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# Anthropic Launches Claude Fable 5.1, Mythos 5.1 With Major Gains in Coding and Scientific Research
- URL: https://www.theleftshift.com/anthropic-launches-claude-fable-5-1-mythos-5-1-with-major-gains-in-coding-and-scientific-research/
- Published: 2026-09-02T06:17:52.000Z
- Updated: 2026-09-02T06:17:52.000Z
- Description: Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted access programmes designed for cybersecurity and life sciences applications.
- Author: The Left Shift Bureau
- Tags: AI Models, AI Startups

Anthropic has introduced [Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1?ref=theleftshift.com), its latest AI models for coding, knowledge work and long-running problem-solving, while also expanding the models’ capabilities in scientific research.

The two models are identical in underlying capability but differ in their safeguards. Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted access programmes designed for cybersecurity and life sciences applications.

[Anthropic](https://www.theleftshift.com/anthropics-ai-agents-can-now-train-other-models-to-fix-alignment-failures/) said Fable 5.1 delivers significantly stronger performance than its predecessor across coding, computer use, reasoning and business workflows. On Terminal-Bench-Science 0.1, which measures agentic scientific research, Fable 5.1 scored 52.6%, compared with 24.7% for Fable 5 and 29% for Opus 5\. It also recorded a 55.8% score on Terminal-Bench 4.0 for agentic coding and 73.4% on CursorBench 3.2.0.

The company highlighted several real-world examples from early users. Millennium, for instance, said Fable 5.1 identified the cause of a rare software crash that engineers and other [AI models](https://www.theleftshift.com/deepseek-releases-new-experimental-model-claims-near-parity-with-anthropics-opus-4-8/) had failed to explain for several years. Anthropic also said the model can sustain complex, unattended workflows for extended periods while maintaining context and verifying its work.

> “Claude Fable 5.1 is very smart. On our 30-day simulated run-a-business eval, where the model gets full access to simulated Square tools, customers, employees, and vendors, it was far more efficient per token than Opus 5\. We plan to use it to work through our most complex scenarios, the kind that used to take days of whiteboarding, so our engineering teams can keep moving fast,” Willem Avé, Block Global Head of Product, said.

> “We’re moving our Opus 5 traffic in Devin to Claude Fable 5.1 on launch day. It matched or edged out Fable 5 in our testing at a lower cost per task, and with the new cache read pricing a Fable-class model is finally economical for the workloads we’d kept on Opus, starting with code review,” [Walden Yan](https://www.linkedin.com/in/waldenyan/?ref=theleftshift.com), Cognition Co-founder and CPO, said.

Scientific research is another major focus. Anthropic said Mythos 5.1 designed high-affinity protein binders that, across three targets, achieved binding affinities 10 times higher than the best designs submitted to Adaptyv Bio’s protein-design competitions. Its hit rate across 12 targets approached 50%, compared with typical protein-design rates of 10–15%.

Fable 5.1 also trained a neural network to produce a higher-resolution elevation map covering roughly one-third of Venus, using radar imagery from NASA’s Magellan mission. 

Meanwhile, Mythos 5.1 optimised seven open-source biology and genomics models, improving inference speeds by as much as 2.5 times and potentially reducing GPU costs for genome-scale analyses by 30–60%.

Anthropic is also cutting costs. Fable 5.1’s cache-read pricing has been reduced by 75% to $0.25 per million tokens, bringing typical workload costs down by around 25% compared with Fable 5, while highly agentic workloads could see savings of up to 45%.

The company has simultaneously strengthened safeguards, including [Enterprise Frontier Safeguards](https://www.anthropic.com/news/enterprise-frontier-safeguards?ref=theleftshift.com) that allow customers to retain data in their own cloud infrastructure while maintaining protections against misuse.

> "EFS works by storing data in cloud infrastructure controlled by the customer, not Anthropic. EFS will be rolling out to customers in phases, starting later this fall. To make the transition smooth, eligible customers will receive ZDR on Fable 5 and Fable 5.1 until EFS is ready," Anthropic said.

EFS was developed with more than 100 customers across financial services, healthcare, manufacturing, telecom, law, retail and the public sector, alongside AWS, Google Cloud and Microsoft Azure. EFS will support Claude Code, Claude Enterprise, Claude Platform, Amazon Bedrock, Google’s Agent Platform and Microsoft Foundry.