writing-subagents

Writing Subagents

Overview

Writing subagents IS creating specialized workers with isolated contexts.

Subagents run via the Agent 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.

Routing

Pattern: Skill Steps Handoff: none Next: none

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
7. REFACTOR - Quality review

Announce: "Created 7 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

Built-in subagent types (consider BEFORE creating custom agents):

Type	Model	Tools	Use Case
Explore	Haiku	Read-only (no Write/Edit)	Fast codebase search, analysis
Plan	Inherit	Read-only	Research and planning
general-purpose	Inherit	All	Complex multi-step tasks

Decision: Can a built-in type handle this? If yes, use it directly via the Agent tool — no agent file needed.

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

Verification: Documented at least 1 issue (context pollution, focus loss, inconsistency) 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 & Tools

See references/agent-spec.md for full configuration fields, model selection, context:fork guidelines, and tool permissions.

Key rules:

• name: lowercase with hyphens, matches filename
• description: include "Use proactively when..." for auto-invoke
• tools: minimal set (principle of least privilege). Subagents CANNOT request permissions at runtime.
• Plugin agents do NOT support hooks, mcpServers, or permissionMode

Agent Type	Recommended 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 Agent tool.

Test:

Agent 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

Task 7: REFACTOR - Quality Review

Goal: Have subagent reviewed by subagent-reviewer.

Agent tool:
- subagent_type: "rcc:subagent-reviewer"
- prompt: "Review subagent at [path/to/agent.md]"

Outcomes:

• Pass → Subagent complete
• Needs Fix → Fix issues, re-run reviewer, repeat until Pass
• Fail → Major problems, return to Task 3

This is the REFACTOR phase: Close loopholes identified by reviewer.

Verification: subagent-reviewer returns "Pass" rating.

Trigger Patterns

• Proactive: Include "Use proactively when..." in description for auto-invoke
• Manual: Omit proactive triggers for explicit-only invocation

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];
    review [label="Task 7: REFACTOR\nQuality review", shape=box, style=filled, fillcolor="#ccccff"];
    review_pass [label="Review\npassed?", shape=diamond];
    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 -> review;
    review -> review_pass;
    review_pass -> done [label="pass"];
    review_pass -> write [label="fail\nfix issues"];
}

References

• references/examples.md - Subagent templates
• references/tools.md - Available tools reference
• See also: advising-architecture skill for component classification guidance