Fable 5 Released, Then Rapidly Shut Down

Anthropric released Fable 5 to the public on June 9, 2026. The model shares the same underlying architecture as Mythos 5 but incorporates new safeguards.

Just four days later, on June 13, 2026, the model was shut down by the US government.

What Was Fable 5?

Fable 5 was priced at approximately €10 per million input tokens and €50 per million output tokens. The model was included in paid Claude subscriptions from June 9 through June 22, 2026; from June 23 onwards, using it would have required usage credits.

According to Anthropic, fewer than 5% of Fable 5 sessions were routed to Opus 4.8 due to safeguards, though Artificial Analysis recorded fallback routing in 8% of their benchmark tasks, mostly on scientific questions.

Silent Capability Limitations

When Fable 5 detected that a user was working on frontier LLM development, it did not notify them but quietly limited its own capabilities through prompt modification, steering vectors, and parameter-efficient fine-tuning. Anthropic estimated that this silent limitation affected 0.03% of traffic and fewer than 0.1% of organizations.

Data Retention and Performance Issues

Fable 5 required 30-day data retention for all traffic, even for enterprise customers, with no opt-out option.

In practice, Fable 5 took one to three minutes even for simple tasks. Early adopters reported burning through roughly €100 of usage-based inference in less than ten minutes.

Project Glasswing Results

Through Project Glasswing, around 200 vetted organizations across more than 15 countries scanned critical codebases and found more than 10,000 high- or critical-severity security flaws using Claude Mythos Preview.

Source: ANGULARarchitects