Claude Code Architecture Explained: Agent Loop, Tool System, and Permission Model (Rust Rewrite Analysis)

Claude Code Deep Dive (Part 1): Architecture Overview and the Core Agent Loop Claude Code’s leaked source code weighs in at over 510,000 lines of TypeScript—far too large to analyze directly. Interestingly, a community-driven Rust rewrite reduced that complexity to around 20,000 lines, while still preserving the core functionality. Starting from this simplified version makes one thing much clearer: What does an AI agent system actually need to work? Why Start with the Rust Rewrite? On March 31, 2026, Claude Code’s full source was unintentionally exposed due to an npm packaging mistake. The package @anthropic-ai/claude-code v2.1.88 included a 59.8MB source map file, which allowed anyone to reconstruct the original TypeScript codebase. To clarify: The official GitHub repo always existed But it only contained compiled bundles and documentation The readable source code was not normally accessible The Problem with the Original Codebase Most analyses focused on the leaked TypeScript code: 51