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Mythos-Class Models

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Mythos-Class Models

A tier of Anthropic Claude models that sit above Opus in capability — a new generation name (Fable) alongside Haiku / Sonnet / Opus. Announced/launched ~June 9, 2026 and covered here via two YouTube reactions: Claude Mythos is Finally Here (Nate Herk) and MYTHOS MYTHOS MYTHOS (Matthew Berman).

Everything here is sourced from creator reactions to Anthropic's announcement/blog, not independently verified. Vendor benchmarks; treat as claims. Note this supersedes older "Opus is current SOTA" lines elsewhere in the vault.

The three models in the tier

Model Access Guardrails Cost vs Opus
Mythos preview Project Glasswing (April 2026) cyber safeguards lifted ~5× Opus
Fable 5 Everyone, today "made safe for general use" ~2× Opus
Mythos 5 Glasswing partners only (security community, US gov, infra/cyber defenders) cyber safeguards lifted ~2× Opus (same model as Fable)

Fable 5 and Mythos 5 are the same model under the hood — the only difference is that Mythos has the cyber guardrails removed. Glasswing preview holders can upgrade to Mythos 5. Berman claims it's a 10-trillion-parameter model, "first of its kind."

Pricing & availability

  • $10 / million input, $50 / million output (≈2× Opus; less than half the price of Mythos preview).
  • Fable 5 free on Pro Max / Team / Seat-based Enterprise plans until June 22, 2026; from June 23 → usage-credit (token) billing. Anthropic aims to restore subscription access "when capacity allows."
  • Fable 5 available in the Claude API; Anthropic expects very high demand.

Capability claims

  • State of the art on nearly all tested benchmarks (SE, knowledge work, vision, scientific research). The longer/more complex the task, the larger Fable's lead. Headline numbers (vendor-reported) tabulated in MYTHOS MYTHOS MYTHOS (Matthew Berman) — e.g. SWE-Bench Pro 80% (Opus 4.8 69%, GPT 5.5 58%), Terminal Bench 88%, new #1 on Humanity's Last Exam.
  • Mythos 5 has "the strongest cybersecurity capabilities of any model in the world"; internal protein-design experts accelerated drug-design steps ~10×.
  • Stripe early test: months of engineering compressed into days on a 50M-line Ruby codebase (codebase-wide migration in a day).
  • Pokémon Fire Red completed with vision alone, no helper harness.
  • More token-efficient than past Claude models, but very verbose and information-dense.
  • Effort levels: both creators advise running at low effort — even that often overkill; Nate pegs Fable-low ≈ Opus-4.8-X-high.

Quirks (Berman, first-hand)

Extreme verbosity, over-eager clarifying questions, and a slow "thinking" start (minutes at ~1,500 tokens, then a burst to ~1.5M) — see MYTHOS MYTHOS MYTHOS (Matthew Berman).

Safety & strategy

  • Conservative safeguards triggering <5% of sessions; distillation-flagged requests fall back to Opus 4.8.
  • New 30-day data-retention policy for Mythos-class traffic (logged human access, deletion after 30 days, not used for training).
  • Both creators read the staggered Fable/Mythos release as Anthropic protecting a lead and accelerating its own research (see Recursive Self-Improvement, Project Glasswing).

Live controversies (per Fable 5 Raises the Bar for AI Ambition)

  • Bio guardrails over-trigger. Users hit "chat paused" on basic biology — "mitochondria," "DNA → RNA," even the word cancer rerouted to Opus 4.8. Anthropic owns this in the blog post: bio/chem/cyber/distillation requests auto-route to Opus; 95% of sessions don't fall back. Real biologists' complaints are substantive; "looking-for-something-to-complain-about" Twitter noise should be filtered out separately.
  • AI-research distillation filter is invisible — WALKED BACK within 24h. Buried on p13 of the 319-page system card: Fable is "deliberately bad" on frontier LLM-development tasks (pre-training pipelines, distributed training infra, ML accelerator design) — and degraded without telling the user. Critics (Prime Intellect's Ellie Bau / Will Brown, Nathan Lambert, Dean Ball) read this as a response to Chinese low-cost distillation work; collateral damage on legitimate research is real. Anthropic blog framing: "using Claude to develop competing models already violates our terms of service."

    warning Superseded per Why Fable 5 is the Most Controversial AI Release Ever: after the firestorm, Anthropic told Wired "We made the wrong trade-off, and we apologize" and made these safeguards visible (explicit refusal or announced rerouting). See Silent Model Degradation for the precedent question that outlives the walkback.

  • 30-day retention is an enterprise NDA issue — STILL OPEN. Mythos-class prompts/outputs retained 30 days for trust-and-safety including human review. Mike Taylor: "if you used Claude Fable 5 today with memory turned on, you just violated all your NDAs." The Information's reporting on Glasswing partners shows the same friction at the eye-watering token-cost level. This is a real GA blocker, not theater. Escalation per Why Fable 5 is the Most Controversial AI Release Ever: the policy applies precisely to zero-data-retention customers, "flagged for potential serious harm" is at Anthropic's sole discretion, and Microsoft restricted employee use of Fable 5 in Copilot within a day (The Verge). Whittemore: this, not the research filter, is the unresolved revenue risk going into the IPO.
  • Anthropic walked back the "public Mythos in a few weeks" line (The Next Wave of Enterprise AI): from Opus 4.8 launch "Mythos-level coming in the next couple of weeks" to "safeguards… we, and to our knowledge all other AI developers, have yet to develop." Confused messaging; Glasswing meanwhile expanded to 150 partners.

Felix Ryberg's "third era" frame

Felix Ryberg (Claude Code lead) on the Fable 5 launch: "With Fable 5, I've personally moved on to responsibilities or loops. I no longer tell Claude to investigate a particular crash report. It runs a loop watching every crash report that comes in." See Tasks to Responsibilities Shift and Loops as Core Primitive — Fable's capability ceiling is what makes the responsibility-mode work.

Connects to your work

  • The launch is the concrete "frontier just moved" event behind the vault's loops / Claude Code Workflows / Token Maxing threads — a model explicitly built for long-horizon, parallel, token-heavy autonomous work.
  • The cost/routing angle is the practical enterprise takeaway: reserve Fable for the hardest problems, route the rest down-tier.

Sources

  • Claude Mythos is Finally Here (Nate Herk)
  • MYTHOS MYTHOS MYTHOS (Matthew Berman)
  • Fable 5 Raises the Bar for AI Ambition — third reaction; biology/distillation/retention controversies + use-case classification + Felix Ryberg's third-era framing
  • The Next Wave of Enterprise AI — Glasswing expansion to 150 partners; walked-back Mythos-public timeline
  • Why Fable 5 is the Most Controversial AI Release Ever — the backlash week: silent-degradation walkback, Microsoft Copilot restriction, lab-power discourse