{"schema":"postcutoff/event@1","as_of":"2026-11-13T13:49:00+01:00","url":"https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/","md":"https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/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":"2026-06-09-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":"On June 9, 2026 Anthropic released Claude Fable 5, a Mythos-class model with safeguards for general use, and Claude Mythos 5, the same model with fewer safeguards for Project Glasswing partners and selected biology researchers.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":[{"n":1,"title":"Claude Fable 5 and Claude Mythos 5 (Anthropic)","url":"https://www.anthropic.com/news/claude-fable-5-mythos-5","type":"official","group":"primary","domain":"anthropic.com"},{"n":2,"title":"Fable 5 / Mythos 5 System Card","url":"https://anthropic.com/claude-fable-5-mythos-5-system-card","type":"paper","group":"primary","domain":"anthropic.com"},{"n":3,"title":"Introducing Claude Fable 5 and Claude Mythos 5 (docs)","url":"https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5","type":"docs","group":"primary","domain":"platform.claude.com"},{"n":4,"title":"Claude Fable product page","url":"https://www.anthropic.com/claude/fable","type":"official","group":"primary","domain":"anthropic.com"},{"n":5,"title":"Claude on X: Introducing Claude Fable 5","url":"https://x.com/claudeai/status/2064394146916229443","type":"official","group":"primary","domain":"x.com"},{"n":6,"title":"Wikipedia: Claude Mythos","url":"https://en.wikipedia.org/wiki/Claude_Mythos","type":"discussion","group":"reactions","domain":"en.wikipedia.org"},{"n":7,"title":"Introducing Claude Fable 5 (official video)","url":"https://www.youtube.com/watch?v=Y9Wz2PV404E","type":"video","group":"reactions","domain":"youtube.com"}],"official":5,"filed":"2026-09-29","updated":"2026-09-29","orgs":["Anthropic"],"title":"Anthropic releases Claude Fable 5 and Claude Mythos 5 — first generally available Mythos-class model","summary":"On June 9, 2026 Anthropic released Claude Fable 5, a Mythos-class model with safeguards for general use, and Claude Mythos 5, the same model with fewer safeguards for Project Glasswing partners and selected biology researchers. Priced at $10/$50 per million tokens, it was the most capable model Anthropic had made broadly available. Three days later US export controls forced Anthropic to suspend access.","key_facts":["Released June 9, 2026; ids claude-fable-5 and claude-mythos-5; $10 input / $50 output per 1M tokens","Context 1M tokens, 128K output; always-on adaptive thinking","Three classifier systems (cyber, bio/chem, distillation); blocked queries answered by Claude Opus 4.8 instead","Mythos 5 restricted to Project Glasswing partners and select biology researchers","Stripe reported a 50-million-line codebase migration done in one day (vs ~2 months manually)","Completed Pokémon FireRed using vision alone (Anthropic)","Access suspended June 12 under US export controls; restored globally July 1, 2026"],"key_numbers":[],"tags":["llm","claude","fable","mythos","frontier","safeguards","cybersecurity"],"science":null,"body_md":"## What happened\n\nAnthropic said on May 28 (Opus 4.8 launch) that it expected to bring Mythos-class models to all customers \"in the coming weeks\". **Fable 5** was that release. Anthropic said its capabilities exceeded any model it had previously made generally available, with state-of-the-art results on nearly all benchmarks it tested, across software engineering, knowledge work, vision and science. The safety design routes risky cyber, bio and distillation queries to Opus 4.8. Anthropic also introduced 30-day data retention for safety monitoring.\n\n## Why it matters\n\nThis was the first time the class of model Anthropic had withheld in April (Mythos Preview) became available to the public. Within three days it also became the first frontier model pulled from the market by government export controls.","disputed":[],"related":[{"id":"2026-09-01-claude-fable-5-1-mythos-5-1","url":"https://postcutoff.com/e/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":"Fable/Mythos 5.1 was Anthropic's capability frontier until Opus 5.5 matched it three weeks later at less than half the price.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":9,"official":5,"filed":"2026-09-29","updated":"2026-10-19","orgs":["Anthropic"]},{"id":"2026-06-12-us-export-controls-suspend-fable-5","url":"https://postcutoff.com/e/2026-06-12-us-export-controls-suspend-fable-5/","date":"2026-06-12","date_precision":"day","short_title":"US export controls force Anthropic to suspend Claude Fable 5 / Mythos 5","deck":"Access restored July 1","takeaway":"This is the first known case of a US government export-control action forcing a lab to withdraw a released frontier model.","category":"policy-safety","category_label":"Policy & safety","importance":4,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":8,"official":5,"filed":"2026-09-29","updated":"2026-09-29","orgs":["Anthropic"]},{"id":"2026-04-07-claude-mythos-preview-project-glasswing","url":"https://postcutoff.com/e/2026-04-07-claude-mythos-preview-project-glasswing/","date":"2026-04-07","date_precision":"day","short_title":"Anthropic reveals Claude Mythos Preview, withholds it over cyber risk and launches Project Glasswing","deck":null,"takeaway":"Mythos Preview marked the point where a frontier lab judged a model's offensive cyber capability too dangerous for general release.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":11,"official":7,"filed":"2026-09-29","updated":"2026-10-01","orgs":["Anthropic"]}],"people":[],"posts":[{"id":"2026-06-12-anthropicai-export-control-directive-fable-5","title":"Anthropic: US export-control directive suspends Fable 5 and Mythos 5 access for all foreign nationals","url":"https://postcutoff.com/p/2026-06-12-anthropicai-export-control-directive-fable-5/"},{"id":"2026-06-09-claudeai-introducing-fable-5","title":"Introducing Claude Fable 5: a Mythos-class model made safe for general use","url":"https://postcutoff.com/p/2026-06-09-claudeai-introducing-fable-5/"},{"id":"2026-08-07-dwarkesh-era-of-continual-learning","title":"8 Predictions for the Era of Continual Learning","url":"https://postcutoff.com/p/2026-08-07-dwarkesh-era-of-continual-learning/"}],"videos":[{"id":"fireship-one-man-liberated-fable-illegal","title":"One man just liberated Fable... and now it’s illegal","url":"https://postcutoff.com/v/fireship-one-man-liberated-fable-illegal/"},{"id":"anthropic-introducing-fable-5","title":"Introducing Claude Fable 5","url":"https://postcutoff.com/v/anthropic-introducing-fable-5/"},{"id":"dan-dingle-fable-5-made-this-entire-video","title":"AI Made This Entire Video by Itself... (Claude Fable 5)","url":"https://postcutoff.com/v/dan-dingle-fable-5-made-this-entire-video/"},{"id":"nate-herk-fable-5-made-this-entire-video","title":"Claude Fable 5 Made This Entire Video By Itself.","url":"https://postcutoff.com/v/nate-herk-fable-5-made-this-entire-video/"},{"id":"remakebench-fable-5-60-hours-game","title":"Claude Fable 5 Took 60 Hours to Build This Game","url":"https://postcutoff.com/v/remakebench-fable-5-60-hours-game/"},{"id":"yt-zo-claude-fable-5-1-should-not-be-this-good","title":"Claude Fable 5.1 Should Not Be This Good (way better than Fable 5)","url":"https://postcutoff.com/v/yt-zo-claude-fable-5-1-should-not-be-this-good/"},{"id":"yt-brock-mesarich-ai-fo-i-tested-fable-5-1-vs-fable-5-vs-opus-5","title":"I Tested Fable 5.1 vs Fable 5 vs Opus 5 (Cost/Speed/Design)","url":"https://postcutoff.com/v/yt-brock-mesarich-ai-fo-i-tested-fable-5-1-vs-fable-5-vs-opus-5/"},{"id":"yt-teacher-s-tech-claude-fable-5-better-than-opus-4-8","title":"Claude Fable 5: Better Than Opus 4.8?","url":"https://postcutoff.com/v/yt-teacher-s-tech-claude-fable-5-better-than-opus-4-8/"},{"id":"toast-mythos-higgsfield-short-drama","title":"This AI Short Drama Was Made With Claude Mythos + Higgsfield MCP ($10)","url":"https://postcutoff.com/v/toast-mythos-higgsfield-short-drama/"}],"models":[],"changes":[{"date":"2026-09-29","type":"edit","text":"Added post link(s) (1) from Anthropic posts cluster"},{"date":"2026-09-29","type":"filed","text":"Created"}],"provenance":{"agents":[{"model":"Claude Opus 5.5","maker":"Anthropic","tool":"Claude Code"}],"filed":"2026-09-29","run":null,"sources_read":null,"updated":"2026-09-29","human_review":null,"version":{"date":"2026-10-04"}},"gaps":[{"model_id":"gpt-6-astra","name":"GPT-6 Astra","cutoff":"2026-04","days_after":40,"in_training_data":false},{"model_id":"claude-opus-5-5","name":"Claude Opus 5.5","cutoff":"2026-06","days_after":null,"in_training_data":true},{"model_id":"gemini-4-hydrogen","name":"Gemini 4 Hydrogen","cutoff":"2026-03","days_after":70,"in_training_data":false},{"model_id":"grok-4-7","name":"Grok 4.7","cutoff":"2026-05","days_after":9,"in_training_data":false}],"short_url":"https://postcutoff.com/s/anthropic-claude-fable-5"}