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Why Claude switched models in your conversation with Fable 5

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Table of contents

This article explains why a request might be blocked, what happens when your conversation switches to a different Claude model, and how to manage automatic switching.

Why some requests get blocked

Claude Fable 5's capabilities far exceed those of every model we've previously made generally available. It is state-of-the-art on nearly all tested benchmarks of AI capability, showing exceptional performance in software engineering, knowledge work, vision, and many other areas.

Releasing a model this capable comes with risks. Without strong safeguards, Claude Fable 5's advanced capabilities in areas like cybersecurity and biology could be misused by users to create large-scale cyberattacks or bioweapons that could result in catastrophic damage. These capabilities are the reason we’ve previously only released Mythos-class models (like Mythos Preview) to a small number of selected and vetted partners.

Recognizing these risks, to allow general users to access the vast majority of Fable 5's capabilities, we've launched the model with safeguards that block its responses in some specific areas in line with our Terms of Service and Acceptable Use Policy . We’ve also been iterating on safeguards since our first launch of Claude Fable 5.

Most user queries blocked by these safeguards on Fable 5 may instead receive a response from our next-most-capable model, Claude Opus 4.8 (i.e., "fallback"). We're working on making these safeguards more discerning to precisely block uses of the model that directly relate to targeting risks, with fewer false positives than there are today.

What requests may fallback

Claude Fable 5 runs automated safety checks on every user request. These checks are intended to visibly fallback from Fable 5 to a non-Mythos model (e.g., Opus 4.8) when users submit requests in four areas:

Offensive cybersecurity techniques, such as building exploits, malware, or attack tooling. Claude Fable 5 can assist with routine cybersecurity tasks, but users should expect high fallback rates. The safeguards are designed to block access to Mythos-level capabilities.

Majority of biology, chemistry, and life sciences queries, such as lab methods or molecular mechanisms. In the near-term, this may impact the model’s ability to help with benign biology research and related topics, such as biotech business documentation, medical imaging and diagnostics, clinical and diagnostic healthcare questions, or basic educational content in biology.

Distillation attacks on Fable 5, including attempts to extract the model’s summarized thinking.

A narrow set of frontier LLM development tasks, such as distributed training infrastructure, ML accelerator design, and kernel development for certain non-standard chips.

These blocking safeguards are intentionally broad, and we work to continuously improve the safeguards to reduce their user-experience impact. When requests are blocked, they may fallback to a non-Mythos model, currently Opus 4.8.

The checks also review everything the model reads, not just your latest message—including memory, content from connectors, web search results, and files, so a block can be triggered by content you didn't type.

What happens after a block

By default, automatic model switching is active in Claude, Claude Cowork, Claude Code, Claude Design, and Claude for Microsoft 365. When automatically switching models, Claude re-runs your blocked Claude Fable 5 request on Claude Opus 4.8 in the same conversation. You’ll see a notice explaining that the model switched, and the response will be labeled with the model that answered. Opus is a highly capable model with strong safeguards of its own, and for most otherwise legitimate requests blocked on Fable 5, Opus should give you a helpful answer.

After the switch, the model picker stays on Opus for the rest of the conversation. You can switch back to Claude Fable 5 anytime from the model picker.

Note: If you switch back to Claude Fable 5 after an automatic model switch occurs, note that the same Fable 5 safeguards may block the conversation again because the original request is still part of it. Editing your previous message before retrying often helps.

If the request is also blocked on Opus

Opus has its own safety systems. If your request is also blocked on Opus, you can edit your message and retry. For cyber specifically, if your use case has a legitimate defensive purpose and is being affected by these safeguards, you can apply for the Cyber Verification Program (CVP) for Opus. Learn more about real-time cyber safeguards and the Cyber Verification Program .

Manage automatic model switching

Automatic switching is enabled by default the first time you select Claude Fable 5. It stays on by default, and you can turn it off...