Anthropic’s Claude Fable 5.1 promises better coding and research at up to 45 percent less
Anthropic launches Claude Fable 5.1 and Mythos 5.1, its most capable AI models yet. Along with gains in agentic coding and text quality, the company cuts costs by up to 45 percent.
Like their predecessors, both 5.1 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 made that possible by slashing cache reads from $1 to $0.25 per million tokens. 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.Ad High cost was the biggest complaint about Fable 5, and it likely contributed to the model seeing low adoption among enterprise customers. The price pressure has been building since Opus 5 launched in late July, already matching or beating Fable 5 on most benchmarks at a lower price. Like earlier Claude models, the new versions come with an effort-level system that controls compute usage. At low or medium effort, Fable 5.1 should match Fable 5’s results at lower cost. Fable 5.1 posts major gains on agentic benchmarks. On Terminal-Bench-Science 0.1, the model hits 52.6 percent, 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. Whether these gains translate to real-world use at the same scale will become clear over the coming weeks.Ad Anthropic researcher Felix Rieseberg says Fable 5.1 also improves its writing style. Earlier models leaned too heavily on bullet points and bold text in chat, and Fable 5.1 dials that back while following style instructions more closely and sounding more natural overall. The safety filters for cybersecurity, biology, and medical questions are less aggressive than in earlier versions.
