At the end of October 2026, a line reading "Claude Mythos 5.5" was spotted in the HTML of the Mythos page. Does that mean that a new Mythos model is coming out, or was that just an error that doesn't mean anything? The line was promptly deleted, but the rumors remain, and today we're breaking down everything we know about the potential new Mythos.
Most likely, yes. The name was spotted on Anthropic's website, which makes it hard to think that it appeared there by accident. In fact, if you check the archived copy of the page, you can verify this for yourself:
Besides that, rumors about a new Mythos checkpoint have been circulating for a bit, but they're not as credible.
| Rumor | Where it appeared | Status |
|---|---|---|
| "Claude Mythos 5.5" in the code of the Mythos page | anthropic.com | Confirmed by an archived copy |
| claude-fable-5-5 in the Azure model list | LiteLLM price list | Removed a day later |
| Opus 5.5 requests routed to a new model | Post on X | Unconfirmed |
| Newer Fable checkpoints in A/B tests | Posts on X and Discord | Unconfirmed |
| Obsidian 3 ahead of Opus 5.5 on FrontierCode 2.0 | Cognition benchmark data | Removed a day later |
| Release on Tuesday, November 3 | Post on X | Didn't happen |
| 20% lower price | Post on X | Unconfirmed |
| Leaked 14T "numbat" checkpoint | Hugging Face | Fake |
| Five exploit chains built by Mythos 5.5 | Post on X | No source |
| Mythos 5.5 benchmark scores | Posts on X, AI-written explainers | Fake |
There's a page on Anthropic's website dedicated to Mythos models, where you can find their descriptions, benchmarks and prices of the current version, Mythos 5.1. In the header section of the HTML, there's an alt text of the Mythos logo — that's where the Mythos 5.5 line was spotted.
In the archived copy from October 26, the header and the logo alt text say "Claude Mythos 5.1". In the copy from October 31, they say "Claude Claude Mythos 5.5". The name actually appears six times in the page code: in the header, in the logo descriptions for desktop and mobile, and in the page data.
This was found on November 1 by the X user Chubby (@kimmonismus), who posted a screenshot of the page code sent in by an anonymous reader, and the post got more than 435,000 views.
Someone from Anthropic was on top of that and by the evening of November 1, the header was promptly changed to "Claude Mythos 5.1". As of now, the page doesn't have any mentions of Mythos 5.5.
A likely explanation is that Anthropic started preparing a section for Mythos 5.5, but part of the change went live too early.
Since Mythos 5, Anthropic has released each Mythos model together with a Claude Fable model for everyone else. Fable 5.1, for example, is Mythos 5.1 with added safeguards for cybersecurity and biology. So when Mythos 5.5 comes out, it is very likely that Fable 5.1 will get an upgrade in the form of Fable 5.5 as well.
The real leak was quickly followed by fakes. These leaks unfortunately were debunked:
Anthropic hasn't given a date, but the pace of the previous Mythos releases points to the second half of November 2026.
So far, Anthropic has released Mythos models three times, always on a Tuesday: Mythos Preview on April 7, Mythos 5 with Fable 5 on June 9, and Mythos 5.1 with Fable 5.1 on September 1, 2026. The gap between the last two releases was 84 days, or exactly 12 weeks. The same gap after Mythos 5.1 points to Tuesday, November 24, 2026.
Some posts on X expected an earlier date. After the leak, Chetaslua wrote that the release could come on Tuesday, November 3, but Anthropic didn't release a new model that day.
Until the leak, most rumors called the next model Fable 5.2. On September 8, Chubby wrote about a new Anthropic pretraining run and used Fable 5.2 as a working name. Two days later, @Mr_Salio, who had first mentioned Fable 5.5 in August, called that run Fable 5.5. The demos of a new model that users shared on X later in September also went around as Fable 5.2.
The 5.5 name fits the rest of the lineup. Claude Opus 5.5, Sonnet 5.5 and Haiku 5.5 came out between September 22 and October 7, and Opus and Sonnet went from version 5 straight to 5.5. After Mythos 5.5 and Fable 5.5, all Claude models would share one version number. The page code and the Azure entry both use 5.5 as well, so it's the most likely final name.
Mythos 5.5 will most likely cost the same as or less than Mythos 5.1, which costs $10 per million input tokens and $50 per million output tokens in the API.
After the leak, Chetaslua wrote that the new models would be 20% cheaper than the previous generation. Anthropic made a cut of the same size with Opus 5.5, which costs $4 per million input tokens and $20 per million output tokens, against $5 and $25 for Claude Opus 5.
Mythos 5.5 will most likely be available only to verified security teams and life science researchers, like Mythos 5.1. Anthropic's Cyber Verification Program already covers Mythos 5.1 "and future models", so teams that have access today will most likely get Mythos 5.5 without a new application.
However, just like with the previous generation, Fable 5.5, a version of Mythos with stricter security guardrails, will become public and will be available on the paid Claude plans, as well as in select third-party platforms such as Overchat AI, where you can access these models without a Claude subscription.
These are the questions people ask most often about Mythos 5.5.
No, Mythos 5.5 isn't out, and Anthropic hasn't announced it. The newest Mythos model is Mythos 5.1, released on September 1, 2026.
They will most likely be one model with different access. Mythos 5.5 will go to verified organizations with fewer restrictions, and Fable 5.5 will be the public version with safeguards for cybersecurity and biology.
No. Mythos is a paid API model for verified organizations, and there's no free plan or consumer app for it. The closest model you can use without verification is Fable 5.1.
Nobody outside Anthropic has tested Mythos 5.5 yet. Among the current models, Opus 5.5 is ahead of Fable 5.1 on general tasks: on the Artificial Analysis Intelligence Index, Opus 5.5 at maximum effort scores 58, and Fable 5.1 scores 53. Mythos has the edge in security and biology tasks, where it has fewer restrictions than the other Claude models.
No. Mythos 5.5 belongs to the Claude 5 generation. Claude 6 is the next generation of Claude models, and Anthropic hasn't announced it yet.
