Claude Mythos 5 — what it is, and how to actually start.

01Two doors into one model

On June 9, 2026, Anthropic shipped a new model family it calls "Mythos-class" — a tier above its previous flagship, Opus. Rather than choosing between shipping it widely or keeping it safe, Anthropic did both, through two doors:

Why bother locking up Mythos? Because the capability is genuinely dual-use. The early results Anthropic reported — roughly 10× faster drug design (9 of 14 protein targets produced strong candidates), molecular-biology hypotheses scientists preferred 80% of the time in blinded tests, and an unattended week of genomics work that beat a recent Science paper — come from the same machinery that could, in the wrong hands, help design something harmful. The condensed announcement breakdown lives on our Fable 5 page.

02Can you get Mythos 5 itself?

Honest answer for almost everyone: not yet, and possibly not ever directly — and that's fine, because Fable 5 is the same model for everyday work. Here's the current access picture:

• Today — Project Glasswing participants only (cybersecurity defenders and infrastructure providers working with Anthropic).
• Coming weeks — a dedicated biology program for biomedical researchers, with cyber safeguards intact but biology/chemistry protections removed.
• Later — Anthropic says a broader trusted-access program with a systematic application process is in development. No dates or criteria published yet; watch the Anthropic newsroom.
• Pricing when you do have access — same as Fable: $10 per million input tokens, $50 per million output tokens.

03Getting started today — the manual

Fable 5 is rolling out across subscription plans June 9–23, so if you don't see it in a menu yet, check back in a day or two — that's expected, not a problem. Three ways in, easiest first:

A. In the Claude app (claude.ai) — easiest

1. Sign in at claude.ai with your existing account (or create one — the free tier works for trying things out, though Fable lands on paid plans first).
2. Open the model picker — the small model name shown near the message box. Click it.
3. Choose "Fable 5." If it isn't listed yet, your plan's rollout day hasn't arrived; Opus 4.8 remains an excellent stand-in until it does.
4. Ask it something real. Don't test with trivia — give it an actual task from your day: "summarize this document," "plan this project," "explain this bill." Capability differences show up on real work, not riddles.

B. In Claude Code — for website and coding work

1. Open Claude Code in your terminal (or the desktop app). If you haven't installed it, the Claude Code docs walk through setup for Windows, Mac, and Linux — and our own OS guides cover the same in plainer language.
2. Type /model and pick Fable 5 from the list. That's the whole switch — it also saves as your default for future sessions.
3. Give it a long-horizon job. Fable's headline strength is working autonomously across millions of tokens — multi-file builds, migrations, overnight runs. The bigger and better-specified the task, the more the new model shows its worth. Stripe reported it "compressed months of engineering into days" on a codebase migration.

C. Through the API — for builders

1. Get an API key from the Claude Platform console if you don't have one.
2. Use the model ID claude-fable-5 — exactly that string, no date suffix.
3. Mind the new rules: Fable 5 accepts adaptive thinking only (thinking: {"type": "adaptive"}). Sending the old budget_tokens style, sampling parameters (temperature, top_p, top_k), or even an explicit thinking: {"type": "disabled"} returns a 400 error — omit what you don't need.
4. Budget deliberately. At $10 in / $50 out per million tokens, Fable costs 2× Opus 4.8. Route the hard, long-horizon work to Fable and keep everyday calls on Opus or Sonnet.

04Quick-reference card

05Resources worth bookmarking

And on this site: the condensed announcement breakdown, the assistant comparison, and the device setup guides for getting Claude running on whatever machine is in front of you.

06Videos

• Anthropic's YouTube channel — launch videos, developer walkthroughs, and the "Claude Plays..." series. Expect Fable 5 launch material here in the days after the announcement; the channel is the reliable first stop for official video.
• Search tip — on YouTube, search "Claude Fable 5" or "Claude Mythos 5" and sort by upload date. Launch-week explainers from independent channels vary in accuracy; when a video's claims conflict with the official announcement, trust the announcement.

07Courses & structured learning

Neither has a Fable/Mythos-specific course yet — the announcement is a day old — but everything in them transfers directly: prompting, tool use, and agent patterns work the same on Fable 5, just better.

08Train in person: Austin

If you're in Austin and want immersion rather than self-paced study, the standout option sits in the downtown tech district, around the corner from Capital Factory:

• Gauntlet AI — an intensive, completely free AI-engineering program founded by Austen Allred (of Lambda School/BloomTech). Roughly ten weeks — a few weeks remote, then in-person in Austin with housing and food covered — at 80–100 hours a week, shipping real AI systems every week. Graduates receive a $200K/yr AI engineering job offer in Austin. Apply at gauntletai.com/apply.
• Is there a Mythos track? No program anywhere has announced Mythos-specific training yet — the model shipped yesterday. Gauntlet's curriculum is "AI-first" engineering on whatever the current frontier models are, which as of this month means Fable 5. Check their site for current cohort dates and curriculum.
• Capital Factory — Austin's startup hub, steps away from Gauntlet, runs a steady calendar of AI meetups, demo days, and talks. The lower-commitment way to stay current: show up to an AI event, see what people are building with the new models, ask questions.
• Fair warning — Gauntlet is built for people who can commit 1,000 hours over ten weeks. For a gentler on-ramp, the free courses in section 07 plus Capital Factory events cover the same ground at your own pace.

09What people miss (so you don't)

• "Mythos" isn't a product you buy — it's the class name plus the restricted variant. If a website or video offers to sell you "Mythos 5 access," it's a scam. The real thing has no public sign-up.
• Fable isn't a downgrade — for everything outside the three guarded domains (advanced cyber, bio/chem, model copying), Fable 5 is the full Mythos-class model. The classifiers fire in under 5% of sessions.
• The phased rollout fools people — June 9–23 means your account may get Fable days after your friend's. Missing from your model picker ≠ broken.
• Premium pricing changes the habit — at 2× Opus, Fable rewards "one big well-specified task" over "lots of small chats." Write the full job up front; let it run long.
• External red-teaming held — zero successful single-turn jailbreaks across 30 published techniques. The split design appears to be working as intended so far.
• Watch for the biology program — if you or someone you know does biomedical research, the coming-weeks researcher program is the first real chance at Mythos itself.

Sources: Anthropic, "Claude Fable 5 and Claude Mythos 5" (June 9, 2026); Gauntlet AI; Capital Factory. Figures and quotes as reported in the official announcement. See also the condensed announcement breakdown — and from the archive, our May 2026 Mythos preview, written before launch.