - choose based on your workflow.

YAML Frontmatter Fields

Required Fields

name (string)

• Unique identifier for the subagent
• Use lowercase letters and hyphens only
• Examples: code-reviewer, test-runner, backend-engineer
• ❌ Avoid: Spaces, underscores, capitals

description (string)

• Natural language description of the subagent's purpose
• Must include when/why to invoke the agent
• Should contain keywords users would mention
• Used by Claude to decide when to delegate tasks

Optional Fields

tools (comma-separated string)

• Specific tools this subagent can use
• If omitted, inherits all tools from main thread
• Examples: Read, Write, Edit, Bash, Grep, Glob
• Use principle of least privilege

model (string)

• AI model for this subagent to use
• Options: sonnet, opus, haiku, inherit
• inherit - Use same model as main conversation
• If omitted, uses default subagent model (sonnet)

Writing Effective Descriptions

The description is critical for automatic invocation. Claude reads descriptions to decide which subagent to use.

Good Descriptions

# Specific + Proactive Trigger
description: Expert code reviewer. Use PROACTIVELY after code changes to review quality, security, and maintainability.

# Clear Purpose + When to Use
description: Debugging specialist for errors, test failures, and unexpected behavior. Use when encountering any issues or when tests fail.

# Multiple Triggers
description: Backend development specialist that enforces KISS, DRY, and YAGNI. Use when creating or modifying Python/FastAPI code, database models, API endpoints, or backend logic.

Bad Descriptions

# Too vague
description: Helps with code

# Missing trigger
description: Reviews code for quality

# No "when to use"
description: Testing expert

Key Elements

1. Role - What kind of expert is this?
2. Purpose - What specific tasks?
3. Triggers - When should Claude invoke it?
4. Keywords - Terms users would mention

Proactive Invocation

Add these phrases to encourage automatic invocation:

• "Use PROACTIVELY"
• "MUST BE USED"
• "Use immediately after"
• "Use automatically when"

Example:

description: Test coverage guardian. Use PROACTIVELY to remind about tests when code changes are made. MUST BE USED when implementing new features.

System Prompt Structure

The system prompt (Markdown content after frontmatter) defines the subagent's behavior.

Essential Sections

# Agent Name

Brief overview of role and capabilities.

## Your Role

What this agent does and its expertise.

## When Invoked

Specific situations that trigger this agent.

## Your Process

Step-by-step workflow:
1. First action
2. Second action
3. Final action

## Output Format (CRITICAL)

**Why Structured Output Matters**:
Your output must be machine-readable so the orchestrator can parse and aggregate results.

Return results in JSON format:
{
  "summary": "Brief overview of findings",
  "issues": [
    {"severity": "high", "location": "file.py:42", "description": "...", "suggestion": "..."}
  ],
  "recommendations": ["...", "..."],
  "metrics": {"files_reviewed": 5, "issues_found": 3}
}

This structured format makes your output reliable and actionable for the orchestrator.

## Best Practices

Guidelines and principles to follow.

## Red Flags

What to watch for or challenge.

Key Addition: Output Format The most important best practice for subagents is defining a structured output format (typically JSON). This ensures:

• Orchestrator can reliably parse subagent results
• Results can be aggregated across multiple subagent calls
• No ambiguity in how to interpret the output
• Enables automated decision-making based on subagent output

For detailed examples of subagents with structured output, see examples.md.

Quick Example: Structured Output

---
name: code-reviewer
description: Expert code reviewer. Use PROACTIVELY after writing or modifying code.
tools: Read, Grep, Glob, Bash
model: sonnet
---

# Code Reviewer

## Your Process
1. Run `git diff` to see changes
2. Review systematically (readability, quality, security)
3. Return results as JSON

## Output Format

```json
{
  "summary": "Reviewed 3 files with 2 critical issues",
  "files_reviewed": ["src/auth.py", "src/api.py"],
  "critical_issues": [{"file": "...", "issue": "...", "recommendation": "..."}],
  "warnings": [...],
  "suggestions": [...]
}

**See [examples.md](examples.md) for the complete code reviewer example with full checklists and detailed output format.**

## Tool Restrictions

Use the `tools` field to limit what the subagent can do (principle of least privilege).

### Quick Reference

```yaml
# Read-only (analysis, review)
tools: Read, Grep, Glob

# Limited write (code generation)
tools: Read, Write, Edit, Grep, Glob

# Selective bash (specific commands only)
tools: Read, Bash(git:*), Bash(pytest:*)

# Full access (omit tools field)
# Inherits all tools from main conversation

See reference.md for detailed tool restriction patterns, security best practices, and when to use each type.

Model Selection

Choose the right model based on task complexity and requirements.

Quick Reference

model: haiku    # Fast, cheap (simple tasks, file search)
model: sonnet   # Balanced (default, most tasks)
model: opus     # Powerful (critical decisions, security)
model: inherit  # Match main conversation

Decision Tree:

• Simple/pattern-based → haiku
• Mission-critical/security → opus
• Default choice → sonnet

See reference.md for detailed model characteristics, use cases, and cost/speed trade-offs.

Common Patterns

Four main subagent patterns have emerged as most effective:

1. Task-Specific - Single focused task (test-runner, file-analyzer)
2. Domain-Specific - Expert in technology (database-engineer, api-designer)
3. Principle-Enforcing - Enforce guidelines (backend-engineer with KISS/DRY/YAGNI)
4. Workflow Orchestrators - Multi-step processes (deployment-engineer)

See examples.md for complete implementations of all four patterns with full system prompts.

Subagent Communication

Subagents can work with Skills and other subagents:

Invoking Skills

## Your Process
1. **Invoke the `code-style-guide` Skill** for language-specific patterns
2. Apply recommendations

Delegating to Other Subagents

## When to Delegate
- Test failures → `test-runner` subagent
- Deployment → `deployment-engineer` subagent

See communication.md for delegation patterns, testing strategies, and debugging communication issues.

Best Practices

1. Single Responsibility - One clear purpose (not "general-helper")
2. Clear Triggers - Specific keywords in description ("Use PROACTIVELY when...")
3. Appropriate Tools - Least privilege (read-only for reviewers, write for generators)
4. Focused Prompts - Under 300 lines, use Skills for complex knowledge
5. Structured Output - Define JSON format for orchestrator parsing
6. TodoWrite Tracking - Include task tracking in system prompt

Common Pitfalls

Too Generic: description: "Helps with coding" → Add specific triggers Too Restrictive: tools: Read only → Add Grep, Glob for useful analysis Missing Triggers: No "Use when..." → Add explicit activation criteria Bloated Prompts: 500+ lines → Use Skills for detailed knowledge

Validation Checklist

Before finalizing a subagent:

• Name uses lowercase and hyphens only
• Description includes "when to use"
• Description contains trigger keywords
• Tools follow least privilege principle
• Model choice is appropriate
• System prompt has clear structure
• Process/workflow is defined
• Examples or checklists included
• No conflicts with existing subagents

Summary

Essential Elements:

1. Focused responsibility (one job, done well)
2. Clear description with triggers
3. Appropriate tool restrictions
4. Structured system prompt
5. Defined workflow/process

Success Criteria:

• Automatically invoked at right time
• Has necessary tools to complete tasks
• System prompt provides clear guidance
• Integrates well with other subagents