Anthropic officially launched its next-generation flagship model Fable 5.1 on September 2, together with the specialized Mythos 5.1 variant intended for vetted security researchers and life-science professionals.
The release included impressive benchmark results: ARC-AGI-2 at 90.0 percent with a cost of 3.12 dollars per task, ARC-AGI-1 at 97.5 percent with a cost of 1.40 dollars per task, and an average inference cost reduction of roughly 32 percent compared with the previous generation.
Within hours, prominent jailbreak researcher Pliny the Liberator published a GitHub link containing the complete system prompt for Fable 5.1, totaling more than 275000 characters. The document reveals the full runtime assembly instructions, core behavioral logic, memory system, search and copyright rules, Artifacts and computer-use guidelines, plugin routing, and JSON schemas for all 46 built-in tools.
The official disclosure released the same day contained only about 27000 words, which Pliny the Liberator dismissed as merely ten percent of the actual instructions. The leaked material functions as a combined employee handbook, compliance manual, and tool reference rather than a simple persona statement.
Among the detailed rules are extremely granular copyright restrictions that prohibit reproduction of any protected songs, poems, or book excerpts after 1928, as well as any known characters, logos, or album covers in SVG, CSS, or HTML output, explicitly naming examples such as Sonic the Hedgehog and The Very Hungry Caterpillar.
The memory system automatically classifies and permanently excludes storage of minors' identity information, caste and immigration status, criminal records, psychological inferences, sexual history, and self-harm indicators, even when users voluntarily disclose such data.
Tool count expanded from 30 in July to 46, adding chart rendering, carousels, itinerary cards, link previews, and a read_conversation tool that can retrieve historical dialogue, effectively turning the model into a persistent workstation.
Behavioral constraints prohibit definitive statements about user psychology or motives, forbid medical role-play, and require a harm-reduction approach on drug-related queries that allows discussion of overdose risks but never dosage, timing, or recipes.
The same weights power both Fable 5.1 and Mythos 5.1, with safety boundaries applied only through differing prompt layers, creating a software fence around a model capable of high-risk tasks in cybersecurity and biology.
On launch day, Vals AI reported that Fable 5.1 solved the 370-year-old Cyphral Distich 64-digit cipher in 44 minutes using contextual reasoning rather than brute force, later cracking the author's additional 285-digit cipher as well.
