Gemini Extension Authoring

Provide expert guidance on creating, packaging, and publishing extensions for the Gemini CLI.

Extension Structure

Organize Gemini CLI extensions as self-contained directories including commands, hooks, MCP servers, and other resources.

Maintain the following extension directory structure:

my-extension/
├── gemini-extension.json  (Required: Manifest)
├── README.md              (Required: Documentation)
├── LICENSE                (Required: License)
├── hooks/
│   ├── hooks.json         (Optional: Hook definitions)
│   └── my_hook.py         (Optional: Hook script)
├── commands/              (Optional: Custom commands)
├── mcp-servers/           (Optional: MCP server code)
├── skills/                (Optional: Agent skills)
└── agents/                (Optional: Sub-agent definitions)

Core Components

The Manifest (gemini-extension.json)

Use the gemini-extension.json manifest to define metadata, configuration settings, dependencies, and entry points for hooks or MCP servers.

Key Fields:

• name: Unique identifier (kebab-case).
• version: Semantic version (e.g., "1.0.0").
• mcpServers: Definitions for bundled MCP servers.
• settings: Configuration options exposed to the user.
• dependencies: Required system tools (e.g., {"python": ">=3.7"}).

Consult references/extension-schema.md for the full schema specification.

Hooks Configuration (hooks.json)

Define hooks in a JSON file to intercept and modify CLI behavior. Reference this file in the manifest.

Structure:

• Group by event name (e.g., AfterAgent, BeforeTool).
• Use matcher to filter events (e.g., * for all, or specific tool names).
• Define the command to execute.

Best Practice: Use ${extensionPath} in command strings to reference files within the extension directory reliably.

"command": "${extensionPath}/hooks/my_script.py"

Consult references/hooks-schema.md for detailed event types and configuration options.

MCP Servers

Bundle MCP servers to expose tools and resources to the model.

Configuration:

Define servers in gemini-extension.json under the mcpServers object.

"mcpServers": {
  "my-server": {
    "command": "node",
    "args": ["${extensionPath}/mcp-server/index.js"],
    "env": {
      "API_KEY": "${env:MY_API_KEY}"
    }
  }
}

• Note: Ensure all standard MCP configuration options are supported except trust.
• Conflict Resolution: Ensure servers compliant with gemini-extension.json override those in settings.json if names collide.

Agent Skills

Provide specialized workflows by placing skills in a skills/ directory.

skills/
└── security-audit/
    └── SKILL.md

The model automatically discovers skills within this directory.

Recommendation: To learn how to write effective skills, activate the Skill Development skill.

activate_skill("Skill Development")

Sub-agents (Experimental)

Bundle sub-agents by placing markdown definitions in an agents/ directory.

agents/
├── security-auditor.md
└── database-expert.md

Consult references/agents-schema.md for configuration details.

Custom Commands

Add slash commands by placing TOML configuration files in a commands/ directory.

commands/
├── deploy.toml  -> /deploy
└── gcs/
    └── sync.toml -> /gcs:sync

Consult references/commands-schema.md for the TOML specification.

Conflict Resolution: Handle name collisions between user commands and extension commands:

• User command: /deploy
• Extension command: /extension-name.deploy

Development Workflow

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Additional Resources

Reference Files

• references/extension-schema.md - Detailed gemini-extension.json schema.
• references/hooks-schema.md - Detailed hooks.json schema and event types.
• references/agents-schema.md - Configuration for sub-agents (agents/).
• references/commands-schema.md - Configuration for custom commands (commands/).

Examples

• examples/gemini-extension.json - Complete manifest example.
• examples/hooks.json - Complete hooks configuration example.