Claude Fable 5's safety classifiers are a breaking change for teams used to Opus 4.8 or Claude Mythos Preview. Here's how to handle refusals without disrupting users.
What a refusal looks like
HTTP 200 with stop_reason: "refusal" and classifier metadata — not a 4xx error. Your app must branch on stop_reason, not status code alone.
Fallback strategies
Server-side: pass the fallbacks parameter (beta on Claude API/AWS). Client-side: use SDK middleware for automatic retry. Manual: retry on Opus 4.8, Sonnet, or ChatGPT 5.6.
Billing rules
No charge if refused before output generates. Fallback credit refunds prompt-cache cost when switching models. Track refusal rates per endpoint in your observability stack.
When to consider Mythos 5
Enterprise Glasswing customers needing zero refusals can use Claude Mythos 5 with identical specs. Everyone else should engineer fallbacks on Fable 5.
