Anthropic’s site briefly named Claude Mythos 5.5. What’s known about it and Fable 5.5

What people saw

At 15:31 UTC on November 1, 2026, TestingCatalog posted: “A hidden Claude Mythos 5.5 mention has been spotted on the official Anthropic website.” @chetaslua then wrote that “Mythos 5.5” was in the page’s HTML six times, as a “hidden h1 + logo alt text,” adding: “The visible text still says 5.1.” @AiBattle_ added that an October 23 archive copy didn’t have it.

We read Anthropic’s Mythos page and the Internet Archive’s copies ourselves:

When (UTC)	The page’s hidden title	“Mythos 5.5” in the HTML
October 23, 02:24	Claude Claude Mythos 5	0 times
October 31, 21:49	Claude Claude Mythos 5.5	6 times
November 1, 20:05	Claude Mythos 5.1	0 times
Live page, November 2, 00:06	Claude Mythos 5.1	0 times

So the reports were right, and the name has since gone. Between the October 31 and November 1 copies, the only change is that title, in those same six places. The page’s newest announcements are still the Cyber Verification Program on October 6 and Mythos 5.1 on September 1.

Three things limit what this tells you:

• Visitors don’t see that title. The logo is an image that reads “Claude Mythos,” with no number, in all four copies. TestingCatalog’s screenshot shows “Claude Mythos 5.5” as a large headline, which we couldn’t reproduce from any of them.
• The field has been wrong before. On October 23 it said Mythos 5, seven weeks after Mythos 5.1 came out. The same title on Anthropic’s Fable page still reads “Claude Claude Fable 5.”
• Anthropic hasn’t explained it. We found no comment on why the name went in or came out.

It’s fair to read this as a sign that a Mythos 5.5 is in someone’s plans. It isn’t a date, a price or an announcement.

What Anthropic has said

Nothing about either model. We checked on November 1:

• The models overview lists Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5. The API pricing and deprecations pages list Fable 5.1 and Mythos 5.1, with no 5.5 of either.
• The platform release notes end on October 30 and the apps release notes on October 21, and neither names one. Nor does Anthropic’s newsroom.
• Claude Code’s model guide says “the fable alias resolves to Fable 5.1,” and its newest release in the changelog, 2.1.316, doesn’t mention Fable.

A Mythos number matters to people who will never get Mythos because of what Anthropic has said about the two lines. “Claude Fable 5.1 is the same underlying model as Claude Mythos 5.1 with safeguards for cybersecurity and biology,” says its Mythos page. It released Fable 5 and Mythos 5 together on June 9, and Fable 5.1 and Mythos 5.1 together on September 1. So people read a new Mythos as a new Fable. That’s their inference, not Anthropic’s statement.

Model	Status on November 1	Who can use it	API price per million tokens
Claude Mythos 5.1	Out since September 1	Only “vetted cyberdefenders and life scientists”	$10 in, $50 out
Claude Fable 5.1	Out since September 1	The API; Max and Ultra, up to half the weekly limit; Team premium seats; Enterprise; Pro with usage credits	$10 in, $50 out
Claude Opus 5.5	Out since September 22	The API and every paid plan	$4 in, $20 out
Claude Mythos 5.5	Not announced	Not announced	Not announced
Claude Fable 5.5	Not announced	Not announced	Not announced

Two other things Anthropic has said get folded into this. It wrote that “an unreleased, internal research model” built working exploits (our memo), and its IPO filing says it “chose to delay pre-training a new frontier base model for more than a month” (our memo). It named neither model, and it hasn’t tied either to a 5.5.

What’s rumored

Everything here is someone’s report, not Anthropic’s word.

• Testing. “Some new Claude models have recently entered early testing. A newer Fable checkpoint + new Opus,” @legit_api wrote on October 29. TestingCatalog says “multiple reports suggest” A/B tests with selected users, and that the comparisons “remain anecdotal, with no public benchmarks.” When BitsMinds went through the first wave in early October, the screenshots still showed the 5.1 label, and the same kind of clip had been credited to Opus 5.5 a day earlier.
• Azure. On October 13, LiteLLM’s price sync added a row for azure_ai/claude-fable-5-5 at $10 in and $50 out, pointing to Azure’s East US 2 model list. The next day its maintainers removed it because it “prices a model that Azure does not list,” and requests returned a 404. MLflow’s catalog still carries the row, with the same numbers as Fable 5.1’s. TestingCatalog cautions that “most of those values appear to have been carried over from Fable 5.1.” Polymarket’s small market had 36% on $50 or more for output when we read it.
• Partners. “Partners got access to the upcoming Claude Fable 5.5,” @lyraxana wrote on October 15, with nothing attached.
• A new Opus. Besides @legit_api’s post, TestingCatalog’s October 17 piece lists early-access codenames found in Cognition’s Devin CLI, and it treats matching them to models as uncertain. Polymarket had 66% on a new Opus by November 30, in a small market.

The dates are the weakest part:

Said on	By	The claim	Outcome
October 1	@imjustnewatai	“expected to release next week… I’m hearing of a Tuesday release”	Passed
October 15	Andrew Curran, as quoted by OrcaRouter; we didn’t read the original	“may actually arrive today,” or “could get pushed to next week”	Passed
October 15	@lyraxana	“I assume next week”	Passed
October 17	TestingCatalog	“Anthropic prepares to release Claude Fable 5.5 next week”	Passed
November 1	TestingCatalog	“could point to a launch as early as next week”	Open
November 1	@chetaslua	“i think tuesday can be launch day”	Open

Tuesday, November 3 is an easy guess. Fable 5, Fable 5.1 and Opus 5.5 all launched on a Tuesday, though Sonnet 5.5 came on a Monday and Haiku 5.5 on a Wednesday. As BitsMinds put it, “Someone with no information at all would also say Tuesday.”

There’s reason not to dismiss it all, either. BitsMinds notes that @synthwavedd reported a Fable successor in testing on August 18, and Fable 5.1 shipped 14 days later. When we read Polymarket on November 1, it had the next Fable at 21% by November 3, 52% by November 12, 83% by November 30 and 94% by December 31, with about $1.3 million traded and every date through October 31 already settled as no. That’s what bettors think, not information.

What to do

1. Don’t buy or change a plan over it. There’s no price and no list of plans. Decide on what Fable 5.1 costs you today: usage credits on Pro, up to half the weekly limit on Max and Ultra. Our comparison goes through the plans.
2. Know what your Claude Code setting will do. Anthropic says aliases “update over time,” and its changelog says that picking Fable in /model on a claude.ai login “follows the newest Fable.” So a new Fable could reach you without a choice. To stay on today’s, use the full name: /model claude-fable-5-1. Fable 5.1 needed Claude Code v2.1.257 or later, so a new one may need claude update too, though Anthropic hasn’t said.
3. On the API, nothing moves by itself. “Every Claude model ID is a pinned snapshot,” says the models overview, and the deprecations page gives Fable 5.1’s retirement as “Not sooner than September 1, 2027.” Test any new model against your own work before you switch.
4. If you do security or life sciences work, apply now. Each Cyber Verification Program tier “includes access to our most capable models, including Claude Opus 5.5, Claude Sonnet 5.5, Claude Mythos 5.1, and new models moving forward.” Its help article aims to answer “within seven business days.” Life sciences researchers have their own program. Neither promises a Mythos 5.5, and of Mythos 5.1 Anthropic says “Access remains limited to a small set of vetted organizations.”
5. Wait for the pages that settle it. A launch shows up as a post in Anthropic’s newsroom, a row in the models overview and a price on the pricing page. Until then, a screenshot is a screenshot.

If a new Fable reaches Claude Code

Enjoy runs Claude Code on your Claude plan and asks Claude Code for its current models each time you open the settings, so a new model appears there without an Enjoy update, once your Claude Code offers it. If it’s missing, update Claude Code. If a new model uses up your limit faster, you can switch the conversation to another agent, such as Codex, and it gets the messages it missed. Enjoy doesn’t add usage to a Claude plan, and it can’t give you a model your account doesn’t have, Mythos included.

What Anthropic hasn’t said

• Whether Mythos 5.5 or Fable 5.5 will ship, under those names, and whether they’d launch together.
• Any date, price, plan, limit or system card for either.
• Why the name went onto the Mythos page, and why it came off.
• Whether some users are testing a newer model while the picker says Fable 5.1.
• Whether a new Opus is coming.

We’ll update this memo when Anthropic fills these in.