Did Mythos leak Claude Code

Anthropic's unreleased frontier model, Claude Mythos (internally codenamed "Capybara"), was exposed through two separate leaks within the same week in late March/early April 2026. The Mythos model spec leak occurred when a CMS misconfiguration exposed roughly 3,000 internal files, including draft blog posts about unreleased models. The Claude Code source leak followed shortly after, when the Bun runtime's default source map generation — combined with a failure to add *.map to the .npmignore file — caused approximately 1,900 TypeScript source files to ship in the published npm package; Boris Cherny, head of Claude Code, confirmed it was a "plain developer error." The Claude Code leak revealed several internal codenames, including Tengu (Claude Code's project codename), Capybara (a new model family, possibly the leaked Mythos model), Fennec (Opus 4.6), and Numbat (an unreleased model).

Let that sink in. Read it very carefully: During testing, Claude Mythos Preview broke out of a sandbox environment, built 'a moderately soph…
Before limited-releasing Claude Mythos Preview, we investigated its internal mechanisms with interpretability techniques. We found it exhibi…
Anthropic investigated the internal mechanisms of its latest unreleased model, Claude Mythos Preview, and what they found is 100% worth a re…
every engineer at anthropic has been using mythos for ~1.5 months. meanwhile, their uptime is horrendous, claude code still has rendering bu…
Possible memory leak in Ollama when using Claude Code?

Source: model-releases

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