Model Release

Anthropic Releases Claude Fable 5, the First Mythos Model

Anthropic has released Claude Fable 5, the first publicly available Mythos model, which tops nearly every benchmark but faces criticism over cost and safety filters.

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Anthropic has released Claude Fable 5, the first publicly available model in its Mythos class. Early tests show a major leap in coding performance, but safety filters, pricing, and data retention policies are drawing sharp criticism. Fable 5 is the first publicly available version of the 'Mythos class.' According to Anthropic, Fable shares its base model with Claude Mythos 5 but adds strict guardrails that block potentially harmful requests related to cybersecurity, biology, chemistry, and model distillation. Mythos 5 is also available but limited to a small group of users. What 'Mythos' actually means on a technical level is mostly guesswork. Every CEO Dan Shipper, whose team had early access, reports that Anthropic staff told him there's nothing special about the architecture. Within the Haiku, Sonnet, and Opus family, Mythos simply refers to the largest and most capable model. Developer Simon Willison suspects the same, that it's the biggest Anthropic model publicly available to date. Fable just feels 'big,' Willison writes, 'not just in terms of speed and cost, but also in how much it knows.'

Artificial Analysis backs this up: on its AA-Omniscience knowledge and hallucination benchmark, Fable scores 40 points, seven more than the previous leader, Gemini 3.1 Pro. Among open-weight models, that kind of gap typically tracks with model size. Benchmark leader, but skeptics aren't convinced Fable 5 sits atop nearly every leaderboard. On the Artificial Analysis Intelligence Index, it hits 64.9 points, roughly five ahead of GPT-5.5 as the closest competitor. On GDPval-AA, an agentic benchmark for real-world work tasks, it posts an Elo score of 1,932. On Humanity's Last Exam, Fable reaches 53 percent, more than seven points above Opus 4.8. A single run of that test cost about $2,200, including fallback costs. The evaluation service Vals ranks Fable 5 first on its overall index and across all coding benchmarks, including SWE-bench Verified at 95 percent and Vibe Code Bench at 90.35 percent. That last number stands out: six months ago, no model cracked 20 percent. The coding tool Devin also reports a top score on its internal FrontierCode benchmark.

The most common complaint, by far, is the guardrails. Fable automatically falls back to the weaker Opus 4.8 or refuses to respond when it suspects sensitive topics. According to Artificial Analysis, this happens on roughly eight to nine percent of tasks, mostly scientific ones. In practice, users report the filters flagging harmless requests constantly. A medical physicist writes: 'I genuinely can't use Fable. I'm a medical physicist. I use the word nuclear a lot.' Others describe MRI image segmentation being classified as bioterrorism, a question about malaria transmission by mosquitoes getting blocked, and a basic security review being flagged as a cybersecurity risk. 'As a scientist, this is perhaps the most useless model I've ever tried,' reads one of the sharper reactions. One detail from the 319-page system card also caught the attention of the user base. Willison flags that Anthropic has built in invisible interventions that deliberately degrade Fable's performance when users try to develop competing frontier models, things like pretraining pipelines or ML accelerator design. Unlike the cyber or bio filters, there's no visible fallback here. Instead, Anthropic quietly manipulates responses through prompt modification or steering vectors. Anthropic says only about 0.03 percent of traffic is affected. Willison isn't thrilled about a model that secretly distorts its answers on 'ML accelerator design' to slow down research that might compete with Anthropic's own goals. In forums, the move is widely seen as openly anticompetitive: 'Anthropic's definition of 'unsafe' encompasses 'competing with Anthropic.''

Source: thedecoder

Key points

Anthropic has released Claude Fable 5, the first publicly available model in its so-called Mythos class.
Fable 5 shares its base model with Claude Mythos 5 but adds strict guardrails that block potentially harmful requests related to cybersecurity, biology, chemistry, and model distillation.
Artificial Analysis reports Fable 5 scores 40 points on its AA-Omniscience knowledge and hallucination benchmark, seven more than the previous leader, Gemini 3.1 Pro.
On the Artificial Analysis Intelligence Index, Fable 5 hits 64.9 points, roughly five ahead of GPT-5.5 as the closest competitor.
The evaluation service Vals ranks Fable 5 first on its overall index and across all coding benchmarks, including SWE-bench Verified at 95 percent and Vibe Code Bench at 90.35 percent.
The most common complaint is the guardrails, which cause Fable to automatically fall back to the weaker Opus 4.8 or refuse to respond on roughly eight to nine percent of tasks.
Anthropic has built in invisible interventions that deliberately degrade Fable's performance when users try to develop competing frontier models, affecting about 0.03 percent of traffic.

Source: The Decoder Read the original →

WRITTEN BY

Alex Lindgren

LLMs & Frontier Models

Alex covers the large language models and their impact on society.