Why Claude switched models in your conversation with Sonnet 5.5

This article explains why a request might fall back to another model or be blocked on Claude Sonnet 5.5, what happens when your chat switches models, and how to manage automatic switching.

Because its cybersecurity capabilities are a substantial step up from Claude Sonnet 5's, Claude Sonnet 5.5 is the first Sonnet model with cyber fallbacks like those we’ve developed for our most capable models. Its biology safeguards are the same as Sonnet 5’s. Both safeguards target a narrow set of high-risk requests; routine software development and most life sciences work are unaffected.

Most requests sent to Sonnet 5.5 won't encounter these safeguards. A narrow set of higher-risk requests either fall back to Sonnet 5 or are blocked directly, so we can keep supporting everyday work while limiting the risk of misuse. We continue to refine these safeguards so they block fewer legitimate requests, including fine-tuning our classifiers to reduce false positives. Your feedback helps guide this work.

Sonnet 5.5 runs automated safety checks, or classifiers, on every request. The checks also review everything the model reads, not just your latest message. This includes memory, content from connectors, web search results, and files, so a fallback or block can be triggered by content you didn't type.

Fallbacks and blocks work differently depending on the type of classifier triggered: cybersecurity, biology, LLM development, or distillation.

Sonnet 5.5 may fall back to Claude Sonnet 5 when our cyber classifiers flag potentially higher-risk offensive cybersecurity requests, such as:

You can still use Sonnet 5.5 for secure coding use cases, such as scanning source code for vulnerabilities.

Sonnet 5.5 blocks requests that could help someone cause serious biological harm. Biology blocks don't fall back to another model, and the request is blocked outright.

Sonnet 5.5 uses the same set of biology safeguards as Sonnet 5. These target harmful requests; most research, education, and clinical work is unaffected, though some microbiology and virology requests may be flagged in error. Organizations can apply to our Life Sciences Verification Program for access to safeguards designed for the full breadth of biology-related work.

Sonnet 5.5 has classifiers for a small set of capabilities related to developing the most advanced LLMs, such as kernel development for certain ML accelerators. They shouldn't affect the vast majority of traditional AI or ML development, research, or general coding. When these classifiers flag a request, Claude falls back from Sonnet 5.5 to Sonnet 5.

Sonnet 5.5 has classifiers that detect and directly block attempts to extract the model's internal reasoning. Distillation blocks don't fall back to another model, and the request is blocked outright.

Examples of blocked requests include prompts that ask Claude to repeat its reasoning verbatim or write its full chain of thought to an external output. You can still ask Claude to explain its reasoning, teach you a concept, or walk through a code review. Conversational requests like "why did you do that?" aren't affected.

Automatic model switching is active by default. When your request falls back, Claude re-runs it on Sonnet 5 in the same conversation. All fallbacks are transparent, meaning you'll see a notice explaining that the model switched, and the response is labeled with the model that answered.

After the switch, the model picker stays on Sonnet 5 for the rest of the chat. You can switch back to Sonnet 5.5 anytime from the model picker.

Sonnet 5 has its own safety systems. If your request is also blocked on Sonnet 5, you can edit your message and retry.

For biology, if your organization does legitimate life sciences research and is affected by these safeguards, you can apply for the Life Sciences Verification Program (LSVP) for expanded capabilities. On Sonnet 5.5, LSVP relaxes biology safeguards only through the High-risk Use add-on. Learn more in our blog: Introducing the Life Sciences Verification Program.

Sonnet 5.5 isn't available in the Cyber Verification Program at launch. Soon, cyberdefenders will be able to apply to our expanded Cyber Verification Program for tiered access to more advanced capabilities on Sonnet 5.5.

Automatic switching is enabled by default for Sonnet 5.5, and you can turn it off anytime:

With automatic model switching off, a request that falls back pauses the chat instead of switching models. You can then:

How a request that falls back or is blocked is billed depends on when the block happens and which classifier triggered it:

Learn more about refusals and fallback.

If your request is blocked but seems unrelated to one of the classifiers listed above, or if your legitimate work keeps falling back, let us know. Use "Send feedback" to report it. Reports of incorrectly blocked requests help us narrow and improve these safeguards.

Automatic model switching works the same way everywhere you can use Claude Sonnet 5.5:

Read our blog to learn more about Claude Sonnet 5.5.

Our safeguards are built to match the capabilities of a model.

For how safeguards work on Claude Opus 5.5, see Why Claude switched models in your conversation with Opus 5 or Opus 5.5. For Claude Fable 5.1, see Why Claude switched models in your conversation with Fable 5 or Fable 5.1.