I Read the Leaked Claude Code Source – Here's What I Found

Summary

Today, the source code for Claude Code — Anthropic’s AI-powered CLI tool — leaked online. Naturally, I did what any curious developer would do.I read it. All of it.And what I found is genuinely fascinating. This isn’t some weekend hack or a thin wrapper around an API. Claude Code is one of the most sophisticated terminal applications I’ve ever seen — and it’s hiding some wild engineering decisions under the hood.Let me walk you through the highlights.The Numbers The leaked source tree. 35 top-level modules, from assistant to voice.First, let’s set the stage:~512,000 lines of TypeScript1,884 files across 35 top-level modules33 MB of source code80+ built-in tools (Read, Write, Edit, Bash, Agent, etc.)main.tsx alone is 803 KBThis is not a small project. This is a production-grade agentic system that happens to live in your terminal.They Built Their Own Terminal UI FrameworkThis is the one that made me sit up.The ink/ directory — roughly 50 files — is not the popular npm ink package. Anthropic built their own React-based terminal rendering engine from scratch.And it’s not a toy. It includes:A pure TypeScript port of Meta’s Yoga flexbox layout engine — no C++ bindings, no native dependencies. Flexbox. In your terminal.A custom React reconciler that goes from Fiber → DOM → Layout → Screen Buffer → Terminal outputMouse tracking with hit-testing (yes, you can click things in Claude Code)Text selection with word and line boundariesOSC 8 hyperlinks — clickable links in the terminalTerminal capability detection for Kitty, iTerm2, xterm.js, and multiplexersWhy build all this instead of using the existing ink package? My guess: performance and control. When your terminal app is running an AI agent that streams tokens in real-time while executing tools concurrently, you need tight control over rendering. Off-the-shelf solutions probably couldn’t keep up.The Dual-Track Permission SystemSecurity in Claude Code isn’t an afterthought — it’s deeply embedded in the architecture. And th...