Writing Subagents

Overview

Writing subagents IS creating specialized workers with isolated contexts.

Subagents run via the Task tool with their own context window, tools, and system prompt. Use for tasks that benefit from focused expertise.

Core principle: One agent, one responsibility. Bloated agents become unfocused.

Violating the letter of the rules is violating the spirit of the rules.

Task Initialization (MANDATORY)

Before ANY action, create task list using TaskCreate:

TaskCreate for EACH task below:
- Subject: "[writing-subagents] Task N: <action>"
- ActiveForm: "<doing action>"

Tasks:

1. Analyze requirements
2. RED - Test without subagent
3. GREEN - Write agent file
4. Validate structure
5. Test invocation
6. Verify behavior

Announce: "Created 6 tasks. Starting execution..."

Execution rules:

1. TaskUpdate status="in_progress" BEFORE starting each task
2. TaskUpdate status="completed" ONLY after verification passes
3. If task fails → stay in_progress, diagnose, retry
4. NEVER skip to next task until current is completed
5. At end, TaskList to confirm all completed

TDD Mapping for Subagents

TDD Phase	Subagent Creation	What You Do
RED	Test without agent	Perform task in main context, note issues
Verify RED	Document problems	Note context pollution, scope creep
GREEN	Write agent	Create focused agent addressing problems
Verify GREEN	Test invocation	Verify agent runs with correct tools/prompt
REFACTOR	Optimize	Reduce tools, sharpen system prompt

Task 1: Analyze Requirements

Goal: Understand what specialized task needs isolation.

Questions to answer:

• What specific task needs a subagent?
• Why can't the main agent do this?
• What tools does this agent need?
• Should it be proactive or manual?

When to use subagents:

• Task needs isolated context (code review, deep analysis)
• Task would pollute main conversation
• Task is repetitive and well-defined
• Task benefits from specialized system prompt

Verification: Can describe the agent's single responsibility in one sentence.

Task 2: RED - Test Without Subagent

Goal: Perform the task in main context. Note issues that arise.

Process:

1. Perform the intended task in main conversation
2. Observe context pollution or scope creep
3. Note where specialized focus would help
4. Document specific problems

Common problems without subagents:

• Context window fills with review comments
• Main agent loses focus on primary task
• Repeated tasks lack consistency
• No clean handoff point

Verification: Documented at least 1 issue from running task in main context.

Task 3: GREEN - Write Agent File

Goal: Create agent file addressing the problems you documented.

Agent Location

.claude/agents/      # Project-level
~/.claude/agents/    # User-level (global)

Agent Format

---
name: agent-name
description: What this agent does. Use proactively when [triggers].
tools: Read, Grep, Glob, Bash
model: sonnet
---

System prompt for the agent.

## Role
[Clear description of the agent's responsibility]

## Process
[Steps the agent should follow]

## Output Format
[Expected output structure]

Configuration Fields

Field	Required	Description
name	Yes	Lowercase with hyphens: code-reviewer
description	Yes	Include "Use proactively when..." for auto-invoke
tools	No	CSV list; omit = inherit all
model	No	sonnet, opus, haiku, inherit
skills	No	Auto-load skills when invoked
permissionMode	No	default, acceptEdits, bypassPermissions, plan

Model Selection

Model	Use Case	Cost
haiku	Fast exploration, simple validation	$
sonnet	Balanced (default)	$$
opus	Complex reasoning, architecture	$$$

Tool Recommendations

Agent Type	Tools
Code reviewer	Read, Grep, Glob
Test runner	Read, Bash, Grep
Explorer	Read, Glob, Grep, Bash
Writer	Read, Edit, Write

Verification:

• Name is lowercase with hyphens
• Description has clear trigger
• Tools are minimal (principle of least privilege)
• System prompt is focused

Task 4: Validate Structure

Goal: Verify agent file structure is correct.

Checklist:

• File is in .claude/agents/ directory
• Frontmatter has name and description
• Name matches filename (minus .md)
• Tools are appropriate for the task
• System prompt is actionable
• No overlapping responsibility with other agents

Verification: All checklist items pass.

Task 5: Test Invocation

Goal: Verify agent can be invoked via Task tool.

Test:

Task tool:
- subagent_type: "[plugin:]agent-name"
- prompt: "Test invocation"

Check:

• Agent loads correctly
• System prompt is applied
• Tool restrictions work
• Model selection honored

Verification: Agent invocation succeeds without errors.

Task 6: Verify Behavior

Goal: Run agent on real task and verify quality.

Process:

1. Invoke agent with real task input
2. Observe agent follows its system prompt
3. Verify output matches expected format
4. Check agent stays within its scope

Verification:

• Agent produces expected output
• Agent uses only granted tools
• Agent doesn't exceed its scope

Trigger Patterns

Proactive (Auto-Invoke)

Include in description:

• "Use proactively when..."
• "Use immediately after..."
• "MUST BE USED when..."

description: Reviews code quality. Use proactively when completing any implementation task.

Manual (Explicit Invoke)

Omit proactive triggers:

description: Deep security analysis of authentication code.

Red Flags - STOP

These thoughts mean you're rationalizing. STOP and reconsider:

• "This agent needs all tools"
• "One agent can handle multiple responsibilities"
• "Main context is fine, don't need isolation"
• "Skip testing, the prompt is simple"
• "Use opus for everything"

All of these mean: You're about to create a weak agent. Follow the process.

Common Rationalizations

Excuse	Reality
"Needs all tools"	More tools = more ways to go wrong. Minimize.
"Multi-purpose agent"	Jack of all trades = master of none. One job.
"Main context works"	Context pollution is invisible until it's a problem.
"Simple prompt"	Simple ≠ correct. Test the behavior.
"Opus is better"	Opus costs 15x haiku. Use appropriate model.

Flowchart: Subagent Creation

digraph subagent_creation {
    rankdir=TB;

    start [label="Need subagent", shape=doublecircle];
    analyze [label="Task 1: Analyze\nrequirements", shape=box];
    need_agent [label="Needs\nisolation?", shape=diamond];
    no_agent [label="Use main\ncontext", shape=box];
    baseline [label="Task 2: RED\nTest without agent", shape=box, style=filled, fillcolor="#ffcccc"];
    write [label="Task 3: GREEN\nWrite agent file", shape=box, style=filled, fillcolor="#ccffcc"];
    validate [label="Task 4: Validate\nstructure", shape=box];
    invoke [label="Task 5: Test\ninvocation", shape=box];
    invoke_pass [label="Invocation\nworks?", shape=diamond];
    verify [label="Task 6: Verify\nbehavior", shape=box];
    done [label="Agent complete", shape=doublecircle];

    start -> analyze;
    analyze -> need_agent;
    need_agent -> no_agent [label="no"];
    need_agent -> baseline [label="yes"];
    baseline -> write;
    write -> validate;
    validate -> invoke;
    invoke -> invoke_pass;
    invoke_pass -> verify [label="yes"];
    invoke_pass -> write [label="no"];
    verify -> done;
}

References

• references/examples.md - Subagent templates
• references/tools.md - Available tools reference