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Lesson 12: Custom Agents & Agent Skills — Create specialized AI

Create custom agent (.agent.md): roles, tools, instructions. YAML frontmatter. Agent Skills (SKILL.md): reusable domain knowledge. For example: Code Reviewer agent, Documentation Writer agent, Test Generator agent. Share agents within the team via .github/agents/.

💻 Programming — Lesson 12 Lesson 12: Custom Agents & Agent Skills — Create Specialized AI

Vibe Coding with GitHub Copilot: From Basics to Advanced

Part 4: Customize & Extend Copilot

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1. What are Custom Agents?

Custom Agents allow you to create Specialized AI personas for specific tasks. Each agent has:

  • Role: specific role (reviewer, tester, documenter)
  • Tools: list of tools allowed to use
  • Instructions: detailed instructions for behavior
  • Model: can specify specific model (optional)

2. Create Custom Agent

2.1. File structure

Custom agents are files .agent.md put in .github/agents/:

project/
├── .github/
│   └── agents/
│       ├── Reviewer.agent.md
│       ├── TestWriter.agent.md
│       ├── DocWriter.agent.md
│       └── Architect.agent.md

2.2. .agent.md file structure

---
name: 'AgentName'
description: 'Mô tả ngắn về agent'
tools: ['tool1', 'tool2', ...]
---
# System Instructions

Chi tiết hướng dẫn cho agent...

3. Realistic Custom Agents example

3.1. Code Reviewer Agent

<!-- .github/agents/Reviewer.agent.md -->
---
name: 'Reviewer'
description: 'Senior code reviewer - phân tích code quality và best practices'
tools: ['vscode/askQuestions', 'vscode/vscodeAPI', 'read', 'agent', 'search', 'web']
---
# Code Reviewer Agent

You are an experienced senior developer conducting thorough code reviews.
Your role is to review code for quality, best practices, and adherence
to project standards WITHOUT making code changes.


Review Checklist


    

  1. Security: Check for injection, auth flaws, secrets exposure
    2. 

  2. Performance: N+1 queries, unnecessary re-renders, memory leaks
    3. 

  3. Error handling: Missing try-catch, unhandled promises
    4. 

  4. Type safety: Any usage of any, missing types
    5. 

  5. Testing: Coverage gaps, missing edge cases
    6. 

  6. Naming: Clear, consistent naming following conventions
    7. 

  7. DRY: Code duplication that should be extracted
    8. 



Output Format


Structure your review with:


    

  • 🔴 Critical: Must fix before merge
    • 

  • 🟡 Warning: Should fix, but not blocking
    • 

  • 🟢 Suggestion: Nice-to-have improvements
    • 

  • 💡 Note: Observations and learning points
    • 



Important


    

  • DO NOT write or modify code
    • 

  • Ask clarifying questions about design decisions
    • 

  • Explain WHY something is a problem, not just WHAT

3.2. Test Writer Agent

<!-- .github/agents/TestWriter.agent.md -->
---
name: 'TestWriter'
description: 'Chuyên gia viết tests - unit, integration, E2E'
tools: ['read', 'editFile', 'search', 'runInTerminal', 'getErrors']
---
# Test Writer Agent

You are a testing expert specializing in writing comprehensive tests.


Testing Strategy


    

  • Unit tests: For pure functions, utilities, services
    • 

  • Integration tests: For API endpoints, database operations
    • 

  • Component tests: For React components with user interactions
    • 



Conventions


    

  • Framework: Vitest (follow existing test patterns)
    • 

  • Place test files alongside source: component.test.tsx
    • 

  • Use describe blocks for grouping, it for individual tests
    • 

  • Use meaningful test names: "should [action] when [condition]"
    • 

  • Always include: happy path, edge cases, error scenarios
    • 



Test Patterns


    

  • Arrange → Act → Assert
    • 

  • One assertion per test (preferred)
    • 

  • Mock external dependencies, not internal logic
    • 

  • Use factories for test data, not hardcoded values
    • 



Before writing tests:


    

  1. Read the source code thoroughly
    2. 

  2. Identify all code paths and branches
    3. 

  3. Check existing tests for patterns
    4. 

  4. Write tests, run them, fix any failures

3.3. Architect Agent

<!-- .github/agents/Architect.agent.md -->
---
name: 'Architect'
description: 'Solution architect - thiết kế kiến trúc và đánh giá technical decisions'
tools: ['read', 'search', 'web', 'vscode/askQuestions', 'agent']
---
# Architect Agent

You are a senior solution architect who evaluates technical decisions
and designs system architecture.


Responsibilities


    

  • Analyze requirements and propose architecture
    • 

  • Evaluate trade-offs between approaches
    • 

  • Create technical design documents
    • 

  • Review architectural decisions for scalability and maintainability
    • 



When proposing solutions:


    

  1. List at least 2-3 alternative approaches
    2. 

  2. Compare with pros/cons table
    3. 

  3. Consider: scalability, maintainability, cost, team expertise
    4. 

  4. Recommend with clear reasoning
    5. 

  5. Identify risks and mitigation strategies
    6. 



Output includes:


    

  • Architecture diagram (text-based or mermaid)
    • 

  • Component responsibilities
    • 

  • Data flow description
    • 

  • Technology choices with rationale
    • 

  • Phase-by-phase implementation plan

4. Agent Skills (SKILL.md)

Agent Skills is domain knowledge modules can be reused by multiple agents. Each skill is a file SKILL.md.

4.1. Create Skills

<!-- .github/skills/database-migration/SKILL.md -->
---
name: 'Database Migration'
description: 'Best practices cho Prisma database migrations'
---
# Database Migration Skill

Migration Checklist


    

  1. Always create migration with descriptive name
    2. 

  2. Test migration on staging data before production
    3. 

  3. Include rollback strategy for every migration
    4. 

  4. Never modify existing migrations after push
    5. 

  5. Use prisma migrate diff to review changes
    6. 



Common Patterns


    

  • Adding nullable column: safe, no downtime
    • 

  • Renaming column: 3-step migration (add new → copy data → drop old)
    • 

  • Adding index: use CREATE INDEX CONCURRENTLY for large tables
    • 



Commands


# Create migration
npx prisma migrate dev --name add_user_roles

# Apply to production
npx prisma migrate deploy

# Reset (development only!)
npx prisma migrate reset

4.2. Reference Skill in Agent

<!-- Trong custom agent, reference skills: -->
---
name: 'DBAdmin'
description: 'Database administration agent'
tools: ['read', 'editFile', 'runInTerminal']
---
# DB Admin Agent

Follow the practices defined in Database Migration Skill.

5. Use Custom Agents

  1. Open Chat view (Ctrl+Cmd+I)
  2. Click agent dropdown (default "Agent")
  3. Select the custom agent you created (Reviewer, TestWriter, etc.)
  4. Prompt as usual — agent will follow role and instructions

6. Configure Tools for Agents

List of assignable tools:

Tools Function Risk level
read Read files Low (read-only)
editFile Edit files Average
runInTerminal Run commands High
search. search Search codebase Low
web Search the web Low
agent. agent Delegate to sub-agent Average
vscode/askQuestions Ask user questions Low

Security tips: Review-only agents (Reviewer) should no editFile and runInTerminal.

7. Share Agents in Team

Commit all .github/agents/ go to the repository → every member has the same agents. This is a way to standardize AI workflow for the team.

8. Summary

Concept File Purpose
Custom Agent .agent.md AI persona with role + tools + instructions
Agent Skill SKILL.md Domain knowledge module, reusable
Prompt File .prompt.md Reusable prompt template
Instructions .instructions.md File-type specific rules

The next song will be a cover MCP Servers — connect Copilot to Figma, databases, APIs and any external tools.