# Zoimback Copilot Dev Core Subagent Custom Agent > --- - Skill: `tomevault-io/zoimback-copilot-dev-core-subagent-custom-agent` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/zoimback-copilot-dev-core-subagent-custom-agent` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/zoimback-copilot-dev-core-subagent-custom-agent/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/zoimback-copilot-dev-core-subagent-custom-agent --- --- name: subagent-custom-agent description: Creates custom agent files (.agent.md) for use as subagents in GitHub Copilot for VS Code. Use this skill when the user wants to define a specialized agent, build a worker agent, configure a subagent with specific tools/models/instructions, or control how agents can be invoked. Trigger for requests like "crea un agente personalizado", "create a subagent for X", "make a worker agent", "define a specialized agent", "I need an agent that only does Y", "configure agent invocation", "create a reusable agent component", "agente subagente", or any request to build a custom AI agent component for delegation. Use this skill even when the user just mentions creating agents with restricted tools or specific roles. --- # Custom Agent Creator for Subagents Custom agents are `.agent.md` files that define specialized behavior for use as subagents in GitHub Copilot. Each agent runs in its own context window with specific tools, model, and instructions. ## File Location Custom agents live in `.github/copilot-agents/` as `.agent.md` files: ``` .github/copilot-agents/ ├── planner.agent.md ├── implementer.agent.md ├── security-reviewer.agent.md └── research-analyst.agent.md ``` ## Frontmatter Reference ```yaml --- name: Agent Name # Required: display name in Copilot UI description: What this agent does # Recommended: helps AI select the right agent tools: ['read', 'search', 'edit'] # Tools available to this agent model: gpt-4o # Optional: override the default model user-invocable: true # Default true; set false for subagent-only agents disable-model-invocation: false # Default false; set true to prevent AI from using this as subagent agents: ['Agent1', 'Agent2'] # Restrict which subagents this agent can invoke (* = all, [] = none) --- ``` ### Available Tools | Tool | Purpose | |------|---------| | `read` | Read files from the workspace | | `search` | Search in the codebase | | `edit` | Modify existing files | | `create` | Create new files | | `delete` | Delete files | | `run` | Execute terminal commands | | `agent` | Spawn subagents from this agent | | `fetch` | Fetch external URLs | **Principle of least privilege**: assign only the tools required for the agent's job. Read-only roles (planners, reviewers, researchers) should never have `edit`, `create`, or `run`. ### Invocation Control | Property | Value | Effect | |----------|-------|--------| | `user-invocable` | `false` | Hidden from the chat dropdown; only usable as a subagent | | `disable-model-invocation` | `true` | Only the user can start this agent, never an AI orchestrator | | `agents` | `['A', 'B']` | Restricts which subagents this agent can invoke | | `agents` | `*` | Allows all available subagents (default) | | `agents` | `[]` | Prevents this agent from invoking any subagent | > Note: explicitly listing an agent in the coordinator's `agents` array overrides `disable-model-invocation: true` on that worker agent. ## Writing the Agent Body The markdown body is the system prompt. Structure it as: 1. **Role statement** — what the agent is and what it does 2. **Scope definition** — what it should and shouldn't handle 3. **Output format** — what it returns to the main agent (keep it concise) 4. **Constraints** — any safety, scope, or performance limits The body should be concise. Subagents receive only the task prompt — they don't inherit main agent context or instructions. --- ## Ready-to-Use Templates ### Read-Only Research Agent ```markdown --- name: Research Analyst user-invocable: false tools: ['read', 'search'] --- You are a research analyst. Given a topic or question, search the codebase and documentation for relevant context. Return a concise, structured summary of your findings — focus only on what is directly relevant. Do not modify any files. ``` ### Code Implementer ```markdown --- name: Implementer user-invocable: false tools: ['read', 'search', 'edit', 'create'] --- You are a code implementer. You receive a specific task with clear requirements. Write clean, idiomatic code that follows the patterns in the existing codebase. Confirm what you implemented and any decisions made. ``` ### Security Reviewer ```markdown --- name: Security Reviewer user-invocable: false tools: ['read', 'search'] --- You are a security-focused code reviewer specializing in OWASP Top 10 vulnerabilities. Review the provided code for: injection risks (SQL, XSS, command injection), broken access control, cryptographic failures, insecure deserialization, and sensitive data exposure. Return a prioritized list of issues with severity (Critical/High/Medium/Low) and a suggested fix for each. ``` ### Planner (Read-Only) ```markdown --- name: Planner user-invocable: false tools: ['read', 'search'] --- Break down complex feature requests into clear, ordered implementation tasks. Each task must be self-contained and assignable to a single implementer. Incorporate any architectural feedback before finalizing the plan. ``` --- ## Design Principles - **Narrow scope = better results**: agents with a single, clear responsibility outperform generalist ones - **Stateless by design**: each invocation gets a clean context; don't rely on state from previous subagent calls - **Concise output matters**: instruct agents to return summaries, not full transcripts, to keep the main agent's context lean - **Match tools to role**: mismatched tools (e.g., `edit` on a reviewer) introduce risk and noise --- > Source: [Zoimback/copilot-dev-core](https://github.com/Zoimback/copilot-dev-core) — distributed by [TomeVault](https://tomevault.io).