Claude Code's Entire Source Code Got Leaked via a Sourcemap in npm, Let's Talk About it — Kuber Mehtaᨒ MindDump

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Claude Code's Entire Source Code Got Leaked via a Sourcemap in npm, Let's Talk About it<br>by Kuber MehtaMar 31, 202617 min read<br>AI<br>ChatGPT<br>Social-Media<br>Artificial-Intelligence<br>GenerativeAI<br>Internet<br>OpenAI<br>Claude<br>Anthropic

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,<br>"sources": ["../src/main.tsx", "../src/tools/BashTool.ts", "..."],<br>"sourcesContent": ["// The ENTIRE original source code of each file", "..."],<br>"mappings": "AAAA,SAAS,OAAO..."<br>That sourcesContent array? That’s everything.<br>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...