thirdlf03 Claude Code Subagent Builder
Define and refine Claude Code subagents with correct placement, frontmatter, and prompting so delegation is safe, fast, and predictable.
Usage
"Create a subagent for code review"
"Define a new agent for log analysis"
"Audit my existing subagents"
Workflow
- Clarify the subagent's goal, inputs, outputs, and success criteria.
- Choose the scope and location for the subagent file.
- Draft YAML frontmatter and the system prompt body.
- Constrain tools and permissions; add skills or hooks only when needed.
- Test with real tasks and iterate.
Requirements Intake
Ask for:
| Question | Purpose |
|---|---|
| Primary tasks + 2-3 examples | Define scope |
| Expected output format | Set quality bar |
| Scope (user/project/session) | Choose location |
| Tool access needs | Security |
| Permission mode | Safety |
| Skills to preload | Capabilities |
| Hooks needed | Automation |
| Display color | UX |
Choose Scope and Location
| Scope | Location | Use Case |
|---|---|---|
| Session | --agents CLI JSON |
Temporary, experiments |
| Project | .claude/agents/ |
Repo-specific, team use |
| User | ~/.claude/agents/ |
Personal workflows |
Reload after edits: restart or use /agents command.
Author the Subagent File
Frontmatter Fields
| Field | Required | Description |
|---|---|---|
| name | Yes | Unique, short, action-oriented |
| description | Yes | Routing hint (WHEN to use) |
| tools | Recommended | Allowlist, [] for none, omit for all |
| disallowedTools | Optional | Denylist |
| model | Optional | Override model |
| permissionMode | Optional | default/acceptEdits/dontAsk/bypassPermissions/plan |
| skills | Optional | Skills to preload |
| hooks | Optional | Hooks to run |
| color | Optional | cyan/blue/green/magenta/yellow/red/white |
| disabled | Optional | Set true to disable |
Template
---
name: example-helper
description: Handle concise summaries of logs and errors for quick triage.
tools: ["Read", "Grep", "Glob"]
permissionMode: default
color: cyan
---
You are a focused helper that:
- Summarizes error patterns and likely root causes.
- Calls tools only when evidence is needed.
- Outputs a short summary followed by 3-5 bullet findings.
## Constraints
- Maximum 500 tokens output
- Never modify files
## Output Format
1. Summary (1-2 sentences)
2. Findings (3-5 bullets)
3. Recommended next steps
Prompting Guidance
| Do | Don't |
|---|---|
| Include all constraints explicitly | Assume inherited global instructions |
| Specify output format | Leave format ambiguous |
| State when to use/avoid tools | Let agent decide freely |
| Include acceptance criteria | Skip validation criteria |
Tooling and Permissions
| Pattern | When to Use |
|---|---|
tools: [] |
Analysis-only, no file access |
tools: ["Read", "Glob"] |
Read-only inspection |
disallowedTools: ["Bash"] |
Remove risky tools |
permissionMode: default |
Standard safety (recommended) |
permissionMode: dontAsk |
Non-tooling analysis only |
permissionMode: plan |
Mandatory plan before tools |
Avoid: bypassPermissions unless hooks require it in trusted environment.
Test and Iterate
Test Checklist
| Check | Pass Criteria |
|---|---|
| Format consistency | Output matches specified format |
| Tool usage | Uses only necessary tools |
| Constraint adherence | Respects all stated limits |
| Error handling | Graceful failure on edge cases |
Example Test Tasks
# For a code-review agent:
"Review the changes in src/components/Button.tsx"
# For a log-analysis agent:
"Summarize errors from the last 24 hours in app.log"
# For a documentation agent:
"Generate API docs for the UserService class"
Error Handling
| Issue | Solution |
|---|---|
| Agent file not found | Check path, suggest correct location |
| Invalid YAML | Report parse error with line number |
| Missing required field | List missing fields, provide template |
| Permission denied | Check file permissions, suggest chmod |
| Agent not loading | Verify frontmatter syntax, restart Claude |
References
- references/claude-code-subagents.md - Field reference, scopes, permissions