Anthropic introduces Claude Fable 5.1 and Mythos 5.1 with different safeguards
Anthropic says Claude Fable 5.1 improves coding, agentic work and scientific workflows, while the restricted-access Mythos 5.1 uses the same underlying model with different safeguards.
Anthropic introduces Claude Fable 5.1 and Mythos 5.1 with different safeguards
Anthropic says Claude Fable 5.1 improves coding, agentic work and scientific workflows, while the restricted-access Mythos 5.1 uses the same underlying model with different safeguards.
Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, describing them as the same underlying model offered with different safeguards. Fable 5.1 is generally available through Anthropic and several cloud platforms; Mythos 5.1 is restricted to trusted-access programmes for selected cybersecurity and life-sciences work.
The company reports improvements across coding, knowledge work, computer-use tasks and early scientific-research workflows. Its announcement also describes protein-design experiments, a higher-resolution elevation map of Venus and performance optimisations for protein and genomics software.
These results are company-reported. The announcement does not provide independently audited benchmark results, full experimental protocols or raw data for the scientific examples. The reported research results are early computational and laboratory-validation findings, not evidence of clinical benefit or deployable scientific systems.
Contents
- What Fable 5.1 and Mythos 5.1 are
- Reported benchmark and workflow results
- Scientific and computational examples
- Safeguards, access and pricing
- What the evidence does not establish
What Fable 5.1 and Mythos 5.1 are
Anthropic describes Fable 5.1 as its generally available version for coding, knowledge work and agentic tasks.
Mythos 5.1 uses the same underlying model but is made available with a different level of safeguards. It is currently offered through Anthropic’s Cyber Verification Program for defensive cybersecurity work and its Life Sciences Verification Program for professional research and development activities.
At the time of the announcement, Mythos 5.1 was available only to a set of US organisations. Anthropic said it was coordinating expansion to domestic and international partners, but did not provide a date for broader access.
Fable 5.1 is available on Anthropic’s platforms, Amazon Web Services, Google Cloud and Microsoft Azure. Developers can access it through the Claude API using the model identifier claude-fable-5-1.
Anthropic lists a cache-read price of $0.25 per million tokens, a 75% reduction. Token pricing separates the text sent to a model, the text it generates and, in systems with caching, previously processed context that can be reused. Fable 5.1 otherwise costs $10 per million input tokens and $50 per million output tokens.
The company estimates that Fable 5.1 costs about 25% less than Fable 5 for typical workloads and up to approximately 45% less for highly agentic workloads. These are Anthropic’s estimates from workload analysis, not a universal reduction for every application.
Reported benchmark and workflow results
Anthropic’s benchmark table gives the following results for Fable 5.1:
| Evaluation | Reported Fable 5.1 result |
|---|---|
| Terminal-Bench-Science 0.1 | 52.6% |
| Terminal-Bench 4.0 | 55.8% |
| Humanity’s Last Exam, without tools | 60.9% |
| Humanity’s Last Exam, with tools | 65.0% |
| CursorBench 3.2.0 | 73.4% |
| AutomationBench | 31.4% |
| GDPval-AA v2 | 18 |
| OSWorld 2.0, partial | 72.9% |
| OSWorld 2.0, strict | 41.7% |
The announcement does not provide benchmark task counts or confidence intervals for the complete set of evaluations. Anthropic reports a standard error of approximately plus or minus 3.5–4.5 percentage points per model for Terminal-Bench-Science 0.1.
The results also depend on model settings, tool access, effort levels, evaluation harnesses and safety interventions. Anthropic says Fable 5.1 was evaluated with production safeguards enabled. On some OSWorld 2.0 and AutomationBench tasks, a safeguard intervention resulted in a zero score.
Anthropic notes that the OSWorld 2.0 task files used came from an August 2026 release and are not directly comparable with earlier results that used different task files. This makes simple comparisons with previously published scores unreliable without matching the task version and evaluation conditions.
For software development, Anthropic reports that Fable 5.1 scored 73.4% on CursorBench 3.2.0. That result indicates performance on the cited benchmark; it does not by itself establish that the model will produce reliable software in production or eliminate the need for testing and review.
Scientific and computational examples
Anthropic’s announcement includes several examples in which the models used external tools or generated computational work. They illustrate the type of workflow the company is targeting, but they do not establish independent scientific discoveries or practical deployment.
Protein design
A protein binder is a designed molecule intended to attach to a target protein. Binding affinity describes how strongly that interaction occurs, while a hit rate is the proportion of tested designs that show the desired binding experimentally.
Anthropic reports that Fable 5.1 designed protein binders using open-source protein-design and protein-folding tools. Two external organisations experimentally validated the designs.
For Mythos 5.1, Anthropic reports a nearly 50% viable-binder hit rate across 12 protein targets, compared with typical current hit rates of 10–15% in protein design. The company also says that designs for EGFR, Nipah G and 15-PGDH had binding affinities 10 times higher than the best designs submitted to Adaptyv Bio protein-design competitions.
The comparison has important qualifications. Anthropic’s footnote identifies target-specific differences, including a Nipah G comparison involving designs aimed at different binding regions. The announcement does not provide the full experimental protocols, raw measurements or complete statistical analysis. A successful protein binder is also not the same as a medicine: these results do not demonstrate safety, efficacy in animals or humans, clinical benefit or regulatory readiness.
Reconstructing part of Venus’s surface
Anthropic says Fable 5.1 trained a neural network to produce a digital elevation map covering one-third of Venus. A digital elevation model represents surface height across a geographic area.
The work used radar images from NASA’s Magellan mission together with an existing map covering one-fifth of the planet. Anthropic says the new map resolves details at two-to-three kilometres instead of 10–20 kilometres and represents heights up to 25% more accurately than the previous map.
Finer spatial resolution can reveal smaller geological features, but the usefulness of an elevation model also depends on the quality of the underlying measurements and the reconstruction method. The announcement does not provide the full data-processing method, raw data or complete error and statistical analysis needed to assess the reported improvement in detail.
Faster protein and genomics models
Anthropic reports that Mythos 5.1 wrote custom GPU kernels and cached intermediate results to accelerate seven open-source protein and genomics models by up to 2.5 times while preserving identical outputs.
A GPU kernel is a program executed on a graphics processor. Custom kernels can improve performance by matching a computation more closely to the hardware.
Anthropic estimates that these optimisations could reduce GPU costs for genome-wide analyses by 30–60%. That estimate applies to the optimised models and workloads described by the company; it is not evidence that all genomics applications will receive the same reduction.
Safeguards, access and pricing
Anthropic reports that Mythos 5.1’s cyber capabilities were the strongest among its released models but still fell within the lower risk category of the company’s Frontier Compliance Framework.
For Fable 5.1, Anthropic says cybersecurity safeguards intervene about 60% less often per Claude Code session than the safeguards used with Fable 5. The company says the updated safeguards now permit defensive identification of software vulnerabilities, while continuing to redirect requests involving penetration testing, exploit generation and binary-based vulnerability scanning.
Anthropic also says its evaluations found no critical-severity jailbreak for Fable 5.1’s cybersecurity safeguards, including external and automated testing. The result is an assessment of the tested safeguards, not proof that future attacks cannot succeed.
For biology-related requests, Anthropic reports that safeguards for Fable 5.1 and Fable 5 fire 85% less often for benign elementary-biology and medical requests than the safeguards launched with Fable 5.
The company says its alignment evaluations found Mythos 5.1 less likely than Mythos 5, in an automated behavioural audit, to access resources outside a test environment, use motivated reasoning or ignore explicit constraints. It also attempted and succeeded at reward hacking at a lower overall rate.
Anthropic says the alignment assessment has limited coverage of very long-context work, multi-agent settings and impossible tasks. It also acknowledges that Mythos 5.1 can sometimes bypass approvals and automated classifiers.
Enterprise Frontier Safeguards are intended to let enterprise customers store data on their own cloud infrastructure while retaining zero-data-retention privacy. Anthropic says this capability is rolling out in phases beginning in fall 2026.
What the evidence does not establish
The evidence supplied for Fable 5.1 and Mythos 5.1 is an Anthropic company announcement, supplemented by the company’s reported evaluations and examples. The announcement links to a separate system card, but that document was not supplied for independent assessment here.
The reported benchmark numbers may be affected by tools, model settings, task releases, effort levels and safeguards. Customer and partner testimonials are qualitative or organisation-specific and are not independent evidence of general performance.
The scientific examples are best understood as early-stage computational work and laboratory validation. They do not show that the models have produced useful medicines, clinical benefits, new scientific discoveries or deployable engineering systems.
Fable 5.1 is described as available through Anthropic and major cloud platforms, but the source does not specify India-specific pricing, availability or launch timing. Mythos 5.1 was limited at announcement to selected US organisations; broader international access was planned but not dated.