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:

Open Command Palette (Ctrl+Shift+P)
Run "Chat: New Custom Agent" or check agents dropdown
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`

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