Hello to all readers out there! Today, I'm going to tell you the story of what happened with **Anthropic** when they accidentally leaked *hundreds of thousands of lines of code*. I actually saw this news when it first broke, but I didn't pay much attention to it. That was until I had some free time and sat down to read about it while practicing for my own upcoming podcast. That's when I realized, *"Whoa! This is actually super interesting and there's so much to talk about."* But since I'm not quite ready to launch a podcast just yet, I'll write it as an article instead. Let's get into it… yeah!
What is Anthropic?
Before we dive in, let's get a brief introduction to the AI company that everyone in the tech industry praises as the **gods of AI model creators**. Anthropic is an AI company similar to **OpenAI** (who made *ChatGPT*) or **Google** (who made *Gemini*). But what makes Anthropic stand out is the sheer brilliance of **Claude AI**, which is incredibly famous among devs. It's so good that when I switched over from Gemini, I literally dropped to my knees thinking, *"…Guys, I'm about to lose my job T-T"*
For anyone who hasn't tried **Claude** yet, go give it a shot. The pricing is about the same as the others. Personally, I was paying for Gemini before, but in late 2025, Google announced they were giving **Gemini Pro for free to all students**. Instead of saving that money, Google's generosity just let me shift my budget to pay for Claude instead, hahaha! But seriously, it's really good. Go try it out.
The Mystery File in the NPM Registry
Going back to the end of last month, on **March 31, 2026**, a user with the Code Name: *Chaofan Shou* (no idea how to pronounce that, but it doesn't matter) found a strange file inside the `claude-code` package released for general users to download on the **NPM Registry**. This was a **60MB** file with a **`.map`** extension — a type of file that we normally don't see exposed to the public eye like this, as it's strictly meant for *internal use only*.
What is Minify Code?
Before we discuss the role of the **`.map`** file in this drama, let me first explain what **Minify Code** is. Minifying code is the process of taking our freshly written code and *compressing it to make it shorter*. This involves removing spaces, changing long function names to simply `a`, `b`, `c`, deleting comments, and stripping out development environment details. This shrinks the code size, allowing it to **load faster over a network**.
Even though many people think this is for security, it really just makes it *harder for anyone trying to reverse-engineer our code to read it*. It's not a true security feature — it just makes the process incredibly painful, kind of like sitting down to decipher ancient Egyptian hieroglyphs.
The Purpose of the .map File
We mentioned in the previous section that minifying code makes it shorter so it loads faster and is harder to read. But what if we want to decode that minified code? How do we do it? The answer is this exact .map file. It acts as a treasure map, telling us, "This piece of code connects here," and the final result gives us the Original code exactly as the developers wrote it.
Now, everyone can probably guess the absolute disaster that was about to unfold. The person who found the .map file cracked it open, took a look, and probably said, "Oh, S**,"*. Keeping something like this to yourself just feels wrong, right? Stuff like this belongs on GitHub! People rushed to download it hundreds of thousands of times, probably setting a new record for the fastest-downloaded open-source leak in history, if I remember correctly.
The Unlocked Treasure
After obtaining the key, Anthropic's code vault containing over 500,000 lines of Claude AI's source code was laid bare. Most of it was written in TypeScript, managing various systems of the Claude Agent, such as how it handles long-term memory, how different Agents communicate with each other, unreleased features, and most importantly: the authentication system between users and the server, commonly known as OAuth.
Buddy
The most talked-about feature in the leaked code was "Buddy." Its character behaves similarly to the Tamagotchis we (well, maybe not me) used to raise as kids. It was designed to have multiple species, and you couldn't choose which Buddy you got; it was randomized based on your User ID. However, this feature was originally planned to launch on April 1, 2026. The Reddit community suspected it was just an April Fools' joke, and ultimately, things went quiet.
Post-Incident Risks
As I mentioned, the blueprint of the OAuth system was exposed. Many might think this is the equivalent of broadcasting our country's top-secret military strategy to our enemies. But hold on! In reality, the client-side OAuth system is designed so that it doesn't need to store any secrets in the first place. The fact that this part of the code leaked doesn't mean hackers can instantly breach the login system. Unless, of course, the team was careless enough to accidentally hardcode an API Key or important Secret inside it (which Anthropic confirmed did not happen; no sensitive data was leaked). Even so, it's still a risk that Anthropic has to deal with and keep a close eye on right now.
Bun
We haven't talked about another fascinating aspect yet: what exactly caused this .map file to leak? Before I tell you that, let me introduce you to Bun. Bun is both a JavaScript Runtime and a Package Manager (think of it as an App Store for JS/TS Devs) used in JavaScript and TypeScript development. It has become incredibly popular among developers over the past 2-3 years, and Anthropic is one of the companies using Bun in this incident.
To elaborate a bit, a JavaScript Runtime is a tool that allows us to run JavaScript code on our computers. Normally, we'd use Node.js, but as I said, over the last 2–3 years, Bun has delivered phenomenal performance, so it's no surprise Anthropic chose to use it.
.ignore
For the developers reading this, feel free to skip this section. But for anyone who hears this file name and feels confused, today I'll explain simply what an .ignore file is for.
If I had to compare an .ignore file to something in daily life, I'd use the example of setting up a keyword filter on Facebook. In that filter, we put banned words like "F**", "S*"*, or "Voldemort". So, when we're about to post something, Facebook reads our keyword file. If it finds one of those words, it will automatically ignore it or block it from showing up right from the start, before we ever release it to the public eye.
The .ignore file does something very similar to those Facebook keywords. It's a file inside our project where devs write down the names of files or folders we don't want to release, telling the system to skip them during the packaging process. Examples include .env, .map, or files containing sensitive information. You can read how to write an .ignore file here: https://linuxize.com/post/gitignore-ignoring-files-in-git/
But in a single project, there are way too many files we want to hide, and typing them out one by one manually is a pain. Therefore, Bun or NPM provides templates right from the start of the project. It's like the system asking, "Hey bro, what kind of project are you building? Oh, you're doing that? I've got the perfect template ready for you, just click and pull it in." This template contains pre-thought-out file and folder names that should be hidden. It's an incredibly useful feature.
The Cause of the Leak
As I said, the .ignore template feature is super useful. But if we trust this template too much and forget to double-check, or if we accidentally put a secret file into the project and that file happens not to be in our template, or if the tool we're using (like Bun) happens to have a bug... that's when absolute disaster strikes.
And yes, this incident was caused by a bug in the Bun Bundler itself, which accidentally generated the .map file alongside the build, even though it was explicitly commanded to run in Production mode! This specific bug, known as Issue #28001, had been reported since March 11, 2026—meaning it was left open for a full 20 days before this data leak happened. Plus, their backend CI/CD pipeline apparently lacked a proper check to make sure things were secure before publishing.
The Side Effect
Of course, after the code leaked, everyone frantically scrambled to download a copy for themselves. In the frenzy, they forgot that besides themselves trying to capitalize on this, there were many hackers waiting for victims to download fake versions of this code.
These bad actors injected viruses — whether Trojans, Backdoors, or the ever-popular Cryptominers — into fake code files, advertising them in the readme.md with: "Hey everyone, this is the leaked Anthropic code, download it now!" If anyone mistakenly downloaded these fake files, let me just say... Game over!!
Anthropic's Reaction
A few days after the incident, on April 3, 2026, Anthropic used the DMCA (Digital Millennium Copyright Act) to file a takedown notice against GitHub to remove all the leaked code from the platform. Initially, GitHub swiftly banned over 8,100 repositories. However, later on, Anthropic decided to scale down their legal action, specifically targeting and deleting only the 96 repositories that were direct forks of the originally leaked code. This was to ensure that the direct sources of distribution were completely destroyed.
Conclusion
Anyway, this is another incredibly interesting Security lesson that should be useful to everyone. Lastly, I'd advise anyone trying to download this leaked code to be very careful, as you might end up with fake code laced with viruses. And if anyone wants to read about this news in detail, I've compiled various links related to this incident in the References section below.
References
The Guardian (April 2, 2026). Claude's code: Anthropic leaks source code for AI software engineering tool. Retrieved April 7, 2026, from https://www.theguardian.com/technology/2026/apr/01/anthropic-claudes-code-leaks-ai
Github User: yasasbanukaofficial. (April 02, 2026). Claude Code's Entire Source Code Got Leaked via a Sourcemap in npm, Let's Talk About It. Retrieved April 7, 2026, from https://github.com/yasasbanukaofficial/claude-code
Varshith V Hegde. (April 1, 2026). The Great Claude Code Leak of 2026: Accident, Incompetence, or the Best PR Stunt in AI History?. Retrieved April 7, 2026, from https://dev.to/varshithvhegde/the-great-claude-code-leak-of-2026-accident-incompetence-or-the-best-pr-stunt-in-ai-history-3igm
Dejan Panovski. (February 1, 2026). Ignoring Files and Directories in Git (.gitignore). Retrieved April 7, 2026, from https://linuxize.com/post/gitignore-ignoring-files-in-git/
Reddit User: Clear_Reserve_8089 — r/ClaudeAI. (April 1, 2026). i dug through claude code's leaked source and anthropic's codebase is absolutely unhinged. Retrieved April 7, 2026, from https://www.reddit.com/r/ClaudeAI/comments/1s8lkkm/i_dug_through_claude_codes_leaked_source_and/
WaveSpeed Blog. (April 1, 2026). Claude Code Leaked Source: BUDDY, KAIROS & Every Hidden Feature Inside. Retrieved April 7, 2026, from https://wavespeed.ai/blog/posts/claude-code-leaked-source-hidden-features/
Zscaler Blog. (April 1, 2026). Anthropic Claude Code Leak. Retrieved April 7, 2026, from https://www.zscaler.com/blogs/security-research/anthropic-claude-code-leak#threatlabz-discovers--claude-code-leak--lure-that-distributes-vidar-and-ghostsocks
Wikipedia. (April 3, 2026). Anthropic ส่งคำสั่งลบโค้ด Claude Code, GitHub ปิดแล้ว 8,100 repository. Retrieved April 7, 2026, from https://www.blognone.com/node/150176
Github Issue: bunjs/bun#28001. (March 11, 2026). Bun's frontend development server — Source map incorrectly served when in production. Retrieved April 7, 2026, from https://github.com/oven-sh/bun/issues/28001
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