GitHub Copilot CLI Extensions: The Most Powerful Feature Nobody's Talking About

There's No Documentation on This I'm going to say something that sounds absurd: GitHub Copilot CLI has a full extension system that lets you create custom tools, intercept every agent action, inject context, block dangerous operations, and auto-retry errors — and there's essentially zero public documentation about it. I'm not talking about MCP servers . I'm not talking about Copilot Extensions (the GitHub App kind). I'm talking about .github/extensions/ — a local extension system baked into the CLI agent harness that runs as a separate Node.js process, communicates over JSON-RPC, and gives you programmatic control over the entire agent lifecycle. You can literally tell the CLI "create me a tool that does X" and it will scaffold the extension file, hot-reload it, and the tool is available in the same session . No restart. No config. No marketplace. Just code. I had to extract this from the Copilot SDK source itself — the .d.ts type definitions, internal docs, and by building extensions