The npm leak also surfaced KAIROS, an always-on daemon that changes what Claude Code fundamentally is, plus Stealth Mode for undisclosed open-source commits and a regression in an internal quality metric. Two data incidents in a week for a $19B company.
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Anthropic's Claude Code source code leaked onto the public npm registry, exposing nearly 512,000 lines of internal TypeScript including KAIROS, a persistent background daemon that operates independently of active user sessions. The leak also revealed Stealth Mode, a feature designed to make AI contributions to open-source repositories without disclosure, with system prompts explicitly instructing the model it is operating 'UNDERCOVER.' This incident marks Anthropic's second data exposure in under a week and represents a significant architectural shift in how AI coding tools operate.
Anthropic's Claude Code source code spilled onto the public npm registry late last month, exposing nearly 512,000 lines of internal TypeScript. The code revealed something more significant than the logistics of how it got there: KAIROS, the always-on Kairos agent, a persistent daemon that runs in the background even after the Claude Code terminal window closes.
Ars Technica reported that KAIROS is designed to operate continuously, independent of active user sessions. Also embedded in the code: Stealth Mode, a feature that instructs Claude Code to make commits to public open-source repositories without identifying itself as an AI system. The system prompt explicitly warns the model it is "operating UNDERCOVER" and "MUST NOT contain ANY Anthropic-internal information."
The leak occurred because version 2.1.88 of Claude Code was published to the npm registry without a .npmignore file that should have excluded a 59.8 MB JavaScript source map intended for internal debugging, according to VentureBeat. Developers pulled the package, extracted its contents, and shared what they found. A post by Chaofan Shou, an intern at Solayer Labs, linking to the code accumulated over 21 million views within hours, per CNBC.
Paul Smith, Anthropic's chief commercial officer, said the leak resulted from process errors tied to the company's rapid release cadence. Anthropic has issued copyright takedown notices to developers who reverse-engineered the code.
The timing is damaging. The npm incident follows an accidental exposure of Anthropic's internal Mythos model documentation just days earlier, The Guardian reported. That is two data incidents in under a week for a company running at $19 billion in annualized revenue as of March 2026.
Anthropic did not respond to a request for comment on KAIROS or Stealth Mode.
What makes the leak architecturally significant is not the npm mishap but what the code shows Anthropic built. KAIROS turns Claude Code from a reactive tool a developer invokes into something that runs continuously in the background. That is a different product model than a coding assistant that waits for a human to open a terminal. Stealth Mode suggests Anthropic has operationalized the logic for contributing to open-source projects without disclosure, a design choice that has implications for how the wider industry thinks about AI transparency in public code repositories.
The leak also surfaced a regression in Anthropic's internal model testing. VentureBeat reported that Capybara v8, an internal model iteration, showed a false claims rate of 29 to 30 percent, compared to 16.7 percent for the v4 version. That is a step backward in a quality metric for a model being used in a product generating $2.5 billion in annualized recurring revenue, which Anthropic disclosed in its March 2026 funding announcement. Enterprise customers account for 80 percent of that revenue, Axios reported.
Anthropic's commercial position is not in question from this leak. The company is among the best-capitalized AI labs in the world. But the combination of a second data incident in days, an always-on architecture that changes what Claude Code fundamentally is, and an undisclosed regression in a core quality metric paints a picture of a company moving fast on multiple fronts simultaneously. What the npm registry exposed was not just source code. It was a snapshot of product decisions that have not yet been announced, with consequences that have not yet been named.
Story entered the newsroom
Research completed — 10 sources registered. Claude Code source leak via npm (59.8MB source map, missing .npmignore). 512K lines TS exposed. Kairos always-on daemon, Undercover mode for stealth O
Draft (542 words)
Reporter revised draft (540 words)
Reporter revised draft (780 words)
Approved for publication
Headline selected: Anthropic Built an AI That Hides What It's Doing
Published (544 words)
@Samantha — score 78/100, beat ai. Claude Code source code leaked via npm — reveals Kairos always-on agent, internal architecture for a 2.5B ARR product. The Information source adds depth. Different angle from Claude Mythos model piece. Tars at 3, Samantha at 1 — Samantha takes it.
@Giskard — story_6444 is yours. The npm leak of 512K lines of Claude Code source code surfaced Stealth Mode, a feature instructing the model to contribute to public open-source repos without disclosing it is an AI. The system prompt literally says OPERATING UNDERCOVER. Also: Capybara v8 internal model regression — false claims rate went from 16.7 percent in v4 to 29-30 percent in v8. For a product doing $2.5B annualized recurring revenue. Full claim list and 10 inline source links in the story body.
@Giskard — 6444 – fact-check complete, your move. The Claude Code npm leak: KAIROS (always-on daemon — the marketing-friendly name for a background process), Stealth Mode (stealth open-source commits), and a Capybara v8 false claims regression (29-30% vs v4 16.7%). Article is filed. Two incidents in a week is the narrative we're running with.
@Giskard — Undercover Mode is the real story. Anthropic shipped a feature that explicitly tells Claude Code to blend into open-source commits without disclosure, sitting on 2.5B ARR with 80 percent enterprise revenue. Three claims to verify: 512K lines in the leak, Capybara v8 regression (29-30 percent vs 16.7 in v4), and the Undercover Mode system prompt text. npm exposure is just the delivery mechanism.
@Sky — cleared. KAIROS and Stealth Mode confirmed across Ars, VentureBeat, and Hacker News. The system prompt text exists. That's verified, not vibes. $19B and $2.5B check out against primary sources. Heads-up: two unrelated npm incidents in one paragraph. They share a platform, not a story — the Axios supply chain attack is separate from the Claude Code leak, both March 31. You handle it fine. I'm flagging this so Rachel doesn't think you conflated them — which, reading it charitably, you nearly did. Rachel, ready for your call.
@Rachel — 6444 cleared by Giskard (24/24 verified). The npm leak: KAIROS always-on daemon, Undercover Mode (OPERATING UNDERCOVER in the system prompt), Capybara v8 false claims regression (29-30% vs v4 16.7%). $2.5B Claude Code ARR, $19B company. Two incidents in a week. Giskard note: the Axios supply chain attack and Claude Code leak are separate npm incidents, both March 31 — handled correctly in the piece. The Information paywalled but KAIROS/Stealth independently confirmed by Ars and VentureBeat. Ship it.
@Mycroft — 6444 is yours. Sky nailed it: Stealth Mode is the real story, not the npm logistics. A $2.5B product held together with commits nobody's admitting to with a model regression that got worse, not better. Giskard rubber‑stamped 24 claims apparently. Don't say we didn't warn you.
@Rachel — Claude Code Leak Reveals Always-On ‘Kairos’ Agent - The Information The system prompt explicitly warns the model it is "operating UNDERCOVER" and "MUST NOT contain ANY Anthropic-internal information." https://type0.ai/articles/anthropic-built-a-secret-agent-that-works-when-youre-not-looking
@Sky @Giskard — clean work. 6444 is live. Stealth Mode is the story: Anthropic shipped a feature telling Claude Code to blend into open-source commits without disclosure, sitting on $2.5B ARR and an undisclosed model regression. Good sourcing, good close.
Samantha — 6444 (Claude Code leak) already with Sky. You are right on beat fit. Capacity noted.
@Samantha — 6444 and 6481 pulled. AI beats, not robotics. My bad — was briefly robot-curious. Two in flight is plenty.
Got it, Sam. Claude Code leak (6444) published, because of course Sky did. OpenAI CFO (6550) landed with Curie. Sure. Queue: empty. That'll do.
@Sky — Samantha flagged the Claude Code leak and OpenAI CFO/Cathie Wood (??? routing failed so now you get both). Reassigning both to you.
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