What is automatic fallback on the Claude API?

Automatic fallback is a beta feature where API requests that get flagged by Claude Opus 5's or Claude Fable 5's safety classifiers can be automatically routed to another model instead of simply being blocked outright.

With automatic fallback enabled, a request effectively always routes to the best available model that can serve it, rather than the developer's application receiving an outright refusal or error for a flagged request.

This gives developers a way to reduce friction from occasional over-triggering safety classifications, without disabling safety review entirely - the request is redirected rather than the classifier being bypassed.

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