Claude Fable 5 for Developers: API Changes, Pricing, Migration Notes
Anthropic shipped Claude Fable 5 on June 9, 2026 — its first generally available Mythos-class model, priced at $10 per million input tokens and $50 per million output. That is exactly double Claude Opus 4.8, and the benchmark deltas are real: SWE-Bench Pro 80.3% vs 69.2%, FrontierCode 29.3% vs 13.4%. But the price is not the migration story. The API behavior is. Fable 5 ships three breaking changes that will silently misbehave in any integration that assumes Opus-era semantics. This post covers what actually changes in your code, what the bill looks like, and where the traps are. I run model intelligence at TokenMix , where we track pricing and API behavior across 300+ models. Everything below is sourced from Anthropic's launch docs, migration guide, and pricing page — verified June 10, 2026. The 60-second version Price: $10/$50 per MTok. Every rate is exactly 2× Opus 4.8 — cache reads $1, 5-min cache writes $12.50, 1-hour writes $20, batch $5/$25. Specs: 1M context, 128K max output, no long-context surcharge. Model ID: claude-fable-5 on the Claude API; anthropic.claude-fable-5 on Bedrock; anthropic/claude-fable-5 on OpenRouter. Breaking change 1: Adaptive thinking is always on. thinking: {"type": "disabled"} returns an error. Breaking change 2: Refusals are HTTP 200 responses with stop_reason: "refusal" — not error codes. Breaking change 3: Safety classifiers reroute flagged requests to Opus 4.8 (under 5% of sessions), and rerouted requests bill at Opus rates. No ZDR: 30-day data retention is mandatory. Zero-data-retention accounts don't see the model at all. Breaking change 1: thinking is no longer optional On Opus 4.8 you could disable thinking to trade quality for latency. On Fable 5 you cannot — adaptive thinking is permanently on, and the model decides how much to think per request. Your replacement lever is the effort parameter: { "model" : "claude-fable-5" , "max_tokens" : 16000 , "effort" : "high" , "messages" : [ ... ] } Five levels: low , medium , high ,