Anthropic · llm model

Claude Mythos 5.1

Claude Mythos 5.1 is an Anthropic language model (limited access) released Sep 1, 2026; Anthropic's newest language model in our dataset is Claude Haiku 5.5. Proprietary, API access only. Anthropic's access-limited Claude Mythos successor for Project Glasswing participants, launched with Claude Fable 5.1 (release-notes date Sep 1, 2026). Verified against vendor sources on Oct 26, 2026.

Reported Last verified Oct 26, 2026 · All models · JSON · Markdown

ReleasedSep 1, 2026
Context window1M1,000,000 tokens
Max output128K128,000 tokens
LicenceProprietaryProprietary (API)

Modalities not verified

Key facts

Claude Mythos 5.1: key facts with sources
Fact	Value	Source
Vendor	Anthropic	n/a
Released	Sep 1, 2026 Date of the launch entry in the Claude Platform release notes (API availability).	source
Status	limited access	n/a
API id	claude-mythos-5-1	source
Context window	1,000,000 tokens (1M)	source
Max output	128,000 tokens	source
Licence	Proprietary (API)	source
Pricing	$10 in / $50 out per 1M tokens	source

Where Claude Mythos 5.1 sits in our data

Released Sep 1, 2026 (week 36 of 2026), in September 2026. Other models in our data released that month: GPT-6.1 Sol, Claude Sonnet 5.5, Claude Opus 5.5, Gemini 3.8 Flash TTS, Gemini 3.8 Flash-Lite TTS, GPT-6 Luna.
The previous Anthropic model we track is Claude Opus 5 (Jul 24, 2026), 39 days earlier.
Of the 60 models in our data with a published context window, Claude Mythos 5.1 has the number 30 largest (1M tokens). Our data is not every model on the market.
Anthropic tools we cover: Claude Code, Claude.
All releases by date: AI model releases by month · new AI models this week. Choosing for code: best LLM for coding.

How Claude Mythos 5.1 differs from Claude Fable 5.1

Read from the recorded facts of the two models, not from a head-to-head test. We have not run either.

Listed price is the same as Claude Fable 5.1: $10 in and $50 out (per 1M tokens).
Both list a 1M-token context window.
Status is limited access, against generally available.
Claude Mythos 5.1 came out 14 days before Claude Fable 5.1.
Facts for the other model: Claude Fable 5.1.

What we tested

Nothing yet. We have not run Claude Mythos 5.1 ourselves, so every fact on this page is Reported from the sources below. When we do test it, the scope will be stated here and in our test logs. See methodology.

Not verified, left out

We could not confirm these on a vendor page, so we do not state them.

Anthropic's newsroom announcement for Fable 5.1 and Mythos is dated Sep 15, 2026 on our Fable 5.1 page; this page uses the API release-notes date.