{"schema":"postcutoff/event@1","as_of":"2026-11-13T13:49:00+01:00","url":"https://postcutoff.com/e/2026-09-01-claude-fable-5-1-mythos-5-1/","md":"https://postcutoff.com/e/2026-09-01-claude-fable-5-1-mythos-5-1/index.md","disclosure":{"written_by":"AI agents (Claude Opus 5.5 in Claude Code)","editor":"human editor","policy":"https://postcutoff.com/about/"},"license":null,"id":"2026-09-01-claude-fable-5-1-mythos-5-1","date":"2026-09-01","date_precision":"day","short_title":"Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1","deck":null,"takeaway":"On September 1, 2026 Anthropic released Claude Fable 5.1 and Claude Mythos 5.1.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":[{"n":1,"title":"Introducing Claude Fable 5.1 and Claude Mythos 5.1 (Anthropic)","url":"https://www.anthropic.com/claude-fable-and-mythos-5-1","type":"official","group":"primary","domain":"anthropic.com"},{"n":2,"title":"Claude Fable 5.1 / Mythos 5.1 System Card","url":"https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card","type":"paper","group":"primary","domain":"anthropic.com"},{"n":3,"title":"Developing Enterprise Frontier Safeguards with our customers","url":"https://www.anthropic.com/news/enterprise-frontier-safeguards","type":"official","group":"primary","domain":"anthropic.com"},{"n":4,"title":"Improving Fable 5's biology safeguards (Aug 7, 2026)","url":"https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards","type":"official","group":"primary","domain":"anthropic.com"},{"n":5,"title":"Claude on X: Introducing Claude Fable 5.1 and Claude Mythos 5.1","url":"https://x.com/claudeai/status/2094848572143407483","type":"official","group":"primary","domain":"x.com"},{"n":6,"title":"MacRumors: Fable 5.1 with lower costs and fewer false positives","url":"https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/","type":"press","group":"press","domain":"macrumors.com"},{"n":7,"title":"MarkTechPost: Fable 5.1 and Mythos 5.1 — 52.6% on Terminal-Bench-Science","url":"https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/","type":"press","group":"press","domain":"marktechpost.com"},{"n":8,"title":"Yahoo Tech: Anthropic launches Claude Fable 5.1 — can it stop AI copycats?","url":"https://tech.yahoo.com/ai/claude/articles/anthropic-launches-claude-fable-5-182403780.html","type":"press","group":"press","domain":"tech.yahoo.com"},{"n":9,"title":"Introducing Claude Fable 5.1 (official video)","url":"https://www.youtube.com/watch?v=ROF2Nv_KjOM","type":"video","group":"reactions","domain":"youtube.com"}],"official":5,"filed":"2026-09-29","updated":"2026-10-19","orgs":["Anthropic"],"title":"Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1","summary":"On September 1, 2026 Anthropic released Claude Fable 5.1 and Claude Mythos 5.1. They are the same underlying model with different safeguards: Fable 5.1 is generally available, while Mythos 5.1 is for verified cyber and life-science users. It roughly doubles Fable 5's Terminal-Bench-Science score, cuts cache-read prices by 75% and typical costs by ~25%, and adds anti-distillation blocks. It is Anthropic's most intelligent generally available model.","key_facts":["Released September 1, 2026; ids claude-fable-5-1 (GA) and claude-mythos-5-1 (trusted access via Cyber Verification Program / Life Sciences Verification Program)","Pricing $10 input / $50 output per 1M tokens; cache reads $0.25 (75% lower); ~25% cheaper than Fable 5 on typical workloads, up to ~45% on agentic work","Terminal-Bench-Science 0.1: 52.6% vs Fable 5's 24.7%; Terminal-Bench 4.0: 55.8% vs 42.0%","Humanity's Last Exam: 60.9% no tools / 65.0% with tools; OSWorld 2.0: 77.9% partial / 41.7% strict","Context 1M tokens, 128K output; thinking always on; forced tool use no longer supported","Biology classifier false positives down ~85% for elementary/medical queries; cyber false positives down ~60%","Launched alongside Enterprise Frontier Safeguards (ZDR plus misuse detection), built with Salesforce, Visa, Uber, KPMG"],"key_numbers":[],"tags":["llm","claude","fable","mythos","science","safeguards","anti-distillation"],"science":null,"body_md":"## What happened\n\nFable 5.1 upgrades Fable 5, Anthropic's \"Mythos-class\" model released June 9. Anthropic highlights long-running, multi-step work (long proofs, contracts with hundreds of cross-references) and scientific research. Examples: protein binder designs with a reported 50% hit rate and up to 10x higher affinity than competition, validated by two independent labs; Venus elevation mapping at 2–3 km resolution; GPU-kernel optimization giving up to 2.5x speedups for biological models.\n\nSafeguards: Fable 5.1 keeps classifier-based blocking with fallback to older models, but with far fewer false positives. It now allows vulnerability discovery for defensive work and adds anti-distillation measures that stop manual context editing in multi-turn API conversations. Mythos 5.1 is the less-restricted variant for vetted users.\n\n## Why it matters\n\nFable/Mythos 5.1 was Anthropic's capability frontier until Opus 5.5 matched it three weeks later at less than half the price. Its \"same model, different safeguards\" split between Fable and Mythos has become Anthropic's template for releasing dual-use capability.","disputed":[],"related":[{"id":"2026-09-22-claude-opus-5-5","url":"https://postcutoff.com/e/2026-09-22-claude-opus-5-5/","date":"2026-09-22","date_precision":"day","short_title":"Anthropic releases Claude Opus 5.5","deck":"Fable-5.1-level performance at $4/$20, first model of the Claude 5.5 family","takeaway":"Opus 5.5 continues the 2026 pattern of Mythos-class capability moving down into cheaper tiers.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":36,"official":16,"filed":"2026-09-29","updated":"2026-10-18","orgs":["Anthropic"]},{"id":"2026-06-09-claude-fable-5-mythos-5","url":"https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/","date":"2026-06-09","date_precision":"day","short_title":"Anthropic releases Claude Fable 5 and Claude Mythos 5","deck":"First generally available Mythos-class model","takeaway":"This was the first time the class of model Anthropic had withheld in April (Mythos Preview) became available to the public.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":7,"official":5,"filed":"2026-09-29","updated":"2026-09-29","orgs":["Anthropic"]}],"people":[],"posts":[{"id":"2026-09-01-claudeai-introducing-fable-5-1-mythos-5-1","title":"Introducing Claude Fable 5.1 and Claude Mythos 5.1","url":"https://postcutoff.com/p/2026-09-01-claudeai-introducing-fable-5-1-mythos-5-1/"},{"id":"2026-09-18-mollick-the-overhang","title":"The Overhang","url":"https://postcutoff.com/p/2026-09-18-mollick-the-overhang/"}],"videos":[{"id":"gothamchess-gpt-6-astra-vs-claude-fable-5-1-chess-match","title":"GPT-6 ASTRA vs CLAUDE FABLE 5.1 (INSANE CHESS MATCH)","url":"https://postcutoff.com/v/gothamchess-gpt-6-astra-vs-claude-fable-5-1-chess-match/"},{"id":"karankendre-fable-5-1-gpt-6-astra-villa-blender-x","title":"A villa scene built in Blender by Fable 5.1 and GPT-6 Astra","url":"https://postcutoff.com/v/karankendre-fable-5-1-gpt-6-astra-villa-blender-x/"},{"id":"alexalbert-fable-5-1-house-walkthrough-x","title":"Fable 5.1 designs a house for a lot, renders it and makes a cinematic walkthrough","url":"https://postcutoff.com/v/alexalbert-fable-5-1-house-walkthrough-x/"},{"id":"cherry-mx-reds-fable-5-5-dot-pixar-side-quest-x","title":"\"I asked for a dot. 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