# Anthropic Goes Big: Claude Fable 5 and Mythos 5 Are Here

> On June 9, 2026, Anthropic shipped Claude Fable 5 and Mythos 5 simultaneously: the same underlying model, differing only in guardrails. Fable 5 is SOTA on nearly every benchmark, with the lead growing on longer tasks (Stripe migrated a 50-million-line codebase in one day). Priced at $10/$50 — less than half of Mythos Preview — and already live on Amazon Bedrock. Plus: why did the hidden Mythos suddenly come out of the vault?

- Author: zhuermu
- Published: 2026-06-10
- Web version: https://zhuermu.com/en/blog/claude-fable-5-mythos-launch/

---
**Mythos-class capability · Built-in safety guardrails · Available now on AWS**

On June 9, 2026, Anthropic released two models at once: **Claude Fable 5 and Claude Mythos 5**. The same day, AWS announced Fable 5 was live on Amazon Bedrock. This article covers everything in one pass: where it's strong, what's new, what it costs, how to use it on AWS — and then a more interesting question: why did the Mythos model, hidden away for months, suddenly get released?

## First, Sort Out: What Exactly Is Fable vs Mythos?

Above the Opus series, Anthropic carved out a higher capability tier: **Mythos-class**. In April, through "Project Glasswing," they handed the first Mythos-class model (Mythos Preview) to a small group of cyber-defense organizations only. Ordinary users couldn't touch it.

This time, Fable 5 and Mythos 5 are **the same underlying model**. The difference is one word: guardrails.

- **Fable 5**: guardrails on, available to everyone.
- **Mythos 5**: guardrails removed in certain domains, available only to a handful of vetted cybersecurity / life-science partners.

> 💡 Trivia: "Fable" comes from the Latin *fabula* — "a story that is told" — cognate with the Greek *mythos*. Different names, different guardrails, same soul.

## Performance: The Longer the Task, the More Absurd the Lead

![Claude Mythos 5 / Fable 5 benchmark comparison](/images/blog/claude-fable-5-mythos-launch/benchmarks.jpg)
*Mythos 5 / Fable 5 vs mainstream models (source: Anthropic)*

Anthropic's own words: **Fable 5 is SOTA on nearly every benchmark tested**. And there's a crucial pattern — **the longer and more complex the task, the wider the gap it opens over other models.**

Early customer reports say more than benchmark tables:

- **Stripe**: compressed months of engineering into days. A full-repository migration in a 50-million-line Ruby codebase — over two months of manual work for an entire team — done by Fable 5 in one day.
- **Coding**: highest score on Cognition's FrontierCode eval, while using fewer tokens — it takes first place even at medium effort.
- **Financial analysis**: top score among all models on Hebbia's senior-level financial reasoning benchmark, with dramatically improved chart and table reading.

> ✦ **One vivid example**: previous Claudes needed a pile of auxiliary tools — maps, navigation aids — to barely play Pokémon FireRed. Fable 5 beat the entire game from raw screenshots alone. Pure vision.

## The New Capabilities: It Can Do "Long Work" Now

### Long-Horizon Autonomous Execution

The biggest change. Fable 5 executes complex coding and knowledge tasks for extended periods, asynchronously, without a human watching — the kind of work previous models couldn't sustain.

### Stronger Vision

It reads charts and tables nested inside PDFs and files, and can even reverse-engineer a webpage's source code from a screenshot. While coding, it uses its "eyes" to check whether its output matches the goal — document-heavy fields like finance, law, analytics, and architecture benefit most.

### Self-Verification and Self-Evolution

It updates its own skills based on what it learns, even building its own test harnesses and evaluation tools. Combined with file-based long-term memory, its memory-driven improvement in games like Slay the Spire is three times that of Opus 4.8.

### Research Capability (Mostly Showcased on Mythos 5)

With Mythos 5, Anthropic's in-house protein-design experts accelerated parts of drug-design workflows roughly 10x. In molecular biology, scientists preferred Mythos-generated hypotheses about 80% of the time in blind tests. It also ran over a week of independent genomics research, training a model that beat recent *Science*-published results at 1/100th the size.

## Safety Guardrails: The Greater the Power, the Stronger the Backstop

A model this capable, running naked, could be weaponized in cybersecurity or biology. So Fable 5 ships with an independent classifier AI system dedicated to detecting abuse and jailbreak attempts.

The mechanism is elegant: when it detects sensitive requests involving **cybersecurity, biochemistry, or model distillation**, it **doesn't refuse — it automatically downgrades the request to Claude Opus 4.8**, and tells you the switch happened.

Anthropic admits the guardrails are tuned conservative and occasionally hit legitimate requests. But the trigger rate averages below 5% — meaning **over 95% of sessions are never downgraded**, and in those sessions Fable 5 performs essentially identically to Mythos 5.

Mythos 5 is the version with those guardrails removed. It currently holds the strongest cybersecurity capability in the world, and is available only through Project Glasswing (in partnership with the US government) to a limited set of cyber defenders and infrastructure partners.

## Pricing: Less Than Half of Mythos Preview

| Item | Price |
|---|---|
| Input (per million tokens) | $10 |
| Output (per million tokens) | $50 |

**Fable 5 and Mythos 5 cost the same.** To be clear, these are first-release models — there's no "price-cut iteration" here. The "cheaper" comparison is against April's first Mythos-class model, Claude Mythos Preview: less than half its price (per Anthropic). One detail: on AWS, if your request gets downgraded to Opus 4.8, that portion bills at **Opus rates**; if a conversation is intercepted mid-stream, earlier tokens bill at Fable rates and later ones at Opus rates.

## How to Use It on AWS

Two paths: **Amazon Bedrock**, or **Claude Platform on AWS** (the native Anthropic platform experience). Bedrock example below.

### Step 1: You Must Enable Data Sharing First

This is a hard gate. Before calling the model, enable `provider_data_sharing` via the Data Retention API. Anthropic requires **30-day data retention** on all Mythos-class traffic, including human review — the goal is catching abuse patterns invisible within a single conversation. Note: once enabled, your data leaves AWS's data and security boundary.

```bash
curl -X PUT \
  https://bedrock-mantle.us-east-1.api.aws/v1/data_retention \
  -H "x-api-key: <your-bedrock-api-key>" \
  -H "Content-Type: application/json" \
  -d '{ "mode": "provider_data_share" }'
```

### Step 2: Call the Model

Option A: the Anthropic SDK over the Messages API (`bedrock-mantle` endpoint). First `pip install anthropic`:

```python
import anthropic

client = anthropic.Anthropic(
    base_url="https://bedrock-mantle.us-east-1.api.aws/anthropic",
    api_key="<your-bedrock-api-key>",
)
message = client.messages.create(
    model="anthropic.claude-fable-5",
    max_tokens=4096,
    messages=[{"role": "user", "content": "..."}],
)
print(message.content[0].text)
```

Option B: stick with the familiar Invoke / Converse API (`bedrock-runtime` endpoint) via Boto3:

```python
import boto3

rt = boto3.client("bedrock-runtime", region_name="us-east-1")
resp = rt.converse(
    modelId="global.anthropic.claude-fable-5",
    messages=[{"role": "user", "content": [{"text": "..."}]}],
    inferenceConfig={"maxTokens": 4096},
)
print(resp["output"]["message"]["content"][0]["text"])
```

Key points:

- Regions: currently live in **US East (N. Virginia)** and **Europe (Stockholm)**.
- Access is rolling out to all AWS accounts; if you need it urgently, contact AWS Support.
- Mythos 5 on Bedrock is a **Limited Preview**, aimed at dual-use scenarios like cybersecurity and life sciences.

## One More Thing: Why Did the Hidden Mythos Suddenly Come Out?

Put bluntly: in April, Mythos Preview was government-and-security-labs-only, wearing a "too dangerous for you" posture. Two months later, with a new name and a layer of guardrails, it's open to everyone. That turn deserves scrutiny. A few observations — strictly my own reading:

- **The official story is "the guardrails matured."** Anthropic's narrative has always been: lock capability away first, release when safety measures are strong enough. This time they ran thousands of hours of external bug bounties and red-teaming, and claim no "universal jailbreak" was found. The story fits their persona, and it does have technical substance.
- **But competitive pressure is the unavoidable reality.** The frontier arms race never pauses. Keeping your strongest capability in a vault means ceding the market to rivals. Cutting the price by more than half and racing onto AWS / GCP / Azure simultaneously — that is an unambiguous land-grab move.
- **Is it IPO staging, then?** Can't be concluded, but the timing is suggestive. Shipping the "world's strongest" flagship while telling the "we're the most responsible" safety story — for any company looking to raise its valuation and narrate a growth curve, that's textbook material.

> ✦ **My take**: rather than "IPO staging," call it a precise confluence of safety and commerce. Technically, the guardrails did improve — that's the external confidence. Commercially, sitting on unmonetized frontier capability is an enormous opportunity cost. **"Hide it first, then release it" is itself the best marketing script** — it manufactures the scarcity of "too powerful to release," then cashes in at the right window. Whether the timing tracks a capital-markets calendar, outsiders can only infer — but the commercial intent is written on its face.

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## Frequently asked

### What is the relationship between Claude Fable 5 and Mythos 5?

They are the same underlying model; the only difference is guardrails. Fable 5 ships with safety guardrails for everyone; Mythos 5 removes them in domains like cybersecurity and life sciences and is available only to a small set of vetted partners. Anthropic defines 'Mythos-class' as a capability tier above Opus.

### How much does Claude Fable 5 cost?

Input $10 per million tokens, output $50 per million — Fable 5 and Mythos 5 are priced identically, less than half of April's Mythos Preview. Note: requests downgraded by the safety classifier to Opus 4.8 are billed at Opus rates.

### How do I use Claude Fable 5 on AWS?

Two paths: Amazon Bedrock or Claude Platform on AWS. On Bedrock you must first enable provider_data_sharing via the Data Retention API (Anthropic requires 30-day retention for all Mythos-class traffic), then call the model via the Anthropic SDK (bedrock-mantle endpoint) or the familiar Converse API (bedrock-runtime endpoint). Currently live in US East (N. Virginia) and Europe (Stockholm).

### How do Fable 5's safety guardrails work?

An independent classifier AI detects abuse and jailbreak attempts. On sensitive requests involving cybersecurity, biochemistry, or model distillation, it doesn't refuse — it automatically downgrades the request to Claude Opus 4.8 and tells you the switch happened. Trigger rate averages under 5%, so over 95% of sessions are never downgraded.