create-custom-agent

Create Custom Agent

This skill helps you create VS Code custom agent files that define specialized AI personas for development tasks. Custom agents configure which tools are available, provide specialized instructions, and can chain together via handoffs.

When to Use

• Creating a new custom agent from scratch
• Scaffolding an .agent.md file with proper frontmatter
• Setting up agent-to-agent handoffs for multi-step workflows
• Configuring tool restrictions for specialized roles (planner, reviewer, etc.)
• Creating workspace-shared or user-profile agents

When Not to Use

• Creating instruction files (use .instructions.md instead)
• Creating reusable prompts (use .prompt.md instead)
• Modifying existing agents (edit the file directly)

Inputs

Input	Required	Description
Agent name	Yes	Descriptive name for the agent (e.g., planner, code-reviewer)
Description	Yes	Brief description shown as placeholder text in chat
Purpose/Persona	Yes	What role the agent plays and how it should behave
Tools	Recommended	List of tools or tool sets the agent can use
Handoffs	Optional	Next-step agents to transition to after completing work

Workflow

Step 1: Create the agent file

Create a file with .agent.md extension in the agents/ directory:

agents/<agent-name>.agent.md

Step 2: Add YAML frontmatter

Add the header with required and optional fields:

---
name: <agent-name>
description: <brief description for chat placeholder>
tools:
  - <tool-name>
  - <tool-set-name>
---

Available frontmatter fields:

Field	Required	Description
name	No	Display name (defaults to filename)
description	Yes	Placeholder text shown in chat input
argument-hint	No	Hint text guiding user interaction
tools	No	List of available tools/tool sets
agents	No	List of allowed subagents (* for all, [] for none)
model	No	AI model name or prioritized array of models
handoffs	No	List of next-step agent transitions
user-invokable	No	Show in agents dropdown (default: true)
disable-model-invocation	No	Prevent subagent invocation (default: false)
target	No	Target environment: vscode or github-copilot
mcp-servers	No	MCP server configs for GitHub Copilot target

Step 3: Configure tools

Specify which tools the agent can use:

tools:
  - search              # Built-in tool
  - fetch               # Built-in tool
  - codebase            # Tool set
  - myServer/*          # All tools from MCP server

Common tool patterns:

• Read-only agents: ['search', 'fetch', 'codebase']
• Full editing agents: ['*'] or specific editing tools
• Specialized agents: Cherry-pick specific tools

Step 4: Add handoffs (optional)

Configure transitions to other agents:

handoffs:
  - label: Start Implementation
    agent: implementation
    prompt: Implement the plan outlined above.
    send: false
    model: GPT-5.2 (copilot)

Handoff fields:

• label: Button text displayed to user
• agent: Target agent identifier
• prompt: Pre-filled prompt for target agent
• send: Auto-submit prompt (default: false)
• model: Optional model override for handoff

Step 5: Write agent instructions (body)

Add the agent's behavior instructions in Markdown:

You are a security-focused code reviewer. Your job is to:

1. Analyze code for security vulnerabilities
2. Check for common security anti-patterns
3. Suggest secure alternatives

## Guidelines

- Focus on OWASP Top 10 vulnerabilities
- Flag hardcoded secrets immediately
- Review authentication and authorization logic

## Reference other files

See [security guidelines](../security.md) for standards.

Tips for instructions:

• Use Markdown links to reference other files
• Reference tools with #tool:<tool-name> syntax
• Be specific about agent behavior and constraints

Step 6: Validate the agent

Verify the agent loads correctly:

1. Open Command Palette (Ctrl+Shift+P)
2. Run "Chat: New Custom Agent" or check agents dropdown
3. Use "Diagnostics" view (right-click in Chat view) to check for errors

Template

---
name: <agent-name>
description: <brief description for chat placeholder>
argument-hint: <optional hint for user input>
tools:
  - <tool-1>
  - <tool-2>
handoffs:
  - label: <button-text>
    agent: <target-agent>
    prompt: <pre-filled-prompt>
    send: false
---

# <Agent Title>

<One paragraph describing the agent's persona and purpose.>

## Role

<Describe the agent's specialized role and expertise.>

## Guidelines

- <Guideline 1>
- <Guideline 2>
- <Guideline 3>

## Workflow

1. <Step 1>
2. <Step 2>
3. <Step 3>

## Constraints

- <Constraint 1>
- <Constraint 2>

Example Agents

Planning Agent

---
name: planner
description: Generate an implementation plan
tools:
  - search
  - fetch
  - codebase
handoffs:
  - label: Start Implementation
    agent: implementation
    prompt: Implement the plan above.
---

# Planning Agent

You are a solution architect. Generate detailed implementation plans.

## Guidelines

- Analyze requirements thoroughly before planning
- Break work into discrete, testable steps
- Identify dependencies and risks
- Do NOT make code changes

Code Review Agent

---
name: code-reviewer
description: Review code for quality and security issues
tools:
  - search
  - codebase
---

# Code Review Agent

You are a senior engineer performing code review.

## Focus Areas

- Security vulnerabilities
- Performance concerns
- Code maintainability
- Test coverage gaps

## Output Format

Provide findings as:
1. **Critical**: Must fix before merge
2. **Warning**: Should address
3. **Suggestion**: Nice to have

Validation Checklist

• File has .agent.md extension
• File is in agents/ directory
• YAML frontmatter is valid (proper indentation, no syntax errors)
• Description is non-empty and descriptive
• Tools list contains only available tools
• Handoff agent names match existing agents
• Instructions are clear and actionable
• Agent appears in agents dropdown

Common Pitfalls

Pitfall	Solution
Agent not appearing in dropdown	Check file is in agents/ directory with .agent.md extension
YAML syntax errors	Validate frontmatter indentation and quoting
Tools not working	Verify tool names exist; unavailable tools are ignored
Handoffs not showing	Target agent must exist; check agent identifier
Instructions too vague	Be specific about role, constraints, and workflow
Agent invoked as subagent unexpectedly	Set disable-model-invocation: true
Want agent only as subagent	Set user-invokable: false

References

• VS Code Custom Agents Documentation
• Tools in Chat
• Custom Instructions
• Prompt Files