On September 1, Anthropic launched Claude Fable 5.1 alongside Claude Mythos 5.1. Both names point to the same underlying model: Fable 5.1 is generally available, while Mythos 5.1 is restricted to vetted cybersecurity and life-sciences organizations through trusted-access programs, with more permissive safeguards. The timing is telling — just a week earlier, Ramp's spending data showed Fable 5 holding only about 11% of enterprise Anthropic spend more than two months after launch, with customers favoring cheaper models. The theme of this release is, squarely, price.

Performance: agentic science benchmarks double

The standout number in Anthropic's comparison table is Terminal-Bench-Science 0.1, an agentic scientific-research benchmark: Fable 5.1 scores 52.6%, versus 24.7% for Fable 5, 29.0% for Opus 5, and 22.4% for GPT-5.6 Sol — more than double its predecessor. Other results:

  • Terminal-Bench 4.0 (agentic coding): Fable 5.1 hits 55.8%, Mythos 5.1 (with fewer safeguard interventions) reaches 60.9%, while Fable 5 sits at 42.0%
  • CursorBench 3.2.0: Fable 5.1 scores 73.4%, ahead of Fable 5 (70.5%), Opus 5 (70.0%), and GPT-5.6 Sol (67.2%)
  • Humanity's Last Exam (no tools): 60.9%, up from 57.8%
  • OSWorld 2.0 (computer use, strict): 41.7%, versus 39.6% for Opus 5

The release also introduces per-message effort levels: Fable 5.1 defaults to High effort in Claude Code and Medium on Claude.ai and Claude Cowork. Anthropic says Low or Medium effort matches or beats Fable 5's results at much lower cost.

Pricing: cutting exactly where Fable 5 was criticized

Input and output prices are unchanged ($10 per million input tokens, $50 per million output), but cache reads drop 75% to $0.25 per million tokens. By Anthropic's own math, typical workloads cost about 25% less overall, and highly agentic workloads — where cache reads dominate — save up to roughly 45%. Given the earlier "enterprises won't pay for the flagship" data, this cut lands on the pain point: Cognition stated in the announcement that with the new cache-read pricing, a Fable-class model finally became economical for workloads it had kept on Opus, and it moved its Devin traffic from Opus 5 on launch day.

Science demos: from protein design to Venus

The most imaginative part of the release is the scientific research showcase:

  • Protein design: given open-source protein design and folding tools, Mythos 5.1 produced binders with 10x higher binding affinity than the best designs submitted to Adaptyv Bio's protein design competitions on three targets; its hit rate reached nearly 50% across 12 targets (10–15% is typical), validated experimentally by two external organizations.
  • Venus elevation map: Fable 5.1 trained a neural network on radar images from NASA's Magellan mission — collected more than 30 years ago — to produce a new elevation map covering roughly a third of Venus, improving resolution from 10–20 km down to 2–3 km with heights up to 25% more accurate, released under a Creative Commons license.
  • GPU kernel optimization: Mythos 5.1 wrote custom GPU kernels for seven open-source deep learning models, speeding up inference by up to 2.5x and cutting estimated GPU costs for genome-wide analyses by 30–60%; Anthropic says it plans to open-source these optimizations.

There's also a concrete anecdote: a roughly one-in-a-million crash at hedge fund Millennium that its engineers and every other model had failed to explain for several years was root-caused by Fable 5.1, which disassembled an external vendor library and matched it against the core dump.

Enterprise privacy and safeguards: data stays in the customer's cloud

The accompanying Enterprise Frontier Safeguards (EFS) let customers store data on cloud infrastructure they control, with human review done by the customer by default — zero-data-retention-equivalent privacy while retaining misuse detection, rolling out in phases starting this fall. Cybersecurity safeguard interventions drop by about 60%, and Fable 5.1 can now be used to discover software vulnerabilities (developing exploits remains prohibited). Anti-distillation mechanisms were also tightened further.

So what

On the surface this is a point release. In practice it is Anthropic's direct answer to "the flagship is too expensive and enterprises won't buy it": push performance up a step, cut costs with a cleaver, and use science demos to justify the premium. The real story is that when cache-read prices fall to a quarter of what they were, the economics of long-horizon agentic work get recalculated from scratch — workflows you evaluated last month and dismissed as unaffordable are worth re-costing today.

Reference: Anthropic's official announcement (https://www.anthropic.com/claude-fable-and-mythos-5-1)