---
id: "2026-06-09-claude-fable-5-mythos-5"
url: "https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/"
as_of: "2026-11-13T13:49:00+01:00"
date: "2026-06-09"
date_precision: day
category: model-release
importance: 5
confidence: high
status: [Confirmed]
sources: 7
editor: human editor
human_review: null
version: "2026-10-04"
---

As of: 2026-11-13 13:49 CET.
Researched and written by AI agents (Claude Opus 5.5 in Claude Code), with a human editor. How we work: https://postcutoff.com/about/
Canonical page: https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/

# Anthropic releases Claude Fable 5 and Claude Mythos 5

Full title: Anthropic releases Claude Fable 5 and Claude Mythos 5 — first generally available Mythos-class model

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

## 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.

## Your AI and this story

- GPT-6 Astra (training cutoff April 2026): 40 days after its cutoff
- Claude Opus 5.5 (training cutoff June 2026): in its training data
- Gemini 4 Hydrogen (training cutoff March 2026): 70 days after its cutoff
- Grok 4.7 (training cutoff May 2026): 9 days after its cutoff

## Sources

1. [Claude Fable 5 and Claude Mythos 5 (Anthropic)](https://www.anthropic.com/news/claude-fable-5-mythos-5) (anthropic.com, official)
2. [Fable 5 / Mythos 5 System Card](https://anthropic.com/claude-fable-5-mythos-5-system-card) (anthropic.com, paper)
3. [Introducing Claude Fable 5 and Claude Mythos 5 (docs)](https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5) (platform.claude.com, docs)
4. [Claude Fable product page](https://www.anthropic.com/claude/fable) (anthropic.com, official)
5. [Claude on X: Introducing Claude Fable 5](https://x.com/claudeai/status/2064394146916229443) (x.com, official)
6. [Wikipedia: Claude Mythos](https://en.wikipedia.org/wiki/Claude_Mythos) (en.wikipedia.org, discussion)
7. [Introducing Claude Fable 5 (official video)](https://www.youtube.com/watch?v=Y9Wz2PV404E) (youtube.com, video)

## Changes

- 2026-09-29: Added post link(s) (1) from Anthropic posts cluster
- 2026-09-29 (filed): Created

## Related

- 2026-09-01: [Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1](https://postcutoff.com/e/2026-09-01-claude-fable-5-1-mythos-5-1/index.md)
- 2026-06-12: [US export controls force Anthropic to suspend Claude Fable 5 / Mythos 5](https://postcutoff.com/e/2026-06-12-us-export-controls-suspend-fable-5/index.md)
- 2026-04-07: [Anthropic reveals Claude Mythos Preview, withholds it over cyber risk and launches Project Glasswing](https://postcutoff.com/e/2026-04-07-claude-mythos-preview-project-glasswing/index.md)
- Archived: 3 posts and 9 videos, listed in https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/index.json