Claude Code Under the Hood: How It Actually Works

In March 2026, Anthropic accidentally published a source map in an npm package. That map pointed to a zip on R2. That zip contained unobfuscated TypeScript for Claude Code. Someone posted it on X. And suddenly, a lot of us had front-row seats to how a modern coding agent is actually built. I spent some time digging through the code, docs, and community notes. This post is the version I wish I had on day one: practical, opinionated. No hype. Just architecture, tradeoffs, and what this means if you build AI tooling. Quick take Claude Code is not magic. It is a very disciplined system around a very capable model. The model is the brain. The product quality comes from the nervous system: loop orchestration, permissions, context compaction, tool contracts, caching, retries, and UI responsiveness. You can clone the outer loop in a weekend. You cannot clone the reliability story in a weekend. The moment it clicked for me I had that "oh" moment while tracing one simple task: "fix failing tests