Claude Code Source Code Analysis

Earlier today (March 31st, 2026) - Chaofan Shou on X discovered something that Anthropic probably didn’t want the world to see: the entire source code of Claude Code, Anthropic’s official AI coding CLI, was sitting in plain sight on the npm registry via a sourcemap file bundled into the published package. I’ve maintained a backup of that code on GitHub here but that’s not the fun part Let’s dive deep into what’s in it, how the leak happened and most importantly, the things we now know that were never meant to be public. How Did This Even Happen? This is the part that honestly made me go “…really?” When you publish a JavaScript/TypeScript package to npm, the build toolchain often generates source map files (.map files). These files are a bridge between the minified/bundled production code and the original source, they exist so that when something crashes in production the stack trace can point you to the actual line of code in the original file, not some unintelligible line 1, column 48293 of a minified blob. But the fun part is source maps contain the original source code. The actual, literal, raw source code, embedded as strings inside a JSON file. The structure of a .map file looks something like this: { "version": 3, "sources": ["../src/main.tsx", "../src/tools/BashTool.ts", "..."], "sourcesContent": ["// The ENTIRE original source code of each file", "..."], "mappings": "AAAA,SAAS,OAAO..." } That sourcesContent array? That’s everything. Every file. Every comment. Every internal constant. Every system prompt. All of it, sitting right there in a JSON file that npm happily serves to anyone who runs npm pack or even just browses the package contents. This is not a novel attack vector. It’s happened before and honestly it’ll happen again. The mistake is almost always the same: someone forgets to add *.map to their .npmignore or doesn’t configure their bundler to skip source map generation for production builds. With Bun’s bundler (which Claude Code uses), source maps are generated by default unless you explicitly turn them off. The funniest part is, there’s an entire system called “Undercover Mode” specifically designed to prevent Anthropic’s internal information from leaking. They built a whole subsystem to stop their AI from accidentally revealing internal codenames in git commits… and then shipped the entire source in a .map file, likely by Claude. What’s Claude Under The Hood? If you’ve been living under a rock, Claude Code is Anthropic’s official CLI tool for coding with Claude and the most popular AI coding agent. From the outside, it looks like a polished but relatively simple CLI. From the inside, It’s a 785KB main.tsx entry point, a custom React terminal renderer, 40+ tools, a multi-agent orchestration system, a background memory consolidation engine called “dream,” and much more Enough yapping, here’s some parts about the source code that are genuinely cool that I found after an afternoon deep dive: BUDDY - A Tamagotchi Inside Your Terminal I am not making this up. Claude Code has a full Tamagotchi-style companion pet system called “Buddy.” A deterministic gacha system with species rarity, shiny variants, procedurally generated stats, and a soul description written by Claude on first hatch like OpenClaw. The entire thing lives in buddy/ and is gated behind the BUDDY compile-time feature flag. The Gacha System Your buddy’s species is determined by a Mulberry32 PRNG, a fast 32-bit pseudo-random number generator seeded from your userId hash with the salt 'friend-2026-401': // Mulberry32 PRNG - deterministic, reproducible per-user function mulberry32(seed: number): () => number { return function() { seed |= 0; seed = seed + 0x6D2B79F5 | 0; var t = Math.imul(seed ^ seed >>> 15, 1 | seed); t = t + Math.imul(t ^ t >>> 7, 61 | t) ^ t; return ((t ^ t >>> 14) >>> 0) / 4294967296; } } Same user always gets the same buddy. 18 Species (Obfuscated in Code) The species names are hidden via String.fromCharCode() arrays - Anthropic clearly didn’t want these showing up in string searches. Decoded, the full species list is: RaritySpecies Common (60%)Pebblecrab, Dustbunny, Mossfrog, Twigling, Dewdrop, Puddlefish Uncommon (25%)Cloudferret, Gustowl, Bramblebear, Thornfox Rare (10%)Crystaldrake, Deepstag, Lavapup Epic (4%)Stormwyrm, Voidcat, Aetherling Legendary (1%)Cosmoshale, Nebulynx On top of that, there’s a 1% shiny chance completely independent of rarity. So a Shiny Legendary Nebulynx has a 0.01% chance of being rolled. Dang. Stats, Eyes, Hats, and Soul Each buddy gets procedurally generated: 5 stats: DEBUGGING, PATIENCE, CHAOS, WISDOM, SNARK (0-100 each) 6 possible eye styles and 8 hat options (some gated by rarity) A “soul” as mentioned, the personality generated by Claude on first hatch, written in character The sprites are rendered as 5-line-tall, 12-character-wide ASCII art with multiple animation frames. There are idle animations, reaction animations, and they sit next to your input prompt. The Lore The code references April 1-7, 2026 as a teaser window (so probably for easter?), with a full launch gated for May 2026. The companion has a system prompt that tells Claude: A small {species} named {name} sits beside the user's input box and occasionally comments in a speech bubble. You're not {name} - it's a separate watcher. So it’s not just cosmetic - the buddy has its own personality and can respond when addressed by name. I really do hope they ship it. KAIROS - “Always-On Claude” Inside assistant/, there’s an entire mode called KAIROS i.e. a persistent, always-running Claude assistant that doesn’t wait for you to type. It watches, logs, and proactively acts on things it notices. This is gated behind the PROACTIVE / KAIROS compile-time feature flags and is completely absent from external builds. How It Works KAIROS maintains append-only daily log files - it writes observations, decisions, and actions throughout the day. On a regular interval, it receives prompts that let it decide whether to act proactively or stay quiet. The system has a 15-second blocking budget, any proactive action that would block the user’s workflow for more than 15 seconds gets deferred. This is Claude trying to be helpful without being annoying. Brief Mode When KAIROS is active, there’s a special output mode called Brief, extremely concise responses designed for a persistent assistant that shouldn’t flood your terminal. Think of it as the difference between a chatty friend and a professional assistant who only speaks when they have something valuable to say. Exclusive Tools KAIROS gets tools that regular Claude Code doesn’t have: ToolWhat It Does SendUserFilePush files directly to the user (notifications, summaries) PushNotificationSend push notifications to the user’s device SubscribePRSubscribe to and monitor pull request activity ULTRAPLAN - 30-Minute Remote Planning Sessions Here’s one that’s wild from an infrastructure perspective. ULTRAPLAN is a mode where Claude Code offloads a complex planning task to a remote Cloud Container Runtime (CCR) session running Opus 4.6, gives it up to 30 minutes to think, and lets you approve the result from your browser. The basic flow: Claude Code identifies a task that needs deep planning It spins up a remote CCR session via the tengu_ultraplan_model config Your terminal shows a polling state - checking every 3 seconds for the result Meanwhile, a browser-based UI lets you watch the planning happen and approve/reject it When approved, there’s a special sentinel value ULTRAPLAN_TELEPORT_LOCAL that “teleports” the result back to your local terminal The “Dream” System - Claude Literally Dreams Okay this is genuinely one of the coolest things in here. Claude Code has a system called autoDream (services/autoDream/) - a background memory consolidation engine that runs as a forked subagent. The naming is very intentional. It’s Claude… dreaming. This is extremely funny because I had the same idea for LITMUS last week - OpenClaw subagents creatively having leisure time to find fun new papers The Three-Gate Trigger The dream doesn’t just run whenever it feels like it. It has a three-gate trigger system: Time gate: 24 hours since last dream Session gate: At least 5 sessions since last dream Lock gate: Acquires a consolidation lock (prevents concurrent dreams) All three must pass. This prevents both over-dreaming and under-dreaming. The Four Phases When it runs, the dream follows four strict phases from the prompt in consolidationPrompt.ts: Phase 1 - Orient: ls the memory directory, read MEMORY.md, skim existing topic files to improve. Phase 2 - Gather Recent Signal: Find new information worth persisting. Sources in priority: daily logs → drifted memories → transcript search. Phase 3 - Consolidate: Write or update memory files. Convert relative dates to absolute. Delete contradicted facts. Phase 4 - Prune and Index: Keep MEMORY.md under 200 lines AND ~25KB. Remove stale pointers. Resolve contradictions. The prompt literally says: “You are performing a dream - a reflective pass over your memory files. Synthesize what you’ve learned recently into durable, well-organized memories so that future sessions can orient quickly.” The dream subagent gets read-only bash - it can look at your project but not modify anything. It’s purely a memory consolidation pass. Undercover Mode - “Do Not Blow Your Cover” This one is fascinating from a corporate strategy perspective. Anthropic employees (identified by USER_TYPE === 'ant') use Claude Code on public/open-source repositories. Undercover Mode (utils/undercover.ts) prevents the AI from accidentally revealing internal information in commits and PRs. When active, it injects this into the system prompt: ## UNDERCOVER MODE - CRITICAL You are operating UNDERCOVER in a PUBLIC/OPEN-SOURCE repository. Your commit messages, PR titles, and PR bodies MUST NOT contain ANY Anthropic-internal information. Do not blow your cover. NEVER include in commit messages or PR descriptions: - Internal model codenames (animal names like Capybara, Tengu, etc.) - Unreleased model version numbers (e.g., opus-4-7, sonnet-4-8) - Internal repo or project names - Internal tooling, Slack channels, or short links (e.g., go/cc, #claude-code-…) - The phrase "Claude Code" or any mention that you are an AI - Co-Authored-By lines or any other attribution The activation logic: CLAUDE_CODE_UNDERCOVER=1 forces it ON (even in internal repos) Otherwise it’s automatic: active UNLESS the repo remote matches an internal allowlist There is NO force-OFF - “if we’re not confident we’re in an internal repo, we stay undercover.” So this confirms: Anthropic employees actively use Claude Code to contribute to open-source - and the AI is told to hide that it’s an AI Internal model codenames are animal names - Capybara, Tengu, etc. “Tengu” appears hundreds of times as a prefix for feature flags and analytics events - it’s almost certainly Claude Code’s internal project codename All of this is dead-code-eliminated from external builds. But source maps don’t care about dead code elimination. Makes me wonder how much are they internally causing havoc to open source repos Multi-Agent Orchestration - “Coordinator Mode” Claude Code has a full multi-agent orchestration system in coordinator/, activated via CLAUDE_CODE_COORDINATOR_MODE=1. When enabled, Claude Code transforms from a single agent into a coordinator that spawns, directs, and manages multiple worker agents in parallel. The coordinator system prompt in coordinatorMode.ts is a masterclass in multi-agent design: PhaseWhoPurpose ResearchWorkers (parallel)Investigate codebase, find files, understand problem SynthesisCoordinatorRead findings, understand the problem, craft specs ImplementationWorkersMake targeted changes per spec, commit VerificationWorkersTest changes work The prompt explicitly [truncated for AI cost control]