How to build a Gemini CLI extension including MCP and slash commands
👨💻 Build Your Own Gemini-CLI Extension from Scratch (Python MCP Tutorial)
Stop just using AI—start building with it! This video is a complete, step-by-step guide on how to create a fully functional, custom extension for the open-source Gemini CLI. We'll use a Subnet Calculator as our example, showing you how to wire a Python application into Gemini's powerful Model Context Protocol (MCP).
If you want to integrate a legacy tool, a custom script, or a specific API into your AI workflow, this is the tutorial for you!
🎬 What You'll Learn & See:
Project Setup (The Foundation):
Creating a dedicated Python venv and managing dependencies with requirements.txt.
Setting up the extension's file structure (including the commands/ directory).
The MCP Server (The Engine):
Writing the core FastMCP Python server using FastMCP and pydantic.
Defining the subnet_calculator function as a tool the AI can understand.
Custom Slash Commands (The User Interface):
Creating multiple .toml files for specialized slash commands (e.g., /subnet:calc, /subnet:hosts, /subnet:next).
Mapping user intent directly to tool execution for a fast, predictable CLI experience.
AI System Instructions (The Brain):
Writing the GEMINI.md file to define the AI's behavior, ensuring it knows when to use a slash command versus when to use the raw tool.
Testing and Deployment:
Running the extension locally and testing all custom commands within the Gemini CLI.
🔑 Key Concepts Covered:
Model Context Protocol (MCP): Understanding how to expose Python tools to the Gemini agent.
Structured Data with Pydantic: Using pydantic to define clear, typed input/output schemas for the AI.
Slash Command Design: Best practices for creating user-friendly, focused commands.
System Prompt Engineering: Using GEMINI.md to control the AI's reasoning and tool usage.
🔗 Resources & Code:
Final Extension Source Code: [Link to your extension's GitHub repository]
Gemini CLI Installation: [Link to the official Gemini CLI documentation]
Required Dependencies: fastmcp, pydantic, python-dotenv, ipaddress
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Install and try it yourself:
gemini extensions install https://github.com/automateyournetwor...
Source code:
https://github.com/automateyournetwor...