Claude Mythos 5.1
Anthropic's top cyber and biology model, open only to vetted groups. Most people should use Opus 5.5.
This product is made by Anthropic, whose Claude models help draft our reviews. How we keep that from tilting a score
Should you use it?
Pick it if
Vetted security teams and life science researchers accepted into Anthropic's programs.
Skip it if
You are not in one of those programs; you cannot get access.
The short version
- What this is
- Anthropic's most capable model for security and biology research, offered only to vetted organizations through its Cyber and Life Sciences Verification Programs and Project Glasswing.
- Why it matters
- It gets a lot of coverage, but almost no reader can use it; this tells you what to use instead.
- Released 1 Sep 2026 for vetted security and science groups only s1.
- API list price $10 / $50 per 1M tokens, for those with access s3.
- No public leaderboard results yet, so no score yet.
Good at
- Anthropic calls it its most capable model for cybersecurity and biology research s1.
Our take
Mythos 5.1 is Anthropic's strongest model for security and biology work, and Anthropic announced it on 1 Sep 2026 s1. It is listed as active in the API s6, but only vetted organizations can use it, through Anthropic's Cyber and Life Sciences Verification Programs and Project Glasswing s1 s2. Anthropic says Fable 5.1 is the same underlying model with added safeguards for broad release s1.
Because it is not on any public leaderboard we checked s4 s5, we can't score it. For nearly everyone, the useful choice is Fable 5.1 or Opus 5.5.
Not scored yet
This is a First look: the call above comes from the maker's own pages, so there is no score yet. Once it is open to the public and arena.ai or Artificial Analysis list it.
ResearchCodingAgents
New to this? What the scores and labels mean
- 0 to 100
- Our score for one job, like writing or coding, compared with everything else out today. 100 is the best available today. For AI models we build it from public data, such as Arena's blind votes and Epoch AI's independent tests; apps and tools are scored on our own checklist. The same tool can score differently for different jobs, and where two likely ranges overlap, we call it a tie.
- Our verdict
- Use it: a top choice for most people. Worth trying: good, and worth a go. Depends: good for some people or one kind of job. Wait: too early to tell, or not out for everyone yet. Skip: something else does the job better.
- First look
- Just released. We say what it is and whether to bother, and give it a score once public results exist.
- No score yet
- None of the public sources we use covers it yet, or the only evidence is from a source whose numbers we can't use, so we describe it in words instead.
- Best buy
- Nothing cheaper scores as well for that job.
- Best free
- The best score for that job that costs nothing, through a free plan or by running on your own computer.
- Pay as you go
- No monthly plan: you pay for each use, usually through a developer account.
- Checked
- The day we last confirmed the score and the price against the sources. Prices change often, so check the date.
- AI model
- The engine underneath, like GPT or Claude. You use it through an app or, if you code, directly.
- Open source
- Free to download and run on your own computer, so nothing leaves your machine.
Alternatives
Better or cheaper for research, from our scores, then the ones we'd also look at. The same kind of product comes first.
- Gemini 4 ArgonTop scoredText model95$4.99 a monthScores 95 in research, first in our ranking; $4.99 a month. Side by side
- MiMo-V2.6-ProFreeText model94FreeScores 94 in research, with a free plan. Side by side
- Claude Opus 5.5Text model94$20 a monthFirst on arena.ai WebDev and Agent today s4, and open to everyone. Side by side
- Claude Fable 5.1Text model88$100 a monthAnthropic says it is the same underlying model with added safeguards; our Fable page points most people to Opus 5.5. Side by side
- GPT-6 AstraText model93$20 a monthOpenAI's top model if you want a non-Anthropic option. Side by side
Evidence
Everything behind the call and the scores, folded so the page stays short. Spotted a wrong price, score or link? Report a mistake.
How we scored this
This product is made by Anthropic, whose Claude models help draft our reviews.
Reviews are drafted with help from AI models, including Anthropic's Claude. Model scores come only from outside leaderboards and independent evaluations, by a written rule.
No one paid for this review or gave us free access, and we earn nothing if you sign up. How we score
Sources 6
- s1Anthropic: Claude Mythos
- s2Anthropic: Project Glasswing
- s3Claude API pricing
- s4arena.ai leaderboards
- s5Artificial Analysis LLM leaderboard
- s6Claude model deprecations (model status table)
Something here out of date or wrong? Report a mistake and we will check it against the source.
History 2 changes
Every change to a rating, the price or our call, with the reason. Scores are against what exists on the day, so they drift down as better things ship. Older entries come from before we moved to scores.
- First look: restricted access, not rated.
- Audit: put Opus 5.5 first, since our Fable 5.1 page recommends Opus 5.5 for most people.
Use it well 3 guides on this site
Pages on this site that help with the jobs above.