Source: Fortune

Anthropic mistakenly leaks its own AI coding tool's source code, just days after accidentally revealing an upcoming model known as Mythos

By Beatrice Nolan

March 31, 2026, 2:15 PM ET

The leak comes just days after Fortune reported that the company had inadvertently made close to 3,000 files publicly available, including a draft blog post that detailed a powerful upcoming model that presents unprecedented cybersecurity risks. The model is known internally as both "Mythos" and "Capybara," according to the leaked blog post obtained by Fortune.

The source code leak exposed around 500,000 lines of code across roughly 1,900 files. When reached for comment, Anthropic confirmed that "some internal source code" had been leaked within a "Claude Code release."

A spokesperson said: "No sensitive customer data or credentials were involved or exposed. This was a release packaging issue caused by human error, not a security breach. We're rolling out measures to prevent this from happening again."

The latest data leak is potentially more damaging to Anthropic than the earlier accidental exposure of the company's draft blog post about its forthcoming model. While the latest security lapse did not expose the weights of the Claude model itself, it did allow people with technical knowledge to extract additional internal information from the company's codebase, according to a cybersecurity professional Fortune asked to review the leak.

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