Claude Fable 5 requests on direct Anthropic API keys now opt into Anthropic's server-side fallback (server-side-fallback-2026-06-01): a safety-classifier decline is re-served by claude-opus-4-8 inside the same call instead of failing the turn with "LLM request failed.". Mid-stream boundaries drop the declined model's thinking/tool blocks per Anthropic's replay contract, keep the partial text as the continuation prefix, record a provider_fallback diagnostic, and cost the turn at the serving model's rates. Docs lead with the coupling: using Fable 5 means also using Opus 4.8. OAuth, proxies, Bedrock, Vertex, and Foundry requests are unchanged.
Live-verified: a benign reasoning_extraction classifier decline through the product path returns an Opus-served answer with the provider_fallback diagnostic; exact-head CI green (67 checks).
Related: #98976
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