# Claude Fable 5: Anthropic's New Flagship AI Model

> Claude Fable 5 is Anthropic's flagship Mythos-class AI model, with a 1-million-token context window and agentic autonomy for long, complex tasks.

Source: https://camnangai.com/en/news/claude-fable-5  
Published: 2026-06-10

**Claude Fable 5** is Anthropic's new Mythos-class flagship AI model, released on June 9, 2026. It moves past the previous Opus-class hierarchy and is tuned for long-horizon autonomous work and complex reasoning. It is built for long-term planning, self-verification, and multi-stage execution across datasets larger than earlier models could handle.


  As the generally available configuration of the Mythos architecture, Claude Fable 5 is a robust
  agentic worker rather than a standard task-based assistant.

It supports a default 1-million-token context window and can generate up to 128k output tokens per
request. It includes a tiered safety architecture that routes high-risk frontier queries to a secure
fallback model while keeping top-tier performance on standard enterprise and engineering workloads.

![Cover image introducing Claude Fable 5 — Anthropic's new flagship Mythos-class AI model.](https://camnangai.com/articles/claude-fable-5/01-infographic-en.webp)

## What is Claude Fable 5?


  **Claude Fable 5** is the generally available implementation of Anthropic's Mythos-class
  intelligence.

It goes beyond the Opus-class in one concrete way: it can manage a 1-million-token context window
and produce 128k output tokens per request. The shift is from "giving AI tasks" to "assigning it
responsibilities" — the model plans across multiple stages, delegates to sub-agents, and verifies
its own execution over multi-day sessions.

A core technical requirement for Fable 5 is the use of "adaptive thinking" as the mandatory reasoning mode. Developers control reasoning depth through an `effort` parameter (options: low, medium, high, xhigh). Note that raw thinking content is never returned to the user; `thinking.display` defaults to `"omitted"`, but can be set to `"summarized"` to receive readable reasoning summaries. Integration also supports beta features such as task budgets (via the `task-budgets-2026-03-13` header) and context management (via the `context-management-2025-06-27` header).

## How Fable 5 and Mythos 5 differ

Claude Fable 5 and Claude Mythos 5 share the same underlying architecture but are distinguished by their safety tuning and access protocols:

- **Claude Fable 5:** the public-facing model using active safety classifiers. If a query is flagged for high-risk frontier capabilities, the system uses a "fallback" mechanism — these requests are routed to [Claude Opus 4.8](https://camnangai.com/en/news/claude-opus-4-8-release), which provides a safe response without a hard refusal.
- **Claude Mythos 5:** a restricted version available to trusted partners in Project Glasswing. This version has specific safeguards lifted — particularly in cybersecurity — to allow offensive security research and critical-infrastructure protection.

![Comparison of Claude Fable 5 (public, full safety classifiers) and Claude Mythos 5 (restricted, safeguards lifted, via Project Glasswing).](https://camnangai.com/articles/claude-fable-5/02-en.webp)

Anthropic reports that over 95% of Fable 5 sessions do not trigger the safety fallback, meaning the vast majority of users experience the full Mythos-class capability.

## Where Fable 5 excels

The model leads by a wide margin in large-scale engineering and dense visual synthesis:

- **Software engineering:** in a documented case study, the model executed a codebase-wide migration of a 50-million-line Ruby codebase for Stripe. This task, which would typically require a human team over two months, was completed in a single day.
- **Vision and simulation:** Fable 5 reached game completion in Pokémon FireRed using only raw screenshots and a minimal vision-only harness, whereas previous models required complex tool-assisted scaffolding. It can also derive physics simulations, such as solar-system models, from first principles to predict astronomical events.
- **Enterprise document processing:** the model excels at interpreting nested diagrams, complex tables, and charts within financial and legal PDFs, often "one-shotting" the creation of full applications from high-level specifications.

## The numbers: benchmarks and performance

The following data reflects the Mythos-class architecture's performance across validated evaluations:

| Benchmark                            | Score |
| ------------------------------------ | ----- |
| SWE-bench Pro                        | 80.3% |
| Terminal-Bench 2.1                   | 88.0% |
| OSWorld-Verified                     | 85.0% |
| Humanity's Last Exam (with tools)    | 64.5% |
| Humanity's Last Exam (without tools) | 59.0% |
| ExploitBench CTFs                    | 78.0% |
| HealthBench Professional             | 66.0% |

![Benchmark scores for Claude Fable 5 on SWE-bench Pro, Terminal-Bench 2.1, OSWorld-Verified and other evaluations.](https://camnangai.com/articles/claude-fable-5/03.webp)

_Source: [Claude Fable 5 Benchmark Scores — Weights & Biases](https://wandb.ai/byyoung3/ml-news/reports/Claude-Fable-5-Benchmark-Scores--VmlldzoxNzE3NTE3MQ)_

In frontier physics research evaluations, Fable 5 matched GPT-5.5's results in 36 hours, against the four days GPT-5.5 took — and used only a third of the reasoning tokens its competitor needed.

## Safety system and fallback

To mitigate the risk of "uplift" — where the model provides information that helps malicious actors in ways standard resources cannot — Anthropic implemented specialized classifiers targeting three domains:

1. **Cybersecurity:** detection of offensive techniques, malware development, and agentic hacking.
2. **Biology and chemistry:** sensitive molecular mechanisms and laboratory methods.
3. **Distillation:** detection of attempts to extract internal reasoning to train competing models.

![Fallback flow: a high-risk request is flagged by a classifier, the API returns HTTP 200 with stop_reason "refusal", then automatically routes to Claude Opus 4.8.](https://camnangai.com/articles/claude-fable-5/04-en.webp)

When a classifier flags a request, the API returns `stop_reason: "refusal"` within a successful HTTP 200 response. The request is then automatically served by Claude Opus 4.8. Users are informed of the fallback and are not charged Mythos-tier pricing for these rerouted responses. All Mythos-class models are "Covered Models," requiring a 30-day data retention policy. For privacy, Anthropic logs all human access to this safety data and deletes it after the 30-day window.

## Pricing, release date, and access

Claude Fable 5 was released on June 9, 2026, with the API model ID `claude-fable-5`.

- **Token pricing:** $10 per million input tokens and $50 per million output tokens.
- **Prompt caching:** a 90% discount applies to input tokens that use prompt caching, which cuts the total cost of ownership (TCO) for long-context agentic workflows.
- **Regional premiums:** workloads requiring US-only inference are subject to a 1.1x pricing premium.
- **Rollout schedule:** Pro, Max, and Team subscribers have free access until June 22, 2026. Starting June 23, the model transitions to a usage-based credit system for these tiers.

## Who should use Claude Fable 5?

Fable 5 is a premium tool designed for high-value, high-complexity workflows. Ideal usage profiles include:

- **System architects:** deploying multi-agent orchestrations that require autonomous planning and sub-agent management.
- **Engineering leads:** managing legacy codebase migrations, multi-day debugging, or large-scale refactoring.
- **Research scientists:** synthesizing novel hypotheses from dense visual data and unstructured research repositories.

For routine tasks like short-form summarization or basic Q&A, lower-tier models such as Claude Haiku or Sonnet remain the more cost-effective engineering choice.

## References

- [Claude Fable 5 and Claude Mythos 5 — Anthropic](https://www.anthropic.com/news/claude-fable-5-mythos-5)
- [Introducing Claude Fable 5 and Claude Mythos 5 — Claude API Docs](https://platform.claude.com/docs/en/about-claude/models/introducing-claude-fable-5-and-claude-mythos-5)
- [Claude Fable — Anthropic](https://www.anthropic.com/claude/fable)
- [Prompting Claude Fable 5 — Claude API Docs](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5)
- [Felix Rieseberg on X: Claude Fable 5 and Mythos 5 announcement](https://x.com/felixrieseberg/status/2064392202504310900)
- [Claude Fable 5 and Claude Mythos 5 Just Launched: Everything You Need to Know — Medium](https://medium.com/@dremfind/claude-fable-5-and-claude-mythos-5-just-launched-everything-you-need-to-know-68ca1c31b26c)