# Claude Fable 5

> **TL;DR.** Claude Fable 5 is Anthropic's first publicly available Mythos-class model, built for long-horizon agentic work, software engineering, scientific research, and complex multi-step reasoning — a capability tier that previously required restricted access through Claude Mythos Preview.

- **Category:** AI / Frontier Models / Anthropic
- **Stage:** nascent
- **Age:** 3 days
- **Origin date:** 2026-06-09
- **First detected:** 2026-06-12
- **Canonical URL:** https://earlyterms.com/term/claude-fable-5
- **Sources:** 7 primary URLs

## Definition

Claude Fable 5 is Anthropic's first publicly available Mythos-class model, built for long-horizon agentic work, software engineering, scientific research, and complex multi-step reasoning — a capability tier that previously required restricted access through Claude Mythos Preview.

Released on [June 9, 2026](https://www.anthropic.com/news/claude-fable-5-mythos-5), Fable 5 shares the same underlying architecture as Claude Mythos 5 but adds three safety classifiers — blocking cybersecurity exploitation, dual-use biology/chemistry, and model distillation — that fall back to Claude Opus 4.8 in less than 5% of sessions. Pricing is $10 per million input tokens and $50 per million output tokens, double Opus 4.8.

## Analogy

Think of it as Mythos with a safety cage: same engine, narrower blast radius.

## Why it's emerging now

Anthropic opened Mythos-class capability to the general public on June 9, 2026 — hitting 95% SWE-bench Verified and 80.3% SWE-bench Pro at $50/M output, half the price of Mythos Preview. Within 48 hours, a controversy over silent behavior modification for AI researchers triggered a public apology and emergency transparency fixes.

## Related terms

- *parent:* Claude Mythos
- *parent:* claude-mythos-preview
- *related:* claude-opus-4-8
- *related:* claude-opus-4-7
- *child:* adaptive thinking
- *related:* managed-agents
- *related:* agent-harness
- *related:* agentic-coding
- *related:* context-window
- *related:* claude-code
- *related:* long-running-agents
- *competitor:* GPT-5.5

## Sources

1. [Anthropic — Claude Fable 5 and Claude Mythos 5 launch announcement](https://www.anthropic.com/news/claude-fable-5-mythos-5)
2. [Anthropic API Docs — Introducing Claude Fable 5 and Claude Mythos 5](https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5)
3. [Hacker News — Claude Fable 5 launch thread (2,604 pts / 2,144 comments)](https://news.ycombinator.com/item?id=48463808)
4. [Simon Willison — Initial impressions of Claude Fable 5](https://simonwillison.net/2026/Jun/9/claude-fable-5/)
5. [TechCrunch — Anthropic's Claude Fable 5, released days after AI danger warning](https://techcrunch.com/2026/06/09/anthropic-released-claude-fable-5-its-most-powerful-model-publicly-days-after-warning-ai-is-getting-too-dangerous/)
6. [Endor Labs — Claude Fable 5: mid-tier results on real-world coding tasks](https://www.endorlabs.com/learn/claude-fable-5-mythos-grade-hype)
7. [The Register — Claude Fable 5 refuses innocuous prompts](https://www.theregister.com/ai-and-ml/2026/06/10/anthropic-claude-fable-5-refuses-innocuous-prompts/5253754)

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_Generated by EarlyTerms · https://earlyterms.com/term/claude-fable-5_