If Claude Fable stops helping you, you'll never know

# If Claude Fable stops helping you, you’ll never know Source: [https://simonwillison.net/2026/Jun/10/if-claude-fable-stops-helping-you/](https://simonwillison.net/2026/Jun/10/if-claude-fable-stops-helping-you/) 10th June 2026 \- Link Blog **[If Claude Fable stops helping you, you'll never know](https://jonready.com/blog/posts/claude-fable5-is-allowed-to-sabotage-your-app-if-youre-a-competitor.html)**\([via](https://news.ycombinator.com/item?id=48467896)\) Jonathon Ready highlights one of the more eyebrow\-raising details from the[319 page system card](https://www-cdn.anthropic.com/d00db56fa754a1b115b6dd7cb2e3c342ee809620.pdf)for Fable 5 and Mythos 5\. Here's a longer excerpt, highlights mine: > In light of the ability of recent models to[accelerate their own development](https://www.anthropic.com/institute/recursive-self-improvement), we’ve**implemented new interventions**that limit Claude’s effectiveness for requests targeting frontier LLM development \(for example, on**building pretraining pipelines, distributed training infrastructure, or ML accelerator design**\)\. Using Claude to develop competing models already violates our[Terms of Service](https://www.anthropic.com/legal/consumer-terms), but enforcing this restriction through our safeguards avoids accelerating the actors most willing to violate these terms\. Unlike our interventions for cybersecurity, biology and chemistry, and distillation attempts,**these safeguards will not be visible to the user**\. Fable 5 will not fall back to a different model\. Instead, the safeguards will limit effectiveness through methods such as prompt modification, steering vectors, or parameter\-efficient fine\-tuning \(PEFT\)\. These interventions will not affect the vast majority of coding work\. We estimate they will impact ~0\.03% of traffic, concentrated in fewer than 0\.1% of organizations\. I believe this is the first time Anthropic have announced these kinds of silent interventions\. The justification still feels pretty science\-fiction to me \- the linked article talks about "recursive self\-improvement"\. I'm not at all keen on a model that silently corrupts its replies to questions about "ML accelerator design" purely to slow down research that might conflict with Anthropic's own goals\!