make-custom-agent

Create Custom Agent

This skill guides you through creating a custom GitHub Copilot agent — an @-invokable chat participant that extends Copilot with domain-specific expertise. Custom agents are distinct from Agent Skills: skills provide reusable instructions loaded on demand, while agents own the full conversational interaction and can orchestrate tools, call APIs, and maintain their own prompt strategies.

When Not to Use

• Adding reusable, invokable workflows — use Agent Skills (.agents/skills/) instead
• Adding background coding guidelines — use file-based instructions (.github/instructions/) instead
• Adding project-wide context for Copilot — use .github/copilot-instructions.md instead
• Creating reusable prompts — use .prompt.md instead

Workflow

Step 1: Choose the agent type

If the agent only needs a scoped system prompt and doesn't require custom code, start with a declarative agent.

Step 2: Create a declarative agent (prompt file)

Declarative agents are Markdown files in .github/agents/. VS Code and GitHub Copilot discover them automatically.

.github/agents/
└── <agent-name>.md        # Agent definition

Template:

---
name: my-agent
description: A short description of what this agent does and when to use it.
---

# <Agent Title>

You are an expert in <domain>. Your job is to:
- <behavior 1>
- <behavior 2>

## Guidelines

- <guideline 1>
- <guideline 2>

## Workflow

1. <step 1>
2. <step 2>

## Constraints

- <constraint 1>
- <constraint 2>

Supported frontmatter fields:

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 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, VS Code only)

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: Create an extension-based chat participant (VS Code only)

For full control, implement a VS Code extension with a chat participant:

1. Define the participant in package.json:

"contributes": {
    "chatParticipants": [
        {
            "id": "my-extension.my-agent",
            "name": "my-agent",
            "fullName": "My Agent",
            "description": "Short description shown in chat input",
            "isSticky": false,
            "commands": [
                {
                    "name": "explain",
                    "description": "Explain the selected code"
                }
            ]
        }
    ]
}

2. Register and implement the request handler in extension.ts:

export function activate(context: vscode.ExtensionContext) {
    const agent = vscode.chat.createChatParticipant('my-extension.my-agent', handler);
    agent.iconPath = vscode.Uri.joinPath(context.extensionUri, 'icon.png');
}

const handler: vscode.ChatRequestHandler = async (
    request: vscode.ChatRequest,
    context: vscode.ChatContext,
    stream: vscode.ChatResponseStream,
    token: vscode.CancellationToken
) => {
    const model = request.model;
    const messages = [
        vscode.LanguageModelChatMessage.User(request.prompt)
    ];
    const response = await model.sendRequest(messages, {}, token);
    for await (const fragment of response.text) {
        stream.markdown(fragment);
    }
};

3. Declare the extension dependency in package.json:

"extensionDependencies": ["github.copilot-chat"]

4. Add tool calling (optional)

Agents can invoke language model tools registered by other extensions:

const tools = vscode.lm.tools.filter(tool => tool.tags.includes('my-domain'));
const result = await chatUtils.sendChatParticipantRequest(request, context, {
    prompt: 'You are an expert in <domain>.',
    tools,
    responseStreamOptions: { stream, references: true, responseText: true }
}, token);
return await result.result;

Step 6: Create a GitHub App (Copilot Extension) for cross-surface availability (optional)

If the agent should be available on GitHub.com, Visual Studio, JetBrains, and VS Code simultaneously, implement a GitHub App that acts as a Copilot Extension. The app registers a webhook endpoint, receives chat requests, and streams responses back.

Key considerations:

• The GitHub App must be installed on the user's account or organization
• Responses are streamed via Server-Sent Events (SSE)
• Use the GitHub Copilot Extensions documentation for the full integration guide
• For VS Code-specific features (editor access, file trees, command buttons), prefer an extension-based participant instead

Step 7: Validate

After creating or modifying an agent, verify:

• name is lowercase, uses hyphens (no spaces), and is unique
• description clearly describes what the agent does and when to invoke it
• Frontmatter YAML is valid (no syntax errors)
• Declarative agent file is in .github/agents/
• Tools list contains only available tools
• Extension-based agent: participant ID matches in package.json and createChatParticipant call
• Agent does not duplicate functionality of built-in agents (@workspace, @vscode, @terminal)
• Handoff agent names match existing agents
• Agent instructions don't include secrets, tokens, or internal URLs

Common Pitfalls

References

• VS Code Chat Participant API
• VS Code AI Extensibility Overview
• VS Code Extension Samples – chat-sample
• GitHub Copilot Extensions documentation
• GitHub Copilot Custom agents configuration
• Agent Skills Specification
• make-skill
• make-instructions