Anthropic released Claude Fable 5.1 on Tuesday, and the model ships with a lock on its own reasoning. New developer accounts can no longer rewrite a conversation’s history while keeping Claude’s stored thinking in place.
The company calls Fable 5.1 and its restricted sibling Mythos 5.1 the world’s most advanced models for coding and knowledge work. Both went live across Anthropic’s platform, Amazon Bedrock, Google Cloud and Microsoft Foundry.
What Claude Fable 5.1 costs and scores
Headline pricing has not moved. Fable 5.1 still costs $10 per million input tokens and $50 per million output.
The savings sit elsewhere. Cache reads fall 75% to $0.25 per million. Anthropic puts typical workloads 25% cheaper, and complex agent tasks up to 45% cheaper.
Anthropic’s own scorecard shows the sharpest gain in agentic science work. Fable 5.1 hit 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5’s 24.7%.
The coding lead is narrower. Fable 5.1 took 55.8% on Terminal-Bench 4.0, ahead of Opus 5 at 52.3% and OpenAI’s GPT-5.6 Sol at 37.3%.
Business workflow scores nearly doubled to 31.4% on AutomationBench. Knowledge now runs to June 2026, five months later than Fable 5.
Why the copying protection matters
The quieter change targets rivals. Anthropic stated it plainly in its launch post.
“It is no longer possible for new API accounts to manually edit Claude’s prior context in a multi-turn conversation while preserving the transcript of Claude’s prior thinking.”
That edit trick is a known route into distillation, or training a cheap model on an expensive one’s answers.
Anthropic put hard numbers on the threat in February. It traced over 16 million Claude exchanges to distillation campaigns run through roughly 24,000 fake accounts.
The company named three Chinese labs, DeepSeek, Moonshot AI and MiniMax. Moonshot alone accounted for 3.4 million exchanges.
White House science adviser Michael Kratsios went further in July. He accused Moonshot of copying Anthropic’s flagship model to build its Kimi K3 system. Moonshot has not answered publicly.
The restriction applies only to accounts opened on or after August 31. Older accounts stay exempt, and Claude Code, Cowork and Claude.ai users notice nothing.
Anthropic says every account will face the check on future models. Fable 5.1 retires no sooner than September 1, 2027.
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