I Read Claude Code's 510K Lines of Source Code — Here's How It Actually Works

I spent the last few weeks reading through Claude Code's source — all 510,000 lines of TypeScript across 1,903 files. The code became available through an accidental npm source map leak, and my team and I documented our findings in a full teardown on GitHub . Here are the five architectural decisions that stuck with me most. 1. The Entire Agent Runs From a Single 1,729-Line File The brain of Claude Code is src/query.ts — one file, 1,729 lines, running the entire agentic loop. No state machine. No event-driven architecture. Just a while(true) loop: while ( true ) { ① Trim context ( 4 - layer cascade ) ② Pre - fetch memory + skills ③ Call Claude API ( streaming ) ④ While receiving stream → detect tool_use blocks → Start executing tools IMMEDIATELY ⑤ Tools called ? → append results → continue loop ⑥ No tools ? → return response → exit } This file handles input processing, API calls, streaming parsing, tool dispatch, error recovery, and context management. It's the textbook definition of a