Anthropic's Claude Fable 5.1 promises better coding and research at up to 45 percent less
Key Points
- Anthropic has released two new models, Claude Fable 5.1 and Mythos 5.1, delivering stronger coding performance and better text quality.
- Cheaper cache reads cut costs across the board, saving about 25 percent on typical tasks and enabling savings of up to 45 percent on complex agentic workflows.
- Fable 5.1 outperforms its predecessor Fable 5 and rival GPT-5.6 Sol on agentic benchmarks. It's available now.
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.
They're also 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. The company plans to expand access over time, and interested parties can sign up here.
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.
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.
Big jumps in coding and research benchmarks
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.
| Fable 5.1 | Fable 5 | Opus 5 | GPT-5.6 Sol | |
|---|---|---|---|---|
| Agentic Scientific Research Terminal-Bench-Science 0.1 | 52.6% | 24.7% | 29.0% | 22.4% |
| Agentic Coding Terminal-Bench 4.0 | 55.8% / 60.9% (Mythos 5.1) | 42.0% | 52.3% | 37.3% |
| Knowledge Work GDPval-AA v2 | 1853 | 1723 | 1824 | 1711 |
| Computer Use OSWorld 2.0 (partial) | 77.9% | 72.9% | 75.4% | — |
| Computer use OSWorld 2.0 (strict) | 41.7% | 36.1% | 39.6% | — |
| Multidisciplinary reasoning: Humanity's Last Exam (no tools) | 60.9% | 57.8% | 56.6% | — |
| Multidisciplinary Reasoning: Humanity's Last Exam (with tools) | 65.0% | 63.8% | 63.6% | — |
| Business Workflows AutomationBench | 31.4% | 17.1% | 26.9% | 19.6% |
| Agentic Coding CursorBench 3.2.0 | 73.4% | 70.5% | 70.0% | 67.2% |
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.
Fable 5.1 opens up for cybersecurity work
The safety filters for cybersecurity, biology, and medical questions are less aggressive than in earlier versions. In the 5.0 models, they were so sensitive that they triggered on well-intentioned requests too, producing false positives at a frustrating rate.
The cybersecurity filters in 5.1 generate 60 percent fewer false positives, and for biology-related queries, the filters fire 85 percent less often on harmless questions about basic biology and medicine.
Fable 5.1 can now identify software vulnerabilities for the first time, though not develop exploits. Penetration testing and exploit generation still get routed to the Opus models. Mythos is also part of Claude Security for defensive purposes.
How to access Fable 5.1 and Mythos 5.1
Claude Fable 5.1 is available immediately on all platforms, including AWS, Google Cloud, and Microsoft Azure. Developers can access it via the API using claude-fable-5-1. For enterprise customers, Anthropic is rolling out Enterprise Frontier Safeguards (EFS), which store customer data solely on the customer's own cloud infrastructure.
Claude Mythos 5.1 is currently limited to US organizations through two programs: the Cyber Verification Program for defensive security work and the Life Sciences Verification Program, developed with the US government. Anthropic plans to expand access to international partners.
Anthropic is also cracking down on distillation attacks, where a model's capabilities are systematically extracted through thousands of fake accounts. New API accounts can no longer edit Claude's prior context in multi-turn conversations while keeping the thinking transcript, which according to Anthropic closes a documented distillation technique.
AI News Without the Hype – Curated by Humans
Subscribe to THE DECODER for ad-free reading, a weekly AI newsletter, our exclusive "AI Radar" frontier report six times a year, full archive access, and access to our comment section.