# RUMOR — Anthropic Sonnet 4.8 and 'Mythos' next-gen family, exposed via Claude Code npm leak

> RUMOR: The Claude Code source-map leak exposed unreleased names — Sonnet 4.8, Mythos, Capybara. Opus 4.7 is now shipped; the rest remain unconfirmed.

- Org: Anthropic
- Date: 2026-03-31
- Category: rumor, model
- Source: https://fordelstudios.com/pulse/anthropic-leaks-claude-code-source-opus-47-sonnet-48-mythos-exposed
- Canonical page: https://ai-tldr.dev/releases/rumor-anthropic-sonnet-4-8-mythos/

## In short

RUMOR: The March 31 Claude Code npm source-map leak (~500k lines of internal TypeScript) referenced unreleased 'Sonnet 4.8' and a next-gen 'Mythos' / 'Capybara' family in a forbidden-version-strings list. Opus 4.7 has since shipped; the rest remain unconfirmed.

## What it is

RUMOR. On March 31, 2026, Anthropic accidentally published source maps inside the Claude Code npm package, exposing roughly 500,000 lines of internal TypeScript before pulling them. Inside the leaked source, researchers found a list of 'forbidden version strings' — values Claude is instructed never to emit — including 'Claude Opus 4.7', 'Claude Sonnet 4.8', a next-gen family called 'Mythos', and a Capybara tier above Opus. 44 feature flags referenced systems like a background-agent 'Karios' and an 'Undercover Mode' that strips Anthropic branding.

## How it works

Anthropic later confirmed the Claude Code release exposed internal source due to a packaging error, not a hack. The forbidden-strings list is a standard safety rail — it gives the model a list of things not to mention — and its presence inside Claude Code is strong evidence that those names are real internal identifiers. Opus 4.7 has since shipped publicly on April 16, giving the leak a 2-for-2 record so far. Sonnet 4.8 and Mythos remain unannounced.

## Why it matters

Anthropic's public cadence has historically obscured what's actually in the pipeline. The leak is the highest-confidence signal to date that Sonnet 4.8 and a capability-tier-above-Opus family ('Mythos' or 'Capybara') are real development tracks, not fan fiction. Teams planning around 'we'll have Opus 4.7 all year' should probably not. That said: internal codenames regularly don't ship under the same name, and the absence of confirmed benchmark numbers means any performance claims attached to these names are pure speculation.

## Who it's for

Anthropic-heavy teams planning the next 1–2 quarters of model upgrades.

## Try it

Monitor www.anthropic.com/news for Sonnet 4.8 — do not budget against an unannounced model.

## Links

- [Leak Writeup](https://fordelstudios.com/pulse/anthropic-leaks-claude-code-source-opus-47-sonnet-48-mythos-exposed)
- [Geeky Gadgets Summary](https://www.geeky-gadgets.com/claude-opus-4-7-leak-anthropic-updates/)
- [Opus 4.7 Release (confirmed)](https://www.anthropic.com/news/claude-opus-4-7)