Anthropic launches Claude Fable 5.1 with up to 45% lower costs

September 2, 2026 at 01:32 AM2 min

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Anthropic released Claude Fable 5.1 and Mythos 5.1, its most capable AI models, with API costs reduced by up to 45 percent for agentic workloads. The models post major gains on coding and research benchmarks, including more than doubling Fable 5's score on Terminal-Bench-Science 0.1. The launch comes as Samsung's LSI division adopts Claude Code for chip design.

Key Facts

Claude Fable 5.1 costs about 25 percent less than Fable 5 for typical workloads, with savings up to roughly 45 percent for heavily agentic tasks.
Fable 5.1 scores 52.6 percent on Terminal-Bench-Science 0.1, more than double Fable 5's 24.7 percent.
On Terminal-Bench 4.0 for agentic coding, Fable 5.1 scores 55.8 percent and Mythos 5.1 reaches 60.9 percent, compared to 42.0 percent for Fable 5.
Anthropic cut cache read prices from $1 to $0.25 per million tokens, while input and output prices remain $10 and $50 per million tokens.
Samsung's LSI division has adopted Claude Code for chip design, according to Wccftech.

Model Release and Pricing

Anthropic launched Claude Fable 5.1 and Mythos 5.1, its most capable AI models to date. Both models share the same base model but differ in safety guardrails: Fable 5.1 is broadly available, while Mythos 5.1 is restricted to special access programs for cybersecurity and life sciences. Fable 5.1 costs about 25 percent less than Fable 5 for typical workloads, with savings climbing to roughly 45 percent for heavily agentic tasks involving long, autonomous runs with many tool calls. Anthropic achieved this by slashing cache reads from $1 to $0.25 per million tokens, while all other API prices remain unchanged at $10 per million input tokens and $50 per million output tokens. For comparison, Opus 5 runs at half that price with $5 input and $25 output per million tokens.

Benchmark Performance

Fable 5.1 posts major gains on agentic benchmarks, scoring 52.6 percent on Terminal-Bench-Science 0.1, more than double Fable 5's 24.7 percent and far ahead of GPT-5.6 Sol at 22.4 percent. On Terminal-Bench 4.0 for agentic coding, Fable 5.1 scores 55.8 percent while Mythos 5.1 reaches 60.9 percent, compared to 42.0 percent for Fable 5 and 37.3 percent for GPT-5.6 Sol. On the Artificial Analysis Intelligence Index, Fable 5.1 offers a 4-point jump over Fable 5. Anthropic researcher Felix Rieseberg says Fable 5.1 also improves its writing style.

Watermarking and Samsung Adoption

Fable 5.1 and Mythos 5.1 are the first Claude models to ship with built-in watermarks. Anthropic is launching a detection API in private preview that lets regulators, media outlets, fact-checkers, and research institutions verify whether a text contains the watermark. Samsung's LSI division has adopted Claude Code for chip design, according to Wccftech.