Claude Mythos 5.5: Leaked on Anthropic's Site, Fable 5.5 Twin
Mythos 5.5 appeared in anthropic.com's HTML on Nov 1, 2026 as Anthropic's top cyber and biology model. How it relates to Fable 5.5, access, price, release date.
Quick answers
- What is Claude Mythos 5.5?
- Anthropic's next Mythos-class model. Its name showed up in the HTML of anthropic.com/claude/mythos on Nov 1, 2026, tagged "Our most capable model class for cybersecurity and biology research."
- Is Mythos 5.5 released?
- No. As of Nov 2, 2026, Anthropic has not announced it. The visible page still shows Claude Mythos 5.1, and the hidden 5.5 text was later removed.
- How is Mythos 5.5 related to Fable 5.5?
- Same model, two versions. Anthropic says Fable 5.1 is the same model as Mythos 5.1 with cyber and bio safeguards. Mythos 5.5 should pair with Fable 5.5 the same way.
- Who can use Mythos 5.5?
- Based on Mythos 5.1, only vetted cyberdefenders and life scientists through Anthropic's trusted access programs. Most people will get Fable 5.5 instead.
Mythos 5.5 is the next Claude Mythos model, and Anthropic's own website leaked it. Late on Nov 1, 2026, the AI account Chubby posted a screenshot of the page source of anthropic.com/claude/mythos. The hidden title and the logo alt text read "Claude Claude Mythos 5.5", with a doubled "Claude" that looks like a placeholder.
Here is what Mythos 5.5 is, how it fits with Fable 5.5, who will get it, and what it may cost.
1. The Mythos 5.5 leak: what was found
Signal | Detail |
|---|---|
Where | anthropic.com/claude/mythos, HTML source (hidden title, alt text) |
When | Nov 1, 2026, late evening (UTC+8) |
Text | "Claude Mythos 5.5 - Our most capable model class for cybersecurity and biology research" |
Visible page | Still Claude Mythos 5.1 (Sep 1, 2026), price $10 / $50 per 1M tokens |
After | The 5.5 text was removed within hours; the title went back to 5.1 |
Official word | None from Anthropic as of Nov 2 |
We checked the page on Nov 2: the source now only mentions Mythos 5.1. TestingCatalog and others saved the earlier version. A model name in production HTML usually means the launch page is staged and ready, which is why many expect Mythos 5.5 within days.
2. Mythos 5.5 vs Fable 5.5: one model, two versions
Anthropic already explains the split on its Mythos page: "Claude Fable 5.1 is the same underlying model as Claude Mythos 5.1 with safeguards for cybersecurity and biology." Fable blocks things like exploit writing and penetration testing. Mythos has those limits lifted for trusted users.
Mythos 5.5 (expected) | Fable 5.5 (expected) | |
|---|---|---|
Model | Same base model | Same base model |
Safeguards | Relaxed for cyber and bio work | Full cyber and bio safeguards |
Who gets it | Vetted security teams and labs | General developers and Claude plans |
Context | 1M tokens (per Fable 5.5 catalog leaks) | 1M input, 128K output |
Price | Same as Fable, per reports | Not announced |
So the Mythos 5.5 leak is also strong news for Fable 5.5. Fable 5.5 has been spotted in Azure and MLflow model catalogs with a 1M-token input window and 128K max output. For the full Fable timeline, see our Fable 5.5 release date tracker.
3. What Mythos 5.5 may already have done
- Linux kernel in 53 minutes: Anthropic described an unreleased internal model that got root on Linux through a serious bug in 53 minutes. Researchers with Mythos 5.1 could not repeat it in three days and $2,000 of usage. Many now link that run to the new model.
- A math paper with no human authors: a paper solving a classic computer science problem, signed only "CLAUDE (ANTHROPIC)", used about 14M tokens. AI watcher Andrew Curran tied it to the same unguarded model.
- Fable 5.5 demos: testers say Fable 5.5 built a full browser Minecraft, with the Nether and the End, from one prompt in a 1–2 hour run.
Anthropic has not tied any of this to a named model. It is the best reading of the leaks right now.
4. Mythos 5.5 release date
No date yet. Testers point to Nov 3 or Nov 4, 2026. Context matters: Anthropic filed its public S-1 on Oct 26 and is expected to list in mid-November, and OpenAI's GPT-6.2 Astra is already in a Codex gray test. Both would likely launch together, since they are one model.
5. Who can get Mythos 5.5 and what it may cost
- Cyber Verification Program: for security teams; already covers Opus 5.5, Sonnet 5.5, Mythos 5.1 "and future models".
- Life Sciences Verification Program: for biology researchers.
- Project Glasswing: Anthropic's partner program, now about 150 more organizations in 15+ countries.
- Price: Mythos 5.1 starts at $10 in and $50 out per 1M tokens. Expect the new version at or above that.
6. Use Claude Opus 5.5 in Sumus today
Yes, you can use Anthropic's strongest public model right now, without waiting for Mythos 5.5 access. In Sumus, Claude Opus 5.5 works as an AI employee in your team chat, next to GPT-6 Astra and other top models. Give it your security review, research or coding task today, and add Fable 5.5 to the same thread the day it ships.
FAQ
Is Mythos 5.5 real?
The name is real: it appeared in Anthropic's own website code on Nov 1, 2026. The model is not released.
Is Mythos 5.5 better than Mythos 5.1?
It should be. It is the next version of the same line, and Anthropic's tagline calls it its most capable class for cyber and biology.
Can I get Claude Mythos 5.5 on Claude Pro or Max?
Unlikely. Mythos models go to vetted partners. Paid Claude plans will most likely get Fable 5.5.
Is Mythos 5.5 on AWS or Azure?
Not yet. Fable 5.5 has appeared in the Azure catalog; Mythos 5.5 has not.
What does Mythos 5.5 cost?
No price yet. Mythos 5.1 costs $10 per 1M input tokens and $50 per 1M output tokens.
Sources
- Chubby on X: Mythos 5.5 in Anthropic's HTML
- Anthropic: Claude Mythos page
- TestingCatalog on X; Xinzhiyuan (WeChat), Nov 2, 2026
Update log
- — Nov 2: First published. Mythos 5.5 found in anthropic.com HTML on Nov 1; not announced.