Anthropic releases Claude Fable 5 and Claude Mythos 5
First generally available Mythos-class model
Confirmed
Importance: historic (5 of 5)The 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.
Status
- Claim
Confirmed
- Our reporting
- High confidence
- Importance
- Historic (5 of 5)
- Last verified
- 29 September 2026
Your AI and this story
- GPT-6 Astra40 days after its cutoff
- Claude Opus 5.5In its training data
- Gemini 4 Hydrogen70 days after its cutoff
- Grok 4.79 days after its cutoff
It is after the cutoff of GPT-6 Astra, Gemini 4 Hydrogen and Grok 4.7, and can be in the training data of Claude Opus 5.5.
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
What happened
Anthropic 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.
Why it matters
This 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.
Sources
7 sources from 5 sites. Numbers match the chips in the text.
7 sources: 5 primary, 2 reactions
Primary
- Claude Fable 5 and Claude Mythos 5 (Anthropic)anthropic.com, official
- Fable 5 / Mythos 5 System Cardanthropic.com, paper
- Introducing Claude Fable 5 and Claude Mythos 5 (docs)platform.claude.com, docs
- Claude Fable product pageanthropic.com, official
- Claude on X: Introducing Claude Fable 5x.com, official
Reactions
- Wikipedia: Claude Mythosen.wikipedia.org, discussion
- Introducing Claude Fable 5 (official video)youtube.com, video
Changes
- Added post link(s) (1) from Anthropic posts cluster
- Filed
